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Sunday, December 19, 2021

Astrology and science

From Wikipedia, the free encyclopedia
 

Astrology consists of a number of belief systems that hold that there is a relationship between astronomical phenomena and events or descriptions of personality in the human world. Astrology has been rejected by the scientific community as having no explanatory power for describing the universe. Scientific testing has found no evidence to support the premises or purported effects outlined in astrological traditions.

Where astrology has made falsifiable predictions, it has been falsified. The most famous test was headed by Shawn Carlson and included a committee of scientists and a committee of astrologers. It led to the conclusion that natal astrology performed no better than chance. Astrologer and psychologist Michel Gauquelin claimed to have found statistical support for "the Mars effect" in the birth dates of athletes, but it could not be replicated in further studies. The organisers of later studies claimed that Gauquelin had tried to influence their inclusion criteria for the study by suggesting specific individuals be removed. It has also been suggested, by Geoffrey Dean, that the reporting of birth times by parents (before the 1950s) may have caused the apparent effect.

Astrology has not demonstrated its effectiveness in controlled studies and has no scientific validity, and is thus regarded as pseudoscience. There is no proposed mechanism of action by which the positions and motions of stars and planets could affect people and events on Earth in the way astrologers say they do that does not contradict well-understood, basic aspects of biology and physics.

Modern scientific inquiry into astrology is primarily focused on drawing a correlation between astrological traditions and the influence of seasonal birth in humans.

Introduction

The majority of professional astrologers rely on performing astrology-based personality tests and making relevant predictions about the remunerator's future. Those who continue to have faith in astrology have been characterised as doing so "in spite of the fact that there is no verified scientific basis for their beliefs, and indeed that there is strong evidence to the contrary". Astrophysicist Neil deGrasse Tyson commented on astrological belief, saying that "part of knowing how to think is knowing how the laws of nature shape the world around us. Without that knowledge, without that capacity to think, you can easily become a victim of people who seek to take advantage of you".

The continued belief in astrology despite its lack of credibility is seen as a demonstration of low scientific literacy, although some continue to believe in it even though they are scientifically literate.

Historical relationship with astronomy

The foundations of the theoretical structure used in astrology originate with the Babylonians, although widespread usage did not occur till the start of the Hellenistic period after Alexander the Great swept through Greece. It was not known to the Babylonians that the constellations are not on a celestial sphere and are very far apart. The appearance of them being close is illusory. The exact demarcation of what a constellation is, is cultural, and varied between civilisations. Ptolemy's work on astronomy was driven to some extent by the desire, like all astrologers of the time, to easily calculate the planetary movements. Early western astrology operated under the ancient Greek concepts of the Macrocosm and microcosm; and thus medical astrology related what happened to the planets and other objects in the sky to medical operations. This provided a further motivator for the study of astronomy. While still defending the practice of astrology, Ptolemy acknowledged that the predictive power of astronomy for the motion of the planets and other celestial bodies ranked above astrological predictions.

During the Islamic Golden Age, astronomy was funded so that the astronomical parameters, such as the eccentricity of the sun's orbit, required for the Ptolemaic model could be calculated to a sufficient accuracy and precision. Those in positions of power, like the Fatimid Caliphate vizier in 1120, funded the construction of observatories so that astrological predictions, fuelled by precise planetary information, could be made. Since the observatories were built to help in making astrological predictions, few of these observatories lasted long due to the prohibition against astrology within Islam, and most were torn down during or just after construction.

The clear rejection of astrology in works of astronomy started in 1679, with the yearly publication La Connoissance des temps. Unlike the west, in Iran, the rejection of heliocentrism continued up towards the start of the 20th century, in part motivated by a fear that this would undermine the widespread belief in astrology and Islamic cosmology in Iran. The first work, Falak al-sa'ada by Ictizad al-Saltana, aimed at undermining this belief in astrology and "old astronomy" in Iran was published in 1861. On astrology, it cited the inability of different astrologers to make the same prediction about what occurs following a conjunction, and described the attributes astrologers gave to the planets as implausible.

Philosophy of science

Philosopher Karl Popper proposed falsifiability as ideas that distinguish science from non-science, using astrology as the example of an idea that has not dealt with falsification during experiment.

Astrology provides the quintessential example of a pseudoscience since it has been tested repeatedly and failed all the tests.

Falsifiability

Science and non-science are often distinguished by the criterion of falsifiability. The criterion was first proposed by philosopher of science Karl Popper. To Popper, science does not rely on induction; instead, scientific investigations are inherently attempts to falsify existing theories through novel tests. If a single test fails, then the theory is falsified.

Therefore, any test of a scientific theory must prohibit certain results that falsify the theory, and expect other specific results consistent with the theory. Using this criterion of falsifiability, astrology is a pseudoscience.

Astrology was Popper's most frequent example of pseudoscience. Popper regarded astrology as "pseudo-empirical" in that "it appeals to observation and experiment", but "nevertheless does not come up to scientific standards".

In contrast to scientific disciplines, astrology does not respond to falsification through experiment. According to Professor of neurology Terence Hines, this is a hallmark of pseudoscience.

"No puzzles to solve"

In contrast to Popper, the philosopher Thomas Kuhn argued that it was not lack of falsifiability that makes astrology unscientific, but rather that the process and concepts of astrology are non-empirical. To Kuhn, although astrologers had, historically, made predictions that "categorically failed", this in itself does not make it unscientific, nor do the attempts by astrologers to explain away the failure by claiming it was due to the creation of a horoscope being very difficult (through subsuming, after the fact, a more general horoscope that leads to a different prediction).

Rather, in Kuhn's eyes, astrology is not science because it was always more akin to medieval medicine; they followed a sequence of rules and guidelines for a seemingly necessary field with known shortcomings, but they did no research because the fields are not amenable to research, and so, "They had no puzzles to solve and therefore no science to practise."

While an astronomer could correct for failure, an astrologer could not. An astrologer could only explain away failure but could not revise the astrological hypothesis in a meaningful way. As such, to Kuhn, even if the stars could influence the path of humans through life astrology is not scientific.

Progress, practice and consistency

Philosopher Paul Thagard believed that astrology can not be regarded as falsified in this sense until it has been replaced with a successor. In the case of predicting behaviour, psychology is the alternative. To Thagard a further criterion of demarcation of science from pseudoscience was that the state of the art must progress and that the community of researchers should be attempting to compare the current theory to alternatives, and not be "selective in considering confirmations and disconfirmations".

Progress is defined here as explaining new phenomena and solving existing problems, yet astrology has failed to progress having only changed little in nearly 2000 years. To Thagard, astrologers are acting as though engaged in normal science believing that the foundations of astrology were well established despite the "many unsolved problems", and in the face of better alternative theories (Psychology). For these reasons Thagard viewed astrology as pseudoscience.

To Thagard, astrology should not be regarded as a pseudoscience on the failure of Gauquelin to find any correlation between the various astrological signs and someone's career, twins not showing the expected correlations from having the same signs in twin studies, lack of agreement on the significance of the planets discovered since Ptolemy's time and large scale disasters wiping out individuals with vastly different signs at the same time. Rather, his demarcation of science requires three distinct foci: "theory, community [and] historical context".

While verification and falsifiability focused on the theory, Kuhn's work focused on the historical context, but the astrological community should also be considered. Whether or not they:

  • are focused on comparing their approach to others.
  • have a consistent approach.
  • try to falsify their theory through experiment.

In this approach, true falsification rather than modifying a theory to avoid the falsification only really occurs when an alternative theory is proposed.

Irrationality

For the philosopher Edward W. James, astrology is irrational not because of the numerous problems with mechanisms and falsification due to experiments, but because an analysis of the astrological literature shows that it is infused with fallacious logic and poor reasoning.

What if throughout astrological writings we meet little appreciation of coherence, blatant insensitivity to evidence, no sense of a hierarchy of reasons, slight command over the contextual force of critieria, stubborn unwillingness to pursue an argument where it leads, stark naivete concerning the efficacy of explanation and so on? In that case, I think, we are perfectly justified in rejecting astrology as irrational. ... Astrology simply fails to meet the multifarious demands of legitimate reasoning.

— Edward W. James

This poor reasoning includes appeals to ancient astrologers such as Kepler despite any relevance of topic or specific reasoning, and vague claims. The claim that evidence for astrology is that people born at roughly "the same place have a life pattern that is very similar" is vague, but also ignores that time is reference frame dependent and gives no definition of "same place" despite the planet's moving in the reference frame of the solar system. Other comments by astrologers are based on severely erroneous interpretations of basic physics, such as the general belief by medieval astrologers that the geocentric solar system corresponded to an atom. Further, James noted that response to criticism also relies on faulty logic, an example of which was a response to twin studies with the statement that coincidences in twins are due to astrology, but any differences are due to "heredity and environment", while for other astrologers the issues are too difficult and they just want to get back to their astrology. Further, to astrologers, if something appears in their favour, they latch upon it as proof, while making no attempt to explore its implications, preferring to refer to the item in favour as definitive; possibilities that do not make astrology look favourable are ignored.

Quinean dichotomy

From the Quinean web of knowledge, there is a dichotomy where one must either reject astrology or accept astrology but reject all established scientific disciplines that are incompatible with astrology.

Tests of astrology

Astrologers often avoid making verifiable predictions, and instead rely on vague statements that let them try to avoid falsification. Across several centuries of testing, the predictions of astrology have never been more accurate than that expected by chance alone. One approach used in testing astrology quantitatively is through blind experiment. When specific predictions from astrologers were tested in rigorous experimental procedures in the Carlson test, the predictions were falsified. All controlled experiments have failed to show any effect.

Carlson's experiment

Shawn Carlson's now renowned experiment was performed by 28 astrologers matching over 100 natal charts to psychological profiles generated by the California Psychological Inventory (CPI) test using double blind methods.

The experimental protocol used in Carlson's study was agreed to by a group of physicists and astrologers prior to the experiment. Astrologers, nominated by the National Council for Geocosmic Research, acted as the astrological advisors, and helped to ensure, and agreed, that the test was fair. They also chose 26 of the 28 astrologers for the tests, the other two being interested astrologers who volunteered afterwards. The astrologers came from Europe and the United States. The astrologers helped to draw up the central proposition of natal astrology to be tested. Published in Nature in 1985, the study found that predictions based on natal astrology were no better than chance, and that the testing "clearly refutes the astrological hypothesis".

Dean and Kelly

Scientist and former astrologer Geoffrey Dean and psychologist Ivan Kelly conducted a large-scale scientific test, involving more than one hundred cognitive, behavioural, physical and other variables, but found no support for astrology. A further test involved 45 confident astrologers, with an average of 10 years' experience and 160 test subjects (out of an original sample size of 1198 test subjects) who strongly favoured certain characteristics in the Eysenck Personality Questionnaire to extremes. The astrologers performed much worse than merely basing decisions off the individuals' ages, and much worse than 45 control subjects who did not use birth charts at all.

Other tests

A meta-analysis was conducted, pooling 40 studies consisting of 700 astrologers and over 1,000 birth charts. Ten of the tests, which had a total of 300 participating, involved the astrologers picking the correct chart interpretation out of a number of others that were not the astrologically correct chart interpretation (usually three to five others). When the date and other obvious clues were removed, no significant results were found to suggest there was any preferred chart.

In 10 studies, participants picked horoscopes that they felt were accurate descriptions, with one being the "correct" answer. Again the results were no better than chance.

In a study of 2011 sets of people born within 5 minutes of each other ("time twins") to see if there was any discernible effect; no effect was seen.

Quantitative sociologist David Voas examined the census data for more than 20 million individuals in England and Wales to see if star signs corresponded to marriage arrangements. No effect was seen.

Mars effect

The initial Mars effect finding, showing the relative frequency of the diurnal position of Mars in the birth charts (N = 570) of "eminent athletes" (red solid line) compared to the expected results [after Michel Gauquelin 1955]

In 1955, astrologer and psychologist Michel Gauquelin stated that although he had failed to find evidence to support such indicators as the zodiacal signs and planetary aspects in astrology, he had found positive correlations between the diurnal positions of some of the planets and success in professions (such as doctors, scientists, athletes, actors, writers, painters, etc.), which astrology traditionally associates with those planets. The best-known of Gauquelin's findings is based on the positions of Mars in the natal charts of successful athletes and became known as the "Mars effect". A study conducted by seven French scientists attempted to replicate the claim, but found no statistical evidence. They attributed the effect to selective bias on Gauquelin's part, accusing him of attempting to persuade them to add or delete names from their study.

Geoffrey Dean has suggested that the effect may be caused by self-reporting of birth dates by parents rather than any issue with the study by Gauquelin. The suggestion is that a small subset of the parents may have had changed birth times to be consistent with better astrological charts for a related profession. The sample group was taken from a time where belief in astrology was more common. Gauquelin had failed to find the Mars effect in more recent populations, where a nurse or doctor recorded the birth information. The number of births under astrologically undesirable conditions was also lower, indicating more evidence that parents choose dates and times to suit their beliefs.

Theoretic obstacles

Beyond the scientific tests astrology has failed, proposals for astrology face a number of other obstacles due to the many theoretical flaws in astrology including lack of consistency, lack of ability to predict missing planets, lack of connection of the zodiac to the constellations in western astrology, and lack of any plausible mechanism. The underpinnings of astrology tend to disagree with numerous basic facts from scientific disciplines.

Lack of consistency

Testing the validity of astrology can be difficult because there is no consensus amongst astrologers as to what astrology is or what it can predict. Dean and Kelly documented 25 studies, which had found that the degree of agreement amongst astrologers' predictions was measured as a low 0.1. Most professional astrologers are paid to predict the future or describe a person's personality and life, but most horoscopes only make vague untestable statements that can apply to almost anyone.

Georges Charpak and Henri Broch dealt with claims from western astrology in the book Debunked! ESP, Telekinesis, and other Pseudoscience. They pointed out that astrologers have only a small knowledge of astronomy and that they often do not take into account basic features such as the precession of the equinoxes. They commented on the example of Elizabeth Teissier who claimed that "the sun ends up in the same place in the sky on the same date each year" as the basis for claims that two people with the same birthday but a number of years apart should be under the same planetary influence. Charpak and Broch noted that "there is a difference of about twenty-two thousand miles between Earth's location on any specific date in two successive years" and that thus they should not be under the same influence according to astrology. Over a 40 years period there would be a difference greater than 780,000 miles.

Lack of physical basis

Edward W. James, commented that attaching significance to the constellation on the celestial sphere the sun is in at sunset was done on the basis of human factors—namely, that astrologers didn't want to wake up early, and the exact time of noon was hard to know. Further, the creation of the zodiac and the disconnect from the constellations was because the sun is not in each constellation for the same amount of time. This disconnection from the constellations led to the problem with precession separating the zodiac symbols from the constellations that they once were related to. Philosopher of science, Massimo Pigliucci commenting on the movement, opined "Well then, which sign should I look up when I open my Sunday paper, I wonder?"

The tropical zodiac has no connection to the stars, and as long as no claims are made that the constellations themselves are in the associated sign, astrologers avoid the concept that precession seemingly moves the constellations because they don't reference them. Charpak and Broch, noting this, referred to astrology based on the tropical zodiac as being "...empty boxes that have nothing to do with anything and are devoid of any consistency or correspondence with the stars." Sole use of the tropical zodiac is inconsistent with references made, by the same astrologers, to the Age of Aquarius, which depends on when the vernal point enters the constellation of Aquarius.

Lack of predictive power

Shown in the image is Pluto and its satellites. Astrology was claimed to work before the discovery of Neptune, Uranus and Pluto and they have now been included in the discourse on an ad hoc basis.

Some astrologers make claims that the position of all the planets must be taken into account, but astrologers were unable to predict the existence of Neptune based on mistakes in horoscopes. Instead Neptune was predicted using Newton's law of universal gravitation. The grafting on of Uranus, Neptune and Pluto into the astrology discourse was done on an ad hoc basis.

On the demotion of Pluto to the status of dwarf planet, Philip Zarka of the Paris Observatory in Meudon, France wondered how astrologers should respond:

Should astrologers remove it from the list of luminars [Sun, Moon and the 8 planets other than earth] and confess that it did not actually bring any improvement? If they decide to keep it, what about the growing list of other recently discovered similar bodies (Sedna, Quaoar. etc), some of which even have satellites (Xena, 2003EL61)?

Lack of mechanism

Astrology has been criticised for failing to provide a physical mechanism that links the movements of celestial bodies to their purported effects on human behaviour. In a lecture in 2001, Stephen Hawking stated "The reason most scientists don't believe in astrology is because it is not consistent with our theories that have been tested by experiment." In 1975, amid increasing popular interest in astrology, The Humanist magazine presented a rebuttal of astrology in a statement put together by Bart J. Bok, Lawrence E. Jerome, and Paul Kurtz. The statement, entitled "Objections to Astrology", was signed by 186 astronomers, physicists and leading scientists of the day. They said that there is no scientific foundation for the tenets of astrology and warned the public against accepting astrological advice without question. Their criticism focused on the fact that there was no mechanism whereby astrological effects might occur:

We can see how infinitesimally small are the gravitational and other effects produced by the distant planets and the far more distant stars. It is simply a mistake to imagine that the forces exerted by stars and planets at the moment of birth can in any way shape our futures.

Astronomer Carl Sagan declined to sign the statement. Sagan said he took this stance not because he thought astrology had any validity, but because he thought that the tone of the statement was authoritarian, and that dismissing astrology because there was no mechanism (while "certainly a relevant point") was not in itself convincing. In a letter published in a follow-up edition of The Humanist, Sagan confirmed that he would have been willing to sign such a statement had it described and refuted the principal tenets of astrological belief. This, he argued, would have been more persuasive and would have produced less controversy.

The use of poetic imagery based on the concepts of the macrocosm and microcosm, "as above so below" to decide meaning such as Edward W. James' example of "Mars above is red, so Mars below means blood and war", is a false cause fallacy.

Many astrologers claim that astrology is scientific. If one were to attempt to try to explain it scientifically, there are only four fundamental forces (conventionally), limiting the choice of possible natural mechanisms. Some astrologers have proposed conventional causal agents such as electromagnetism and gravity. The strength of these forces drops off with distance. Scientists reject these proposed mechanisms as implausible since, for example, the magnetic field, when measured from earth, of a large but distant planet such as Jupiter is far smaller than that produced by ordinary household appliances. Astronomer Phil Plait noted that in terms of magnitude, the sun is the only object with an electromagnetic field of note, but astrology isn't based just off the sun alone. While astrologers could try to suggest a fifth force, this is inconsistent with the trends in physics with the unification of electromagnetism and the weak force into the electroweak force. If the astrologer insisted on being inconsistent with the current understanding and evidential basis of physics, that would be an extraordinary claim. It would also be inconsistent with the other forces which drop off with distance. If distance is irrelevant, then, logically, all objects in space should be taken into account.

Carl Jung sought to invoke synchronicity, the claim that two events have some sort of acausal connection, to explain the lack of statistically significant results on astrology from a single study he conducted. However, synchronicity itself is considered neither testable nor falsifiable. The study was subsequently heavily criticised for its non-random sample and its use of statistics and also its lack of consistency with astrology.

Psychology

It has also been shown that confirmation bias is a psychological factor that contributes to belief in astrology. Confirmation bias is a form of cognitive bias.

From the literature, astrology believers often tend to selectively remember those predictions that turned out to be true and do not remember those that turned out false. Another, separate, form of confirmation bias also plays a role, where believers often fail to distinguish between messages that demonstrate special ability and those that do not.

Thus there are two distinct forms of confirmation bias that are under study with respect to astrological belief.

The Barnum effect is the tendency for an individual to give a high accuracy rating to a description of their personality that supposedly tailored specifically for them, but is, in fact, vague and general enough to apply to a wide range of people. If more information is requested for a prediction, the more accepting people are of the results.

In 1949 Bertram Forer conducted a personality test on students in his classroom. Each student was given a supposedly individual assessment but actually all students received the same assessment. The personality descriptions were taken from a book on astrology. When the students were asked to comment on the accuracy of the test, more than 40% gave it the top mark of 5 out of 5, and the average rating was 4.2. The results of this study have been replicated in numerous other studies.

The study of the Barnum/Forer effect has been focused mostly on the level of acceptance of fake horoscopes and fake astrological personality profiles. Recipients of these personality assessments consistently fail to distinguish between common and uncommon personality descriptors. In a study by Paul Rogers and Janice Soule (2009), which was consistent with previous research on the issue, it was found that those who believed in astrology are generally more susceptible to giving more credence to the Barnum profile than sceptics.

By a process known as self-attribution, it has been shown in numerous studies that individuals with knowledge of astrology tend to describe their personalities in terms of traits compatible with their sun signs. The effect is heightened when the individuals were aware that the personality description was being used to discuss astrology. Individuals who were not familiar with astrology had no such tendency.

Sociology

In 1953, sociologist Theodor W. Adorno conducted a study of the astrology column of a Los Angeles newspaper as part of a project that examined mass culture in capitalist society. Adorno believed that popular astrology, as a device, invariably led to statements that encouraged conformity—and that astrologers who went against conformity with statements that discouraged performance at work etc. risked losing their jobs. Adorno concluded that astrology was a large-scale manifestation of systematic irrationalism, where flattery and vague generalisations subtly led individuals to believe the author of the column addressed them directly. Adorno drew a parallel with the phrase opium of the people, by Karl Marx, by commenting, "Occultism is the metaphysic of the dopes."

False balance is where a false, unaccepted or spurious viewpoint is included alongside a well reasoned one in media reports and TV appearances and as a result the false balance implies "there were two equal sides to a story when clearly there were not". During Wonders of the Solar System, a TV programme by the BBC, the physicist Brian Cox said: "Despite the fact that astrology is a load of rubbish, Jupiter can in fact have a profound influence on our planet. And it's through a force... gravity." This upset believers in astrology who complained that there was no astrologer to provide an alternative viewpoint. Following the complaints of astrology believers, Cox gave the following statement to the BBC: "I apologise to the astrology community for not making myself clear. I should have said that this new age drivel is undermining the very fabric of our civilisation." In the programme Stargazing Live, Cox further commented by saying: "in the interests of balance on the BBC, yes astrology is nonsense." In an editorial in the medical journal BMJ, editor Trevor Jackson cited this incident showing where false balance could occur.

Studies and polling has shown that the belief in astrology is higher in western countries than might otherwise be expected. In 2012, in polls 42% of Americans said they thought astrology was at least partially scientific. This belief decreased with education and education is highly correlated with levels of scientific knowledge.

Some of the reported belief levels are due to a confusion of astrology with astronomy (the scientific study of celestial objects). The closeness of the two words varies depending on the language. A plain description of astrology as an "occult influence of stars, planets etc. on human affairs" had no impact on the general public's assessment of whether astrology is scientific or not in a 1992 eurobarometer poll. This may partially be due to the implicit association amongst the general public, of any wording ending in "-ology" with a legitimate field of knowledge. In Eurobarometers 224 and 225 performed in 2004, a split poll was used to isolate confusion over wording. In half of the polls, the word "astrology" was used, while in the other the word "horoscope" was used. Belief that astrology was at least partially scientific was 76%, but belief that horoscopes were at least partially scientific was 43%. In particular, belief that astrology was very scientific was 26% while that of horoscopes was 7%. This appeared to indicate that the high level of apparent polling support for astrology in the EU was indeed due to confusion over terminology.

Astrology and astronomy

From Wikipedia, the free encyclopedia

Astrology and astronomy were archaically treated together (Latin: astrologia), and were only gradually separated in Western 17th century philosophy (the "Age of Reason") with the rejection of astrology. During the later part of the medieval period, astronomy was treated as the foundation upon which astrology could operate.

Since the 18th century they have come to be regarded as completely separate disciplines. Astronomy, the study of objects and phenomena originating beyond the Earth's atmosphere, is a science and is a widely studied academic discipline. Astrology, which uses the apparent positions of celestial objects as the basis for the prediction of future events, is a form of divination and a pseudoscience having no scientific validity.

Overview

Early science, particularly geometry and astronomy/astrology (astronomia), was connected to the divine for most medieval scholars. The compass in this 13th-century manuscript is a symbol of God's act of creation, as many believed that there was something intrinsically divine or perfect that could be found in circles.

In pre-modern times, most cultures did not make a clear distinction between the two disciplines, putting them both together as one. In ancient Babylonia, famed for its astrology, there were not separate roles for the astronomer as predictor of celestial phenomena, and the astrologer as their interpreter; both functions were performed by the same person. This overlap does not mean that astrology and astronomy were always regarded as one and the same. In ancient Greece, pre-Socratic thinkers such as Anaximander, Xenophanes, Anaximenes, and Heraclides speculated about the nature and substance of the stars and planets. Astronomers such as Eudoxus (contemporary with Plato) observed planetary motions and cycles, and created a geocentric cosmological model that would be accepted by Aristotle. This model generally lasted until Ptolemy, who added epicycles to explain the retrograde motion of Mars. (Around 250 BC, Aristarchus of Samos postulated a proto-heliocentric theory, which would not be reconsidered for nearly two millennia (Copernicus), as Aristotle's geocentric model continued to be favored.) The Platonic school promoted the study of astronomy as a part of philosophy because the motions of the heavens demonstrate an orderly and harmonious cosmos. In the third century BC, Babylonian astrology began to make its presence felt in Greece. Astrology was criticized by Hellenistic philosophers such as the Academic Skeptic Carneades and Middle Stoic Panaetius. However, the notions of the Great Year (when all the planets complete a full cycle and return to their relative positions) and eternal recurrence were Stoic doctrines that made divination and fatalism possible.

In the Hellenistic world, the Greek words 'astrologia' and 'astronomia' were often used interchangeably, but they were conceptually not the same. Plato taught about 'astronomia' and stipulated that planetary phenomena should be described by a geometrical model. The first solution was proposed by Eudoxus. Aristotle favored a physical approach and adopted the word 'astrologia'. Eccentrics and epicycles came to be thought of as useful fictions. For a more general public, the distinguishing principle was not evident and either word was acceptable. For the Babylonian horoscopic practice, the words specifically used were 'apotelesma' and 'katarche', but otherwise it was subsumed under the aristotelian term 'astrologia'.

In his compilatory work Etymologiae, Isidore of Seville noted explicitly the difference between the terms astronomy and astrology (Etymologiae, III, xxvii) and the same distinction appeared later in the texts of Arabian writers. Isidore identified the two strands entangled in the astrological discipline and called them astrologia naturalis and astrologia superstitiosa.

Astrology was widely accepted in medieval Europe as astrological texts from Hellenistic and Arabic astrologers were translated into Latin. In the late Middle Ages, its acceptance or rejection often depended on its reception in the royal courts of Europe. Not until the time of Francis Bacon was astrology rejected as a part of scholastic metaphysics rather than empirical observation. A more definitive split between astrology and astronomy in the West took place gradually in the seventeenth and eighteenth centuries, when astrology was increasingly thought of as an occult science or superstition by the intellectual elite. Because of their lengthy shared history, it sometimes happens that the two are confused with one another even today. Many contemporary astrologers, however, do not claim that astrology is a science, but think of it as a form of divination like the I-Ching, an art, or a part of a spiritual belief structure (influenced by trends such as Neoplatonism, Neopaganism, Theosophy, and Hinduism).

Distinguishing characteristics

Astrologer–astronomer Richard of Wallingford is shown measuring an equatorium with a pair of compasses in this 14th-century work.

The primary goal of astronomy is to understand the physics of the universe. Astrologers use astronomical calculations for the positions of celestial bodies along the ecliptic and attempt to correlate celestial events (astrological aspects, sign positions) with earthly events and human affairs. Astronomers consistently use the scientific method, naturalistic presuppositions and abstract mathematical reasoning to investigate or explain phenomena in the universe. Astrologers use mystical or religious reasoning as well as traditional folklore, symbolism and superstition blended with mathematical predictions to explain phenomena in the universe. The scientific method is not consistently used by astrologers.

Astrologers practice their discipline geocentrically and they consider the universe to be harmonious, changeless and static, while astronomers have employed the scientific method to infer that the universe is without a center and is dynamic, expanding outward per the Big Bang theory.

Astrologers believe that the position of the stars and planets determine an individual's personality and future. Astronomers study the actual stars and planets, but have found no evidence supporting astrological theories. Psychologists study personality, and while there are many theories of personality, no mainstream theories in that field are based on astrology. (The Myers-Briggs personality typology, based on the works of Carl Jung, has four major categories that correspond to the astrological elements of fire, air, earth, and water. This theory of personality is used by career counselors and life coaches but not by psychologists.)

Both astrologers and astronomers see Earth as being an integral part of the universe, that Earth and the universe are interconnected as one cosmos (not as being separate and distinct from each other). However, astrologers philosophically and mystically portray the cosmos as having a supernatural, metaphysical and divine essence that actively influences world events and the personal lives of people. Astronomers, as members of the scientific community, cannot use in their scientific articles explanations that are not derived from empirically reproducible conditions, irrespective of their personal convictions.

Historical divergence

An engraving by Albrecht Dürer featuring Mashallah, from the title page of the De scientia motus orbis (Latin version with engraving, 1504). As in many medieval illustrations, the compass here is an icon of religion as well as science, in reference to God as the architect of creation.

For a long time the funding from astrology supported some astronomical research, which was in turn used to make more accurate ephemerides for use in astrology. In Medieval Europe the word Astronomia was often used to encompass both disciplines as this included the study of astronomy and astrology jointly and without a real distinction; this was one of the original Seven Liberal Arts. Kings and other rulers generally employed court astrologers to aid them in the decision making in their kingdoms, thereby funding astronomical research. University medical students were taught astrology as it was generally used in medical practice.

Astronomy and astrology diverged over the course of the 17th through 19th centuries. Copernicus didn't practice astrology (nor empirical astronomy; his work was theoretical), but the most important astronomers before Isaac Newton were astrologers by profession – Tycho Brahe, Johannes Kepler, and Galileo Galilei.

Also relevant here was the development of better timekeeping instruments, initially for aid in navigation; improved timekeeping made it possible to make more exact astrological predictions—predictions which could be tested, and which consistently proved to be false. By the end of the 18th century, astronomy was one of the major sciences of the Enlightenment model, using the recently codified scientific method, and was altogether distinct from astrology.

History of astronomy

From Wikipedia, the free encyclopedia

Astronomy is the oldest of the natural sciences, dating back to antiquity, with its origins in the religious, mythological, cosmological, calendrical, and astrological beliefs and practices of prehistory: vestiges of these are still found in astrology, a discipline long interwoven with public and governmental astronomy. It was not completely separated in Europe (see astrology and astronomy) during the Copernican Revolution starting in 1543. In some cultures, astronomical data was used for astrological prognostication. The study of astronomy has received financial and social support from many institutions, especially the Church, which was its largest source of support between the 12th century to the Enlightenment.

Ancient astronomers were able to differentiate between stars and planets, as stars remain relatively fixed over the centuries while planets will move an appreciable amount during a comparatively short time.

Early history

Early cultures identified celestial objects with gods and spirits. They related these objects (and their movements) to phenomena such as rain, drought, seasons, and tides. It is generally believed that the first astronomers were priests, and that they understood celestial objects and events to be manifestations of the divine, hence early astronomy's connection to what is now called astrology. A 32,500 year old carved ivory Mammoth tusk could contain the oldest known star chart (resembling the constellation Orion). It has also been suggested that drawing on the wall of the Lascaux caves in France dating from 33,000 to 10,000 years ago could be a graphical representation of the Pleiades, the Summer Triangle, and the Northern Crown. Ancient structures with possibly astronomical alignments (such as Stonehenge) probably fulfilled astronomical, religious, and social functions.

Calendars of the world have often been set by observations of the Sun and Moon (marking the day, month and year), and were important to agricultural societies, in which the harvest depended on planting at the correct time of year, and for which the nearly full moon was the only lighting for night-time travel into city markets.

Sunset at the equinox from the prehistoric site of Pizzo Vento at Fondachelli Fantina, Sicily

The common modern calendar is based on the Roman calendar. Although originally a lunar calendar, it broke the traditional link of the month to the phases of the Moon and divided the year into twelve almost-equal months, that mostly alternated between thirty and thirty-one days. Julius Caesar instigated calendar reform in 46 BCE and introduced what is now called the Julian calendar, based upon the 365 14 day year length originally proposed by the 4th century BCE Greek astronomer Callippus.

Prehistoric Europe

The Nebra sky disk Germany 1600 BC
 
Calendrical functions of the Berlin Gold Hat c. 1000 BC

Since 1990 our understanding of prehistoric Europeans has been radically changed by discoveries of ancient astronomical artifacts throughout Europe. The artifacts demonstrate that Neolithic and Bronze Age Europeans had a sophisticated knowledge of mathematics and astronomy.

Among the discoveries are:

  • Paleolithic archaeologist Alexander Marshack put forward a theory in 1972 that bone sticks from locations like Africa and Europe from possibly as long ago as 35,000 BCE could be marked in ways that tracked the Moon's phases, an interpretation that has met with criticism.
  • The Warren Field calendar in the Dee River valley of Scotland's Aberdeenshire. First excavated in 2004 but only in 2013 revealed as a find of huge significance, it is to date the world's oldest known calendar, created around 8000 BC and predating all other calendars by some 5,000 years. The calendar takes the form of an early Mesolithic monument containing a series of 12 pits which appear to help the observer track lunar months by mimicking the phases of the Moon. It also aligns to sunrise at the winter solstice, thus coordinating the solar year with the lunar cycles. The monument had been maintained and periodically reshaped, perhaps up to hundreds of times, in response to shifting solar/lunar cycles, over the course of 6,000 years, until the calendar fell out of use around 4,000 years ago.
  • Goseck circle is located in Germany and belongs to the linear pottery culture. First discovered in 1991, its significance was only clear after results from archaeological digs became available in 2004. The site is one of hundreds of similar circular enclosures built in a region encompassing Austria, Germany, and the Czech Republic during a 200-year period starting shortly after 5000 BC.
  • The Nebra sky disc is a Bronze Age bronze disc that was buried in Germany, not far from the Goseck circle, around 1600 BC. It measures about 30 cm diameter with a mass of 2.2 kg and displays a blue-green patina (from oxidization) inlaid with gold symbols. Found by archeological thieves in 1999 and recovered in Switzerland in 2002, it was soon recognized as a spectacular discovery, among the most important of the 20th century. Investigations revealed that the object had been in use around 400 years before burial (2000 BC), but that its use had been forgotten by the time of burial. The inlaid gold depicted the full moon, a crescent moon about 4 or 5 days old, and the Pleiades star cluster in a specific arrangement forming the earliest known depiction of celestial phenomena. Twelve lunar months pass in 354 days, requiring a calendar to insert a leap month every two or three years in order to keep synchronized with the solar year's seasons (making it lunisolar). The earliest known descriptions of this coordination were recorded by the Babylonians in 6th or 7th centuries BC, over one thousand years later. Those descriptions verified ancient knowledge of the Nebra sky disc's celestial depiction as the precise arrangement needed to judge when to insert the intercalary month into a lunisolar calendar, making it an astronomical clock for regulating such a calendar a thousand or more years before any other known method.
  • The Kokino site, discovered in 2001, sits atop an extinct volcanic cone at an elevation of 1,013 metres (3,323 ft), occupying about 0.5 hectares overlooking the surrounding countryside in North Macedonia. A Bronze Age astronomical observatory was constructed there around 1900 BC and continuously served the nearby community that lived there until about 700 BC. The central space was used to observe the rising of the Sun and full moon. Three markings locate sunrise at the summer and winter solstices and at the two equinoxes. Four more give the minimum and maximum declinations of the full moon: in summer, and in winter. Two measure the lengths of lunar months. Together, they reconcile solar and lunar cycles in marking the 235 lunations that occur during 19 solar years, regulating a lunar calendar. On a platform separate from the central space, at lower elevation, four stone seats (thrones) were made in north-south alignment, together with a trench marker cut in the eastern wall. This marker allows the rising Sun's light to fall on only the second throne, at midsummer (about July 31). It was used for ritual ceremony linking the ruler to the local sun god, and also marked the end of the growing season and time for harvest.
  • Golden hats of Germany, France and Switzerland dating from 1400–800 BC are associated with the Bronze Age Urnfield culture. The Golden hats are decorated with a spiral motif of the Sun and the Moon. They were probably a kind of calendar used to calibrate between the lunar and solar calendars. Modern scholarship has demonstrated that the ornamentation of the gold leaf cones of the Schifferstadt type, to which the Berlin Gold Hat example belongs, represent systematic sequences in terms of number and types of ornaments per band. A detailed study of the Berlin example, which is the only fully preserved one, showed that the symbols probably represent a lunisolar calendar. The object would have permitted the determination of dates or periods in both lunar and solar calendars.

Ancient times

Mesopotamia

Babylonian tablet in the British Museum recording Halley's comet in 164 BC.

The origins of Western astronomy can be found in Mesopotamia, the 'land between the rivers' Tigris and Euphrates, where the ancient kingdoms of Sumer, Assyria, and Babylonia were located. A form of writing known as cuneiform emerged among the Sumerians around 3500–3000 BC. Our knowledge of Sumerian astronomy is indirect, via the earliest Babylonian star catalogues dating from about 1200 BC. The fact that many star names appear in Sumerian suggests a continuity reaching into the Early Bronze Age. Astral theology, which gave planetary gods an important role in Mesopotamian mythology and religion, began with the Sumerians. They also used a sexagesimal (base 60) place-value number system, which simplified the task of recording very large and very small numbers. The modern practice of dividing a circle into 360 degrees, or an hour into 60 minutes, began with the Sumerians. For more information, see the articles on Babylonian numerals and mathematics.

Classical sources frequently use the term Chaldeans for the astronomers of Mesopotamia, who were, in reality, priest-scribes specializing in astrology and other forms of divination.

The first evidence of recognition that astronomical phenomena are periodic and of the application of mathematics to their prediction is Babylonian. Tablets dating back to the Old Babylonian period document the application of mathematics to the variation in the length of daylight over a solar year. Centuries of Babylonian observations of celestial phenomena are recorded in the series of cuneiform tablets known as the Enūma Anu Enlil. The oldest significant astronomical text that we possess is Tablet 63 of the Enūma Anu Enlil, the Venus tablet of Ammi-saduqa, which lists the first and last visible risings of Venus over a period of about 21 years and is the earliest evidence that the phenomena of a planet were recognized as periodic. The MUL.APIN, contains catalogues of stars and constellations as well as schemes for predicting heliacal risings and the settings of the planets, lengths of daylight measured by a water clock, gnomon, shadows, and intercalations. The Babylonian GU text arranges stars in 'strings' that lie along declination circles and thus measure right-ascensions or time-intervals, and also employs the stars of the zenith, which are also separated by given right-ascensional differences.

A significant increase in the quality and frequency of Babylonian observations appeared during the reign of Nabonassar (747–733 BC). The systematic records of ominous phenomena in Babylonian astronomical diaries that began at this time allowed for the discovery of a repeating 18-year cycle of lunar eclipses, for example. The Greek astronomer Ptolemy later used Nabonassar's reign to fix the beginning of an era, since he felt that the earliest usable observations began at this time.

The last stages in the development of Babylonian astronomy took place during the time of the Seleucid Empire (323–60 BC). In the 3rd century BC, astronomers began to use "goal-year texts" to predict the motions of the planets. These texts compiled records of past observations to find repeating occurrences of ominous phenomena for each planet. About the same time, or shortly afterwards, astronomers created mathematical models that allowed them to predict these phenomena directly, without consulting past records. A notable Babylonian astronomer from this time was Seleucus of Seleucia, who was a supporter of the heliocentric model.

Babylonian astronomy was the basis for much of what was done in Greek and Hellenistic astronomy, in classical Indian astronomy, in Sassanian Iran, in Byzantium, in Syria, in Islamic astronomy, in Central Asia, and in Western Europe.

India

Historical Jantar Mantar observatory in Jaipur, India.

Astronomy in the Indian subcontinent dates back to the period of Indus Valley Civilization during 3rd millennium BCE, when it was used to create calendars. As the Indus Valley civilization did not leave behind written documents, the oldest extant Indian astronomical text is the Vedanga Jyotisha, dating from the Vedic period. Vedanga Jyotisha describes rules for tracking the motions of the Sun and the Moon for the purposes of ritual. During the 6th century, astronomy was influenced by the Greek and Byzantine astronomical traditions.

Aryabhata (476–550), in his magnum opus Aryabhatiya (499), propounded a computational system based on a planetary model in which the Earth was taken to be spinning on its axis and the periods of the planets were given with respect to the Sun. He accurately calculated many astronomical constants, such as the periods of the planets, times of the solar and lunar eclipses, and the instantaneous motion of the Moon. Early followers of Aryabhata's model included Varahamihira, Brahmagupta, and Bhaskara II.

Astronomy was advanced during the Shunga Empire and many star catalogues were produced during this time. The Shunga period is known as the "Golden age of astronomy in India". It saw the development of calculations for the motions and places of various planets, their rising and setting, conjunctions, and the calculation of eclipses.

Indian astronomers by the 6th century believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the 6th century by the astronomers Varahamihira and Bhadrabahu, and the 10th-century astronomer Bhattotpala listed the names and estimated periods of certain comets, but it is unfortunately not known how these figures were calculated or how accurate they were.

Bhāskara II (1114–1185) was the head of the astronomical observatory at Ujjain, continuing the mathematical tradition of Brahmagupta. He wrote the Siddhantasiromani which consists of two parts: Goladhyaya (sphere) and Grahaganita (mathematics of the planets). He also calculated the time taken for the Earth to orbit the Sun to 9 decimal places. The Buddhist University of Nalanda at the time offered formal courses in astronomical studies.

Other important astronomers from India include Madhava of Sangamagrama, Nilakantha Somayaji and Jyeshtadeva, who were members of the Kerala school of astronomy and mathematics from the 14th century to the 16th century. Nilakantha Somayaji, in his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, developed his own computational system for a partially heliocentric planetary model, in which Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits the Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century. Nilakantha's system, however, was mathematically more efficient than the Tychonic system, due to correctly taking into account the equation of the centre and latitudinal motion of Mercury and Venus. Most astronomers of the Kerala school of astronomy and mathematics who followed him accepted his planetary model.

Greece and Hellenistic world

The Antikythera Mechanism was an analog computer from 150–100 BC designed to calculate the positions of astronomical objects.

The Ancient Greeks developed astronomy, which they treated as a branch of mathematics, to a highly sophisticated level. The first geometrical, three-dimensional models to explain the apparent motion of the planets were developed in the 4th century BC by Eudoxus of Cnidus and Callippus of Cyzicus. Their models were based on nested homocentric spheres centered upon the Earth. Their younger contemporary Heraclides Ponticus proposed that the Earth rotates around its axis.

A different approach to celestial phenomena was taken by natural philosophers such as Plato and Aristotle. They were less concerned with developing mathematical predictive models than with developing an explanation of the reasons for the motions of the Cosmos. In his Timaeus, Plato described the universe as a spherical body divided into circles carrying the planets and governed according to harmonic intervals by a world soul. Aristotle, drawing on the mathematical model of Eudoxus, proposed that the universe was made of a complex system of concentric spheres, whose circular motions combined to carry the planets around the earth. This basic cosmological model prevailed, in various forms, until the 16th century.

In the 3rd century BC Aristarchus of Samos was the first to suggest a heliocentric system, although only fragmentary descriptions of his idea survive. Eratosthenes estimated the circumference of the Earth with great accuracy.

Greek geometrical astronomy developed away from the model of concentric spheres to employ more complex models in which an eccentric circle would carry around a smaller circle, called an epicycle which in turn carried around a planet. The first such model is attributed to Apollonius of Perga and further developments in it were carried out in the 2nd century BC by Hipparchus of Nicea. Hipparchus made a number of other contributions, including the first measurement of precession and the compilation of the first star catalog in which he proposed our modern system of apparent magnitudes.

The Antikythera mechanism, an ancient Greek astronomical observational device for calculating the movements of the Sun and the Moon, possibly the planets, dates from about 150–100 BC, and was the first ancestor of an astronomical computer. It was discovered in an ancient shipwreck off the Greek island of Antikythera, between Kythera and Crete. The device became famous for its use of a differential gear, previously believed to have been invented in the 16th century, and the miniaturization and complexity of its parts, comparable to a clock made in the 18th century. The original mechanism is displayed in the Bronze collection of the National Archaeological Museum of Athens, accompanied by a replica.

Depending on the historian's viewpoint, the acme or corruption of physical Greek astronomy is seen with Ptolemy of Alexandria, who wrote the classic comprehensive presentation of geocentric astronomy, the Megale Syntaxis (Great Synthesis), better known by its Arabic title Almagest, which had a lasting effect on astronomy up to the Renaissance. In his Planetary Hypotheses, Ptolemy ventured into the realm of cosmology, developing a physical model of his geometric system, in a universe many times smaller than the more realistic conception of Aristarchus of Samos four centuries earlier.

Egypt

The precise orientation of the Egyptian pyramids affords a lasting demonstration of the high degree of technical skill in watching the heavens attained in the 3rd millennium BC. It has been shown the Pyramids were aligned towards the pole star, which, because of the precession of the equinoxes, was at that time Thuban, a faint star in the constellation of Draco. Evaluation of the site of the temple of Amun-Re at Karnak, taking into account the change over time of the obliquity of the ecliptic, has shown that the Great Temple was aligned on the rising of the midwinter Sun. The length of the corridor down which sunlight would travel would have limited illumination at other times of the year. The Egyptians also found the position of Sirius (the dog star) who they believed was Anubis their Jackal headed god moving through the heavens. Its position was critical to their civilisation as when it rose heliacal in the east before sunrise it foretold the flooding of the Nile. It is also where we get the phrase 'dog days of summer' from.

Astronomy played a considerable part in religious matters for fixing the dates of festivals and determining the hours of the night. The titles of several temple books are preserved recording the movements and phases of the sun, moon and stars. The rising of Sirius (Egyptian: Sopdet, Greek: Sothis) at the beginning of the inundation was a particularly important point to fix in the yearly calendar.

Writing in the Roman era, Clement of Alexandria gives some idea of the importance of astronomical observations to the sacred rites:

And after the Singer advances the Astrologer (ὡροσκόπος), with a horologium (ὡρολόγιον) in his hand, and a palm (φοίνιξ), the symbols of astrology. He must know by heart the Hermetic astrological books, which are four in number. Of these, one is about the arrangement of the fixed stars that are visible; one on the positions of the Sun and Moon and five planets; one on the conjunctions and phases of the Sun and Moon; and one concerns their risings.

The Astrologer's instruments (horologium and palm) are a plumb line and sighting instrument. They have been identified with two inscribed objects in the Berlin Museum; a short handle from which a plumb line was hung, and a palm branch with a sight-slit in the broader end. The latter was held close to the eye, the former in the other hand, perhaps at arm's length. The "Hermetic" books which Clement refers to are the Egyptian theological texts, which probably have nothing to do with Hellenistic Hermetism.

From the tables of stars on the ceiling of the tombs of Rameses VI and Rameses IX it seems that for fixing the hours of the night a man seated on the ground faced the Astrologer in such a position that the line of observation of the pole star passed over the middle of his head. On the different days of the year each hour was determined by a fixed star culminating or nearly culminating in it, and the position of these stars at the time is given in the tables as in the centre, on the left eye, on the right shoulder, etc. According to the texts, in founding or rebuilding temples the north axis was determined by the same apparatus, and we may conclude that it was the usual one for astronomical observations. In careful hands it might give results of a high degree of accuracy.

China

Printed star map of Su Song (1020–1101) showing the south polar projection.

The astronomy of East Asia began in China. Solar term was completed in Warring States period. The knowledge of Chinese astronomy was introduced into East Asia.

Astronomy in China has a long history. Detailed records of astronomical observations were kept from about the 6th century BC, until the introduction of Western astronomy and the telescope in the 17th century. Chinese astronomers were able to precisely predict eclipses.

Much of early Chinese astronomy was for the purpose of timekeeping. The Chinese used a lunisolar calendar, but because the cycles of the Sun and the Moon are different, astronomers often prepared new calendars and made observations for that purpose.

Astrological divination was also an important part of astronomy. Astronomers took careful note of "guest stars"(Chinese: 客星; pinyin: kèxīng; lit.: 'guest star') which suddenly appeared among the fixed stars. They were the first to record a supernova, in the Astrological Annals of the Houhanshu in 185 AD. Also, the supernova that created the Crab Nebula in 1054 is an example of a "guest star" observed by Chinese astronomers, although it was not recorded by their European contemporaries. Ancient astronomical records of phenomena like supernovae and comets are sometimes used in modern astronomical studies.

The world's first star catalogue was made by Gan De, a Chinese astronomer, in the 4th century BC.

Mesoamerica

"El Caracol" observatory temple at Chichen Itza, Mexico.

Maya astronomical codices include detailed tables for calculating phases of the Moon, the recurrence of eclipses, and the appearance and disappearance of Venus as morning and evening star. The Maya based their calendrics in the carefully calculated cycles of the Pleiades, the Sun, the Moon, Venus, Jupiter, Saturn, Mars, and also they had a precise description of the eclipses as depicted in the Dresden Codex, as well as the ecliptic or zodiac, and the Milky Way was crucial in their Cosmology. A number of important Maya structures are believed to have been oriented toward the extreme risings and settings of Venus. To the ancient Maya, Venus was the patron of war and many recorded battles are believed to have been timed to the motions of this planet. Mars is also mentioned in preserved astronomical codices and early mythology.

Although the Maya calendar was not tied to the Sun, John Teeple has proposed that the Maya calculated the solar year to somewhat greater accuracy than the Gregorian calendar. Both astronomy and an intricate numerological scheme for the measurement of time were vitally important components of Maya religion.

Middle Ages

Middle East

Arabic astrolabe from 1208 AD

The Arabic and the Persian world under Islam had become highly cultured, and many important works of knowledge from Greek astronomy and Indian astronomy and Persian astronomy were translated into Arabic, used and stored in libraries throughout the area. An important contribution by Islamic astronomers was their emphasis on observational astronomy. This led to the emergence of the first astronomical observatories in the Muslim world by the early 9th century. Zij star catalogues were produced at these observatories.

In the 10th century, Abd al-Rahman al-Sufi (Azophi) carried out observations on the stars and described their positions, magnitudes, brightness, and colour and drawings for each constellation in his Book of Fixed Stars. He also gave the first descriptions and pictures of "A Little Cloud" now known as the Andromeda Galaxy. He mentions it as lying before the mouth of a Big Fish, an Arabic constellation. This "cloud" was apparently commonly known to the Isfahan astronomers, very probably before 905 AD. The first recorded mention of the Large Magellanic Cloud was also given by al-Sufi. In 1006, Ali ibn Ridwan observed SN 1006, the brightest supernova in recorded history, and left a detailed description of the temporary star.

In the late 10th century, a huge observatory was built near Tehran, Iran, by the astronomer Abu-Mahmud al-Khujandi who observed a series of meridian transits of the Sun, which allowed him to calculate the tilt of the Earth's axis relative to the Sun. He noted that measurements by earlier (Indian, then Greek) astronomers had found higher values for this angle, possible evidence that the axial tilt is not constant but was in fact decreasing. In 11th-century Persia, Omar Khayyám compiled many tables and performed a reformation of the calendar that was more accurate than the Julian and came close to the Gregorian.

Other Muslim advances in astronomy included the collection and correction of previous astronomical data, resolving significant problems in the Ptolemaic model, the development of the universal latitude-independent astrolabe by Arzachel, the invention of numerous other astronomical instruments, Ja'far Muhammad ibn Mūsā ibn Shākir's belief that the heavenly bodies and celestial spheres were subject to the same physical laws as Earth, the first elaborate experiments related to astronomical phenomena, the introduction of exacting empirical observations and experimental techniques, and the introduction of empirical testing by Ibn al-Shatir, who produced the first model of lunar motion which matched physical observations.

Natural philosophy (particularly Aristotelian physics) was separated from astronomy by Ibn al-Haytham (Alhazen) in the 11th century, by Ibn al-Shatir in the 14th century, and Qushji in the 15th century, leading to the development of an astronomical physics.

Western Europe

9th-century diagram of the positions of the seven planets on 18 March 816, from the Leiden Aratea.

After the significant contributions of Greek scholars to the development of astronomy, it entered a relatively static era in Western Europe from the Roman era through the 12th century. This lack of progress has led some astronomers to assert that nothing happened in Western European astronomy during the Middle Ages. Recent investigations, however, have revealed a more complex picture of the study and teaching of astronomy in the period from the 4th to the 16th centuries.

Western Europe entered the Middle Ages with great difficulties that affected the continent's intellectual production. The advanced astronomical treatises of classical antiquity were written in Greek, and with the decline of knowledge of that language, only simplified summaries and practical texts were available for study. The most influential writers to pass on this ancient tradition in Latin were Macrobius, Pliny, Martianus Capella, and Calcidius. In the 6th century Bishop Gregory of Tours noted that he had learned his astronomy from reading Martianus Capella, and went on to employ this rudimentary astronomy to describe a method by which monks could determine the time of prayer at night by watching the stars.

In the 7th century the English monk Bede of Jarrow published an influential text, On the Reckoning of Time, providing churchmen with the practical astronomical knowledge needed to compute the proper date of Easter using a procedure called the computus. This text remained an important element of the education of clergy from the 7th century until well after the rise of the Universities in the 12th century.

The range of surviving ancient Roman writings on astronomy and the teachings of Bede and his followers began to be studied in earnest during the revival of learning sponsored by the emperor Charlemagne. By the 9th century rudimentary techniques for calculating the position of the planets were circulating in Western Europe; medieval scholars recognized their flaws, but texts describing these techniques continued to be copied, reflecting an interest in the motions of the planets and in their astrological significance.

Building on this astronomical background, in the 10th century European scholars such as Gerbert of Aurillac began to travel to Spain and Sicily to seek out learning which they had heard existed in the Arabic-speaking world. There they first encountered various practical astronomical techniques concerning the calendar and timekeeping, most notably those dealing with the astrolabe. Soon scholars such as Hermann of Reichenau were writing texts in Latin on the uses and construction of the astrolabe and others, such as Walcher of Malvern, were using the astrolabe to observe the time of eclipses in order to test the validity of computistical tables.

By the 12th century, scholars were traveling to Spain and Sicily to seek out more advanced astronomical and astrological texts, which they translated into Latin from Arabic and Greek to further enrich the astronomical knowledge of Western Europe. The arrival of these new texts coincided with the rise of the universities in medieval Europe, in which they soon found a home. Reflecting the introduction of astronomy into the universities, John of Sacrobosco wrote a series of influential introductory astronomy textbooks: the Sphere, a Computus, a text on the Quadrant, and another on Calculation.

In the 14th century, Nicole Oresme, later bishop of Liseux, showed that neither the scriptural texts nor the physical arguments advanced against the movement of the Earth were demonstrative and adduced the argument of simplicity for the theory that the Earth moves, and not the heavens. However, he concluded "everyone maintains, and I think myself, that the heavens do move and not the earth: For God hath established the world which shall not be moved." In the 15th century, Cardinal Nicholas of Cusa suggested in some of his scientific writings that the Earth revolved around the Sun, and that each star is itself a distant sun.

Renaissance and Early Modern Europe

Copernican Revolution

During the renaissance period, astronomy began to undergo a revolution in thought known as the Copernican Revolution, which gets the name from the astronomer Nicolaus Copernicus, who proposed a heliocentric system, in which the planets revolved around the Sun and not the Earth. His De revolutionibus orbium coelestium was published in 1543. While in the long term this was a very controversial claim, in the very beginning it only brought minor controversy. The theory became the dominant view because many figures, most notably Galileo Galilei, Johannes Kepler and Isaac Newton championed and improved upon the work. Other figures also aided this new model despite not believing the overall theory, like Tycho Brahe, with his well-known observations.

Brahe, a Danish noble, was an essential astronomer in this period. He came on the astronomical scene with the publication of De nova stella, in which he disproved conventional wisdom on the supernova SN 1572 (As bright as Venus at its peak, SN 1572 later became invisible to the naked eye, disproving the Aristotelian doctrine of the immutability of the heavens.) He also created the Tychonic system, where the Sun and Moon and the stars revolve around the Earth, but the other five planets revolve around the Sun. This system blended the mathematical benefits of the Copernican system with the "physical benefits" of the Ptolemaic system. This was one of the systems people believed in when they did not accept heliocentrism, but could no longer accept the Ptolemaic system. He is most known for his highly accurate observations of the stars and the solar system. Later he moved to Prague and continued his work. In Prague he was at work on the Rudolphine Tables, that were not finished until after his death. The Rudolphine Tables was a star map designed to be more accurate than either the Alfonsine tables, made in the 1300s, and the Prutenic Tables, which were inaccurate. He was assisted at this time by his assistant Johannes Kepler, who would later use his observations to finish Brahe's works and for his theories as well.

After the death of Brahe, Kepler was deemed his successor and was given the job of completing Brahe's uncompleted works, like the Rudolphine Tables. He completed the Rudolphine Tables in 1624, although it was not published for several years. Like many other figures of this era, he was subject to religious and political troubles, like the Thirty Years' War, which led to chaos that almost destroyed some of his works. Kepler was, however, the first to attempt to derive mathematical predictions of celestial motions from assumed physical causes. He discovered the three Kepler's laws of planetary motion that now carry his name, those laws being as follows:

  1. The orbit of a planet is an ellipse with the Sun at one of the two foci.
  2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
  3. The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

With these laws, he managed to improve upon the existing heliocentric model. The first two were published in 1609. Kepler's contributions improved upon the overall system, giving it more credibility because it adequately explained events and could cause more reliable predictions. Before this, the Copernican model was just as unreliable as the Ptolemaic model. This improvement came because Kepler realized the orbits were not perfect circles, but ellipses.

Galileo Galilei (1564–1642) crafted his own telescope and discovered that the Moon had craters, that Jupiter had moons, that the Sun had spots, and that Venus had phases like the Moon. Portrait by Justus Sustermans.

Galileo Galilei was among the first to use a telescope to observe the sky, and after constructing a 20x refractor telescope. He discovered the four largest moons of Jupiter in 1610, which are now collectively known as the Galilean moons, in his honor. This discovery was the first known observation of satellites orbiting another planet. He also found that our Moon had craters and observed, and correctly explained, sunspots, and that Venus exhibited a full set of phases resembling lunar phases. Galileo argued that these facts demonstrated incompatibility with the Ptolemaic model, which could not explain the phenomenon and would even contradict it. With the moons it demonstrated that the Earth does not have to have everything orbiting it and that other parts of the Solar System could orbit another object, such as the Earth orbiting the Sun. In the Ptolemaic system the celestial bodies were supposed to be perfect so such objects should not have craters or sunspots. The phases of Venus could only happen in the event that Venus' orbit is insides Earth's orbit, which could not happen if the Earth was the center. He, as the most famous example, had to face challenges from church officials, more specifically the Roman Inquisition. They accused him of heresy because these beliefs went against the teachings of the Roman Catholic Church and were challenging the Catholic church's authority when it was at its weakest. While he was able to avoid punishment for a little while he was eventually tried and pled guilty to heresy in 1633. Although this came at some expense, his book was banned, and he was put under house arrest until he died in 1642.

Plate with figures illustrating articles on astronomy, from the 1728 Cyclopædia

Sir Isaac Newton developed further ties between physics and astronomy through his law of universal gravitation. Realizing that the same force that attracts objects to the surface of the Earth held the Moon in orbit around the Earth, Newton was able to explain – in one theoretical framework – all known gravitational phenomena. In his Philosophiæ Naturalis Principia Mathematica, he derived Kepler's laws from first principles. Those first principles are as follows:

  1. In an inertial frame of reference, an object either remains at rest or continues to move at constant velocity, unless acted upon by a force.
  2. In an inertial reference frame, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma. (It is assumed here that the mass m is constant)
  3. When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

Thus while Kepler explained how the planets moved, Newton accurately managed to explain why the planets moved the way they do. Newton's theoretical developments laid many of the foundations of modern physics.

Completing the Solar System

Outside of England, Newton's theory took some time to become established. Descartes' theory of vortices held sway in France, and Huygens, Leibniz and Cassini accepted only parts of Newton's system, preferring their own philosophies. Voltaire published a popular account in 1738. In 1748, the French Academy of Sciences offered a reward for solving the perturbations of Jupiter and Saturn which was eventually solved by Euler and Lagrange. Laplace completed the theory of the planets, publishing from 1798 to 1825. The early origins of the solar nebular model of planetary formation had begun.

Edmund Halley succeeded Flamsteed as Astronomer Royal in England and succeeded in predicting the return in 1758 of the comet that bears his name. Sir William Herschel found the first new planet, Uranus, to be observed in modern times in 1781. The gap between the planets Mars and Jupiter disclosed by the Titius–Bode law was filled by the discovery of the asteroids Ceres and 2 Pallas Pallas in 1801 and 1802 with many more following.

At first, astronomical thought in America was based on Aristotelian philosophy, but interest in the new astronomy began to appear in Almanacs as early as 1659.

Stellar astronomy

Cosmic pluralism is the name given to the idea that the stars are distant suns, perhaps with their own planetary systems. Ideas in this direction were expressed in antiquity, by Anaxagoras and by Aristarchus of Samos, but did not find mainstream acceptance. The first astronomer of the European Renaissance to suggest that the stars were distant suns was Giordano Bruno in his De l'infinito universo et mondi (1584). This idea was among the charges, albeit not in a prominent position, brought against him by the Inquisition. The idea became mainstream in the later 17th century, especially following the publication of Conversations on the Plurality of Worlds by Bernard Le Bovier de Fontenelle (1686), and by the early 18th century it was the default working assumptions in stellar astronomy.

The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus. William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction. In addition to his other accomplishments, William Herschel is noted for his discovery that some stars do not merely lie along the same line of sight, but are physical companions that form binary star systems.

Modern astronomy

Mars surface map of Giovanni Schiaparelli.

19th century

Comparison of CMB (Cosmic microwave background) results from satellites COBE, WMAP and Planck documenting a progress in 1989–2013.

In the 19th century, scientists began discovering forms of light which were invisible to the naked eye: X-Rays, gamma rays, radio waves, microwaves, ultraviolet radiation, and infrared radiation. This had a major impact on astronomy, spawning the fields of infrared astronomy, radio astronomy, x-ray astronomy and finally gamma-ray astronomy. With the advent of spectroscopy it was proven that other stars were similar to the Sun, but with a range of temperatures, masses and sizes.

The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types.

The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens. Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.

20th century

The modern version of the stellar classification scheme was developed by Annie J. Cannon during the early 1900s. The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.

Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. In Potsdam in 1906, the Danish astronomer Ejnar Hertzsprung published the first plots of color versus luminosity for these stars. These plots showed a prominent and continuous sequence of stars, which he named the Main Sequence. At Princeton University, Henry Norris Russell plotted the spectral types of these stars against their absolute magnitude, and found that dwarf stars followed a distinct relationship. This allowed the real brightness of a dwarf star to be predicted with reasonable accuracy. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 doctoral thesis. The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined. As evolutionary models of stars were developed during the 1930s, Bengt Strömgren introduced the term Hertzsprung–Russell diagram to denote a luminosity-spectral class diagram. A refined scheme for stellar classification was published in 1943 by William Wilson Morgan and Philip Childs Keenan.

The existence of our galaxy, the Milky Way, as a separate group of stars was only proven in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe seen in the recession of most galaxies from us. The "Great Debate" between Harlow Shapley and Heber Curtis, in the 1920s, concerned the nature of the Milky Way, spiral nebulae, and the dimensions of the universe.


During the 20th century spectroscopy (the study of these lines) advanced, especially because of the advent of quantum physics, which was necessary to understand the observations.

In the 20th century, with the help of the use of photography, fainter objects were observed. The Sun was found to be part of a galaxy made up of more than 1010 stars (10 billion stars). The existence of other galaxies, one of the matters of the great debate, was settled by Edwin Hubble, who identified the Andromeda nebula as a different galaxy, and many others at large distances and receding, moving away from our galaxy.

Physical cosmology, a discipline that has a large intersection with astronomy, made huge advances during the 20th century, with the model of the hot Big Bang heavily supported by the evidence provided by astronomy and physics, such as the redshifts of very distant galaxies and radio sources, the cosmic microwave background radiation, Hubble's law and cosmological abundances of elements.

Identity politics

Although in previous centuries noted astronomers were exclusively male, at the turn of the 20th century women began to play a role in the great discoveries. In this period prior to modern computers, women at the United States Naval Observatory (USNO), Harvard University, and other astronomy research institutions began to be hired as human "computers", who performed the tedious calculations while scientists performed research requiring more background knowledge. A number of discoveries in this period were originally noted by the women "computers" and reported to their supervisors. For example, at the Harvard Observatory Henrietta Swan Leavitt discovered the cepheid variable star period-luminosity relation which she further developed into a method of measuring distance outside of the Solar System.

Annie Jump Cannon, also at Harvard, organized the stellar spectral types according to stellar temperature. In 1847, Maria Mitchell discovered a comet using a telescope. According to Lewis D. Eigen, Cannon alone, "in only 4 years discovered and catalogued more stars than all the men in history put together." Most of these women received little or no recognition during their lives due to their lower professional standing in the field of astronomy. Although their discoveries and methods are taught in classrooms around the world, few students of astronomy can attribute the works to their authors or have any idea that there were active female astronomers at the end of the 19th century.

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