Prebiotic atmosphere

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Prebiotic_atmosphere 

The pale orange dot, an artist's impression of the early Earth which is believed to have appeared orange through its hazy methane rich prebiotic second atmosphere, being somewhat comparable to Titan's atmosphere

The prebiotic atmosphere is the second atmosphere present on Earth before today's biotic, oxygen-rich third atmosphere, and after the first atmosphere (which was mainly water vapor and simple hydrides) of Earth's formation. The formation of the Earth, roughly 4.5 billion years ago, involved multiple collisions and coalescence of planetary embryos. This was followed by a <100 million year period on Earth where a magma ocean was present, the atmosphere was mainly steam, and surface temperatures reached up to 8,000 K (14,000 °F). Earth's surface then cooled and the atmosphere stabilized, establishing the prebiotic atmosphere. The environmental conditions during this time period were quite different from today: the Sun was ~30% dimmer overall yet brighter at ultraviolet and x-ray wavelengths, there was a liquid ocean, it is unknown if there were continents but oceanic islands were likely, Earth's interior chemistry (and thus, volcanic activity) was different, and there was a larger flux of impactors (e.g. comets and asteroids) hitting Earth's surface.

Studies have attempted to constrain the composition and nature of the prebiotic atmosphere by analyzing geochemical data and using theoretical models that include our knowledge of the early Earth environment. These studies indicate that the prebiotic atmosphere likely contained more CO₂ than the modern Earth, had N₂ within a factor of 2 of the modern levels, and had vanishingly low amounts of O₂. The atmospheric chemistry is believed to have been "weakly reducing", where reduced gases like CH₄, NH₃, and H₂ were present in small quantities. The composition of the prebiotic atmosphere was likely periodically altered by impactors, which may have temporarily caused the atmosphere to have been "strongly reduced".

Constraining the composition of the prebiotic atmosphere is key to understanding the origin of life, as it may facilitate or inhibit certain chemical reactions on Earth's surface believed to be important for the formation of the first living organism. Life on Earth originated and began modifying the atmosphere at least 3.5 billion years ago and possibly much earlier, which marks the end of the prebiotic atmosphere.

Environmental context

Establishment of the prebiotic atmosphere

Earth is believed to have formed over 4.5 billion years ago by accreting material from the solar nebula. Earth's Moon formed in a collision, the Moon-forming impact, believed to have occurred 30-50 million years after the Earth formed. In this collision, a Mars-sized object named Theia collided with the primitive Earth and the remnants of the collision formed the Moon. The collision likely supplied enough energy to melt most of Earth's mantle and vaporize roughly 20% of it, heating Earth's surface to as high as 8,000 K (~14,000 °F). Earth's surface in the aftermath of the Moon-forming impact was characterized by high temperatures (~2,500 K), an atmosphere made of rock vapor and steam, and a magma ocean. As the Earth cooled by radiating away the excess energy from the impact, the magma ocean solidified and volatiles were partitioned between the mantle and atmosphere until a stable state was reached. It is estimated that Earth transitioned from the hot, post-impact environment into a potentially habitable environment with crustal recycling, albeit different from modern plate tectonics, roughy 10-20 million years after the Moon-forming impact, around 4.4 billion years ago. The atmosphere present from this point in Earth's history until the origin of life is referred to as the prebiotic atmosphere.

It is unknown when exactly life originated. The oldest direct evidence for life on Earth is around 3.5 billion years old, such as fossil stromatolites from North Pole, Western Australia. Putative evidence of life on Earth from older times (e.g. 3.8 and 4.1 billion years ago) lacks additional context necessary to claim it is truly of biotic origin, so it is still debated. Thus, the prebiotic atmosphere concluded 3.5 billion years ago or earlier, placing it in the early Archean Eon or mid-to-late Hadean Eon.

Environmental factors

Knowledge of the environmental factors at play on early Earth is required to investigate the prebiotic atmosphere. Much of what we know about the prebiotic environment comes from zircons - crystals of zirconium silicate (ZrSiO₄). Zircons are useful because they record the physical and chemical processes occurring on the prebiotic Earth during their formation and they are especially durable. Most zircons that are dated to the prebiotic time period are found at the Jack Hills formation of Western Australia, but they also occur elsewhere. Geochemical data from several prebiotic zircons show isotopic evidence for chemical change induced by liquid water, indicating that the prebiotic environment had a liquid ocean and a surface temperature that did not cause it to freeze or boil. It is unknown when exactly the continents emerged above this liquid ocean. This adds uncertainty to the interaction between Earth's prebiotic surface and atmosphere, as the presence of exposed land determines the rate of weathering processes and provides local environments that may be necessary for life to form. However, oceanic islands were likely. Additionally, the oxidation state of Earth's mantle was likely different at early times, which changes the fluxes of chemical species delivered to the atmosphere from volcanic outgassing.

Environmental factors from elsewhere in the solar system also affected prebiotic Earth. The Sun was ~30% dimmer overall around the time the Earth formed. This means greenhouse gases may have been required in higher levels than present day to keep Earth from freezing over. Despite the overall reduction in energy coming from the Sun, the early Sun emitted more radiation in the ultraviolet and x-ray regimes than it currently does. This indicates that different photochemical reactions may have dominated early Earth's atmosphere, which has implications for global atmospheric chemistry and the formation of important compounds that could lead to the origin of life. Finally, there was a significantly higher flux of objects that impacted Earth - such as comets and asteroids - in the early solar system. These impactors may have been important in the prebiotic atmosphere because they can deliver material to the atmosphere, eject material from the atmosphere, and change the chemical nature of the atmosphere after their arrival.

Atmospheric composition

The exact composition of the prebiotic atmosphere is unknown due to the lack of geochemical data from the time period. Current studies generally indicate that the prebiotic atmosphere was "weakly reduced", with elevated levels of CO₂, N₂ within a factor of 2 of the modern level, negligible amounts of O₂, and more hydrogen-bearing gases than the modern Earth (see below). Noble gases and photochemical products of the dominant species were also present in small quantities.

Carbon dioxide

Carbon dioxide (CO₂) is an important component of the prebiotic atmosphere because, as a greenhouse gas, it strongly affects the surface temperature; also, it dissolves in water and can change the ocean pH. The abundance of carbon dioxide in the prebiotic atmosphere is not directly constrained by geochemical data and must be inferred.

Evidence suggests that the carbonate-silicate cycle regulates Earth's atmospheric carbon dioxide abundance on timescales of about 1 million years. The carbonate-silicate cycle is a negative feedback loop that modulates Earth's surface temperature by partitioning carbon between the atmosphere and the mantle via several surface processes. It has been proposed that the processes of the carbonate-silicate cycle would result in high CO₂ levels in the prebiotic atmosphere to offset the lower energy input from the faint young Sun. This mechanism can be used to estimate the prebiotic CO₂ abundance, but it is debated and uncertain. Uncertainty is primarily driven by a lack of knowledge about the area of exposed land, early Earth's interior chemistry and structure, the rate of reverse weathering and seafloor weathering, and the increased impactor flux. One extensive modeling study suggests that CO₂ was roughly 20 times higher in the prebiotic atmosphere than the preindustrial modern value (280 ppm), which would result in a global average surface temperature around 259 K (6.5 °F) and an ocean pH around 7.9. This is in agreement with other studies, which generally conclude that the prebiotic atmospheric CO₂ abundance was higher than the modern one, although the global surface temperature may still be significantly colder due to the faint young Sun.

Nitrogen

Nitrogen in the form of N₂ is 78% of Earth's modern atmosphere by volume, making it the most abundant gas. N₂ is generally considered a background gas in the Earth's atmosphere because it is relatively unreactive due to the strength of its triple bond. Despite this, atmospheric N₂ was at least moderately important to the prebiotic environment because it impacts the climate via Rayleigh scattering and it may have been more photochemically active under the enhanced x-ray and ultraviolet radiation from the young Sun. N₂ was also likely important for the synthesis of compounds believed to be critical for the origin of life, such as hydrogen cyanide (HCN) and amino acids derived from HCN. Studies have attempted to constrain the prebiotic atmosphere N₂ abundance with theoretical estimates, models, and geologic data. These studies have resulted in a range of possible constraints on the prebiotic N₂ abundance. For example, a recent modeling study that incorporates atmospheric escape, magma ocean chemistry, and the evolution of Earth's interior chemistry suggests that the atmospheric N₂ abundance was probably less than half of the present day value. However, this study fits into a larger body of work that generally constrains the prebiotic N₂ abundance to be between half and double the present level.

Oxygen

Oxygen in the form of O₂ makes up 21% of Earth's modern atmosphere by volume. Earth's modern atmospheric O₂ is due almost entirely to biology (e.g. it is produced during oxygenic photosynthesis), so it was not nearly as abundant in the prebiotic atmosphere. This is favorable for the origin of life, as O₂ would oxidize organic compounds needed in the origin of life. The prebiotic atmosphere O₂ abundance can be theoretically calculated with models of atmospheric chemistry. The primary source of O₂ in these models is the breakdown and subsequent chemical reactions of other oxygen containing compounds. Incoming solar photons or lightning can break up CO₂ and H₂O molecules, freeing oxygen atoms and other radicals (i.e. highly reactive gases in the atmosphere). The free oxygen can then combine into O₂ molecules via several chemical pathways. The rate at which O₂ is created in this process is determined by the incoming solar flux, the rate of lightning, and the abundances of the other atmospheric gases that take part in the chemical reactions (e.g. CO₂, H₂O, OH), as well as their vertical distributions. O₂ is removed from the atmosphere via photochemical reactions that mainly involve H₂ and CO near the surface. The most important of these reactions starts when H₂ is split into two H atoms by incoming solar photons. The free H then reacts with O₂ and eventually forms H₂O, resulting in a net removal of O₂ and a net increase in H₂O. Models that simulate all of these chemical reactions in a potential prebiotic atmosphere show that an extremely small atmospheric O₂ abundance is likely. In one such model that assumed values for CO₂ and H₂ abundances and sources, the O₂ volume mixing ratio is calculated to be between 10⁻¹⁸ and 10⁻¹¹ near the surface and up to 10⁻⁴ in the upper atmosphere.

Hydrogen and reduced gases

The hydrogen abundance in the prebiotic atmosphere can be viewed from the perspective of reduction-oxidation (redox) chemistry. The modern atmosphere is oxidizing, due to the large volume of atmospheric O₂. In an oxidizing atmosphere, the majority of atoms that form atmospheric compounds (e.g. C) will be in an oxidized form (e.g. CO₂) instead of a reduced form (e.g. CH₄). In a reducing atmosphere, more species will be in their reduced, generally hydrogen-bearing forms. Because there was very little O₂ in the prebiotic atmosphere, it is generally believed that the prebiotic atmosphere was "weakly reduced" - although some argue that the atmosphere was "strongly reduced". In a weakly reduced atmosphere, reduced gases (e.g. CH₄ and NH₃) and oxidized gases (e.g CO₂) are both present. The actual H₂ abundance in the prebiotic atmosphere has been estimated by doing a calculation that takes into account the rate at which H₂ is volcanically outgassed to the surface and the rate at which it escapes to space. One of these recent calculations indicates that the prebiotic atmosphere H₂ abundance was around 400 parts per million, but could have been significantly higher if the source from volcanic outgassing was enhanced or atmospheric escape was less efficient than expected. The abundances of other reduced species in the atmosphere can then be calculated with models of atmospheric chemistry.

Post-impact atmospheres

It has been proposed that the large flux of impactors in the early solar system may have significantly changed the nature of the prebiotic atmosphere. During the time period of the prebiotic atmosphere, it is expected that a few asteroid impacts large enough to vaporize the oceans and melt Earth's surface could have occurred, with smaller impacts expected in even larger numbers. These impacts would have significantly changed the chemistry of the prebiotic atmosphere by heating it up, ejecting some of it to space, and delivering new chemical material. Studies of post-impact atmospheres indicate that they would have caused the prebiotic atmosphere to be strongly reduced for a period of time after a large impact. On average, impactors in the early solar system contained highly reduced minerals (e.g. metallic iron) and were enriched with reduced compounds that readily enter the atmosphere as a gas. In these strongly reduced post-impact atmospheres, there would be significantly higher abundances of reduced gases like CH₄, HCN, and perhaps NH₃. Reduced, post-impact atmospheres after the ocean condensed are predicted to last up to tens of millions of years before returning to the background state.

Relationship to the origin of life

The prebiotic atmosphere can supply chemical ingredients and facilitate environmental conditions that contribute to the synthesis of organic compounds involved in the origin of life. For example, potential compounds involved in the origin of life were synthesized in the Miller-Urey experiment. In this experiment, assumptions must be made about what gases were present in the prebiotic atmosphere. Proposed important ingredients for the origin of life include (but are not limited to) methane (CH₄), ammonia (NH₃), phosphate, hydrogen cyanide (HCN), various organics, and various photochemical byproducts. The atmospheric composition will impact the stability and production of these compounds at Earth's surface. For example, the "weakly reduced" prebiotic atmosphere may produce some, but not all, of these ingredients via reactions with lightning. On the other hand, the production and stability of origin of life ingredients in a strongly reduced atmosphere are greatly enhanced, making post-impact atmospheres particularly relevant. It is also proposed that the conditions required for the origin of life could have emerged locally, in a system that is isolated from the atmosphere (e.g. a hydrothermal vent). However, compounds such as cyanides used to make nucleobases of RNA would be too dilute in the ocean, unlike lakes on land. Once life originated and started interacting with the atmosphere, the prebiotic atmosphere transitioned into the post-biotic atmosphere, by definition.

Sumerian literature

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Sumerian_literature

Sumerian inscription on a ceramic stone plaque.

Sumerian literature constitutes the earliest known corpus of recorded literature, including the religious writings and other traditional stories maintained by the Sumerian civilization and largely preserved by the later Akkadian and Babylonian empires. These records were written in the Sumerian language in the 3rd and 2nd millennia BC during the Middle Bronze Age.

The Sumerians invented one of the first writing systems, developing Sumerian cuneiform writing out of earlier proto-writing systems by about the 30th century BC. The Sumerian language remained in official and literary use in the Akkadian and Babylonian empires, even after the spoken language disappeared from the population; literacy was widespread, and the Sumerian texts that students copied heavily influenced later Babylonian literature. The basic genres of Sumerian literature were literary catalogues, narrative/mythological compositions, historical compositions, letters and legal documents, disputation poems, proverbs, and other texts which do not belong to these prior categories.

Poetry

Most Sumerian literature is written in left-justified lines, and could contain line-based organization such as the couplet or the stanza, but the Sumerian definition of poetry is unknown. It is not rhymed, although “comparable effects were sometimes exploited.” Though rhymeless, the intricate patterns of similar and alternating sounds of vowels and consonants and the similar and alternating verb and noun endings give the language a musical resonance. It did not use syllabo-tonic versification, and the writing system precludes detection of rhythm, metre, rhyme, or alliteration. Quantitative analysis of other possible poetic features seems to be lacking, or has been intentionally hidden by the scribes who recorded the writing.

Literary genres and topics

Genre is often the first judgement made of ancient literature; types of literature were not clearly defined, and all Sumerian literature incorporated poetic aspects. Sumerian poems demonstrate basic elements of poetry, including lines, imagery, and metaphor. Humans, gods, talking animals, and inanimate objects were all incorporated as characters. Suspense and humor were both incorporated into Sumerian stories. These stories were primarily shared orally, though they were also recorded by scribes. Some works were associated with specific musical instruments or contexts and may have been performed in specific settings. Sumerian literature did not use titles, instead being referred to by the work's first line.

Based on the categorization work of Miguel Civil, Modern assyriologists have divided the extant corpus of Sumerian literature into broad categories including "Literary Catalogs", "Narratives and Mythological Compositions", "Historical Compositions and Praise Poetry", "Letters, Letter Prayers and Laws", "Hymns and Songs", "Heterogenous Compositions" (including Wisdom literature), and "Proverbs".

Literary catalogs

• Sumerian scribal education focused on a curriculum called the Decad. Manuscripts of these ten texts are some of the best preserved Sumerian literature.

Narrative and mythological compositions

• Narratives featuring heroes include:
  • Stories from the Epic of Gilgamesh, such as Gilgamesh and Huwawa, Gilgamesh and the Bull of Heaven, Gilgamesh and Aga, Gilgamesh, Enkidu, and the Netherworld, and the Death of Gilgamesh.
  • Enmerkar and Lugalbanda: Enmerkar and the Lord of Aratta and Enmerkar and En-suhgir-ana as well as two tales of Lugalbanda during Enmerkar's campaign against Aratta: Lugalbanda in the Mountain Cave and Lugalbanda and the Anzud Bird
  • Inanna's Descent to the Underworld,
  • The Legend of Adapa
• Narratives featuring deities, such as Enki, Enlil (including Enlil and Ninlil), Inanna, Inanna and Dumuzid, and Ninurta (including Lugal-e and Angim)
• Other myths such as the Eridu Genesis

Historical compositions

• Praise Poems for kings
  • Third Dynasty of Ur - Ur-Nammu, Shulgi (including the Self-praise of Shulgi (Shulgi D), Amar-Sin, Shu-Sin, Ibbi-Sin
  • Isin dynasty - Ishbi-Erra, Shu-Ilishu, Iddin-Dagan, Ishme-Dagan, Lipit-Ishtar, Ur-Ninurta, Bur-Suen, Enlil-bani
  • Larsa dynasty - Gungunum, Sin-Iddinam, Sin-Iqisham, Warad-Sin, Rim-Sin
  • First Dynasty of Babylon - Hammurabi, Samsu-iluna, Abi-Eshuh
• City Laments such as Lament for Ur and Lament for Sumer and Ur
• King lists and other historical compositions such as Building of Ningirsu's temple

Letters and laws

• Letters include the Correspondence of the Kings of Ur as well as Isin, Larsa, and other dynasties.
• The Code of Ur-Nammu is attributed to Ur-Nammu, founder of the Third Dynasty of Ur
• Code of Lipit-Ishtar
• Code of Hammurabi

Hymns

• Hymns to deities in the Sumerian pantheon, such as the Hymn to Enlil, as well as Hymns dedicated to specific temples, including The Sumerian Temple Hymns collection and the Kesh Temple Hymn

Disputation poems

Main article: Sumerian disputations

• Debate between the hoe and the plough
• Debate between bird and fish
• Debate between sheep and grain
• Debate between Winter and Summer
• Debate between tree and reed
• Debate between silver and copper

Proverbs

• Anthologies of proverbs from Nippur, Susa, Urim, and Uruk

Heterogeneous compositions

• Instruction literature such as Instructions of Shuruppak
• Dialogue between a Man and His God

Apsis

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Apsis

The apsides refer to the farthest (2) and nearest (3) points reached by an orbiting planetary body (2 and 3) with respect to a primary, or host, body (1)

An apsis (from Ancient Greek ἁψίς (hapsís) 'arch, vault'; pl. apsides /ˈæpsɪˌdiːz/ AP-sih-deez) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides is the line connecting the two extreme values.

Apsides pertaining to orbits around the Sun have distinct names to differentiate themselves from other apsides; these names are aphelion for the farthest and perihelion for the nearest point in the solar orbit. The Moon's two apsides are the farthest point, apogee, and the nearest point, perigee, of its orbit around the host Earth. Earth's two apsides are the farthest point, aphelion, and the nearest point, perihelion, of its orbit around the host Sun. The terms aphelion and perihelion apply in the same way to the orbits of Jupiter and the other planets, the comets, and the asteroids of the Solar System.

General description

The two-body system of interacting elliptic orbits: The smaller, satellite body (blue) orbits the primary body (yellow); both are in elliptic orbits around their common center of mass (or barycenter), (red +).
∗Periapsis and apoapsis as distances: the smallest and largest distances between the orbiter and its host body.

There are two apsides in any elliptic orbit. The name for each apsis is created from the prefixes ap-, apo- (from ἀπ(ό), (ap(o)-) 'away from') for the farthest or peri- (from περί (peri-) 'near') for the closest point to the primary body, with a suffix that describes the primary body. The suffix for Earth is -gee, so the apsides' names are apogee and perigee. For the Sun, the suffix is -helion, so the names are aphelion and perihelion.

According to Newton's laws of motion, all periodic orbits are ellipses. The barycenter of the two bodies may lie well within the bigger body—e.g., the Earth–Moon barycenter is about 75% of the way from Earth's center to its surface. If, compared to the larger mass, the smaller mass is negligible (e.g., for satellites), then the orbital parameters are independent of the smaller mass.

When used as a suffix—that is, -apsis—the term can refer to the two distances from the primary body to the orbiting body when the latter is located: 1) at the periapsis point, or 2) at the apoapsis point (compare both graphics, second figure). The line of apsides denotes the distance of the line that joins the nearest and farthest points across an orbit; it also refers simply to the extreme range of an object orbiting a host body (see top figure; see third figure).

In orbital mechanics, the apsides technically refer to the distance measured between the center of mass of the central body and the center of mass of the orbiting body. However, in the case of a spacecraft, the terms are commonly used to refer to the orbital altitude of the spacecraft above the surface of the central body (assuming a constant, standard reference radius).

Keplerian orbital elements: point G, the nearest point of approach of an orbiting body, is the pericenter (also periapsis) of an orbit; point H, the farthest point of the orbiting body, is the apocenter (also apoapsis) of the orbit; and the red line between them is the line of apsides.

Terminology

The words "pericenter" and "apocenter" are often seen, although periapsis/apoapsis are preferred in technical usage.

• For generic situations where the primary is not specified, the terms pericenter and apocenter are used for naming the extreme points of orbits (see table, top figure); periapsis and apoapsis (or apapsis) are equivalent alternatives, but these terms also frequently refer to distances—that is, the smallest and largest distances between the orbiter and its host body (see second figure).
• For a body orbiting the Sun, the point of least distance is the perihelion (/ˌpɛrɪˈhiːliən/), and the point of greatest distance is the aphelion (/æpˈhiːliən/); when discussing orbits around other stars the terms become periastron and apastron.
• When discussing a satellite of Earth, including the Moon, the point of least distance is the perigee (/ˈpɛrɪdʒiː/), and of greatest distance, the apogee (from Ancient Greek: Γῆ (Gē), "land" or "earth").
• For objects in lunar orbit, the point of least distance are called the pericynthion (/ˌpɛrɪˈsɪnθiən/) and the greatest distance the apocynthion (/ˌæpəˈsɪnθiən/). The terms perilune and apolune, as well as periselene and aposelene are also used. Since the Moon has no natural satellites this only applies to man-made objects.

Etymology

The words perihelion and aphelion were coined by Johannes Kepler to describe the orbital motions of the planets around the Sun. The words are formed from the prefixes peri- (Greek: περί, near) and apo- (Greek: ἀπό, away from), affixed to the Greek word for the Sun, (ἥλιος, or hēlíos).

Various related terms are used for other celestial objects. The suffixes -gee, -helion, -astron and -galacticon are frequently used in the astronomical literature when referring to the Earth, Sun, stars, and the Galactic Center respectively. The suffix -jove is occasionally used for Jupiter, but -saturnium has very rarely been used in the last 50 years for Saturn. The -gee form is also used as a generic closest-approach-to "any planet" term—instead of applying it only to Earth.

During the Apollo program, the terms pericynthion and apocynthion were used when referring to orbiting the Moon; they reference Cynthia, an alternative name for the Greek Moon goddess Artemis. More recently, during the Artemis program, the terms perilune and apolune have been used.

Regarding black holes, the term peribothron was first used in a 1976 paper by J. Frank and M. J. Rees, who credit W. R. Stoeger for suggesting creating a term using the greek word for pit: "bothron".

The terms perimelasma and apomelasma (from a Greek root) were used by physicist and science-fiction author Geoffrey A. Landis in a story published in 1998, thus appearing before perinigricon and aponigricon (from Latin) in the scientific literature in 2002.

Terminology summary

The suffixes shown below may be added to prefixes peri- or apo- to form unique names of apsides for the orbiting bodies of the indicated host/(primary) system. However, only for the Earth, Moon and Sun systems are the unique suffixes commonly used. Exoplanet studies commonly use -astron, but typically, for other host systems the generic suffix, -apsis, is used instead.

Host objects in the Solar System with named/nameable apsides

Astronomical host object	Suffix	Origin of the name
Sun	-helion	Helios
Mercury	-hermion	Hermes
Venus	-cythe	Cytherean
Earth	-gee	Gaia
Moon	-lune -cynthion -selene	Luna Cynthia Selene
Mars	-areion	Ares
Ceres	-demeter	Demeter
Jupiter	-jove	Zeus Jupiter
Saturn	-chron -kronos -saturnium -krone	Cronos Saturn
Uranus	-uranion	Uranus
Neptune	-poseideum -poseidion	Poseidon

Other host objects with named/nameable apsides

Astronomical host object	Suffix	Origin of the name
Star	-astron	Lat: astra; stars
Galaxy	-galacticon	Gr: galaxias; galaxy
Barycenter	-center -focus -apsis
Black hole	-melasma -bothron -nigricon	Gr: melos; black Gr: bothros; hole Lat: niger; black

Perihelion and aphelion

"Perihelion" redirects here. For other uses, see Perihelion (disambiguation).

"Aphelion" redirects here. For other uses, see Aphelion (disambiguation).

Diagram of a body's direct orbit around the Sun with its nearest (perihelion) and farthest (aphelion) points

The perihelion (q) and aphelion (Q) are the nearest and farthest points respectively of a body's direct orbit around the Sun.

Comparing osculating elements at a specific epoch to effectively those at a different epoch will generate differences. The time-of-perihelion-passage as one of six osculating elements is not an exact prediction (other than for a generic two-body model) of the actual minimum distance to the Sun using the full dynamical model. Precise predictions of perihelion passage require numerical integration.

Inner planets and outer planets

The two images below show the orbits, orbital nodes, and positions of perihelion (q) and aphelion (Q) for the planets of the Solar System as seen from above the northern pole of Earth's ecliptic plane, which is coplanar with Earth's orbital plane. The planets travel counterclockwise around the Sun and for each planet, the blue part of their orbit travels north of the ecliptic plane, the pink part travels south, and dots mark perihelion (green) and aphelion (orange).

The first image (below-left) features the inner planets, situated outward from the Sun as Mercury, Venus, Earth, and Mars. The reference Earth-orbit is colored yellow and represents the orbital plane of reference. At the time of vernal equinox, the Earth is at the bottom of the figure. The second image (below-right) shows the outer planets, being Jupiter, Saturn, Uranus, and Neptune.

The orbital nodes are the two end points of the "line of nodes" where a planet's tilted orbit intersects the plane of reference; here they may be 'seen' as the points where the blue section of an orbit meets the pink.

• 
  The perihelion (green) and aphelion (orange) points of the inner planets of the Solar System
• 
  The perihelion (green) and aphelion (orange) points of the outer planets of the Solar System

Lines of apsides

The chart shows the extreme range—from the closest approach (perihelion) to farthest point (aphelion)—of several orbiting celestial bodies of the Solar System: the planets, the known dwarf planets, including Ceres, and Halley's Comet. The length of the horizontal bars correspond to the extreme range of the orbit of the indicated body around the Sun. These extreme distances (between perihelion and aphelion) are the lines of apsides of the orbits of various objects around a host body.

Distances of selected bodies of the Solar System from the Sun. The left and right edges of each bar correspond to the perihelion and aphelion of the body, respectively, hence long bars denote high orbital eccentricity. The radius of the Sun is 0.7 million km, and the radius of Jupiter (the largest planet) is 0.07 million km, both too small to resolve on this image.

Earth perihelion and aphelion

Currently, the Earth reaches perihelion in early January, approximately 14 days after the December solstice. At perihelion, the Earth's center is about 0.98329 astronomical units (AU) or 147,098,070 km (91,402,500 mi) from the Sun's center. In contrast, the Earth reaches aphelion currently in early July, approximately 14 days after the June solstice. The aphelion distance between the Earth's and Sun's centers is currently about 1.01671 AU or 152,097,700 km (94,509,100 mi).

The dates of perihelion and aphelion change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. In the short term, such dates can vary up to 2 days from one year to another. This significant variation is due to the presence of the Moon: while the Earth–Moon barycenter is moving on a stable orbit around the Sun, the position of the Earth's center which is on average about 4,700 kilometres (2,900 mi) from the barycenter, could be shifted in any direction from it—and this affects the timing of the actual closest approach between the Sun's and the Earth's centers (which in turn defines the timing of perihelion in a given year).

Because of the increased distance at aphelion, only 93.55% of the radiation from the Sun falls on a given area of Earth's surface as does at perihelion, but this does not account for the seasons, which result instead from the tilt of Earth's axis of 23.4° away from perpendicular to the plane of Earth's orbit. Indeed, at both perihelion and aphelion it is summer in one hemisphere while it is winter in the other one. Winter falls on the hemisphere where sunlight strikes least directly, and summer falls where sunlight strikes most directly, regardless of the Earth's distance from the Sun.

In the northern hemisphere, summer occurs at the same time as aphelion, when solar radiation is lowest. Despite this, summers in the northern hemisphere are on average 2.3 °C (4 °F) warmer than in the southern hemisphere, because the northern hemisphere contains larger land masses, which are easier to heat than the seas.

Perihelion and aphelion do however have an indirect effect on the seasons: because Earth's orbital speed is minimum at aphelion and maximum at perihelion, the planet takes longer to orbit from June solstice to September equinox than it does from December solstice to March equinox. Therefore, summer in the northern hemisphere lasts slightly longer (93 days) than summer in the southern hemisphere (89 days).

Astronomers commonly express the timing of perihelion relative to the First Point of Aries not in terms of days and hours, but rather as an angle of orbital displacement, the so-called longitude of the periapsis (also called longitude of the pericenter). For the orbit of the Earth, this is called the longitude of perihelion, and in 2000 it was about 282.895°; by 2010, this had advanced by a small fraction of a degree to about 283.067°, i.e. a mean increase of 62" per year.

For the orbit of the Earth around the Sun, the time of apsis is often expressed in terms of a time relative to seasons, since this determines the contribution of the elliptical orbit to seasonal variations. The variation of the seasons is primarily controlled by the annual cycle of the elevation angle of the Sun, which is a result of the tilt of the axis of the Earth measured from the plane of the ecliptic. The Earth's eccentricity and other orbital elements are not constant, but vary slowly due to the perturbing effects of the planets and other objects in the solar system (Milankovitch cycles).

On a very long time scale, the dates of the perihelion and of the aphelion progress through the seasons, and they make one complete cycle in 22,000 to 26,000 years. There is a corresponding movement of the position of the stars as seen from Earth, called the apsidal precession. (This is closely related to the precession of the axes.) The dates and times of the perihelions and aphelions for several past and future years are listed in the following table:

Year	Perihelion		Aphelion
	Date	Time (UT)	Date	Time (UT)
2010	January 3	00:09	July 6	11:30
2011	January 3	18:32	July 4	14:54
2012	January 5	00:32	July 5	03:32
2013	January 2	04:38	July 5	14:44
2014	January 4	11:59	July 4	00:13
2015	January 4	06:36	July 6	19:40
2016	January 2	22:49	July 4	16:24
2017	January 4	14:18	July 3	20:11
2018	January 3	05:35	July 6	16:47
2019	January 3	05:20	July 4	22:11
2020	January 5	07:48	July 4	11:35
2021	January 2	13:51	July 5	22:27
2022	January 4	06:55	July 4	07:11
2023	January 4	16:17	July 6	20:07
2024	January 3	00:39	July 5	05:06
2025	January 4	13:28	July 3	19:55
2026	January 3	17:16	July 6	17:31
2027	January 3	02:33	July 5	05:06
2028	January 5	12:28	July 3	22:18
2029	January 2	18:13	July 6	05:12

Other planets

The following table shows the distances of the planets and dwarf planets from the Sun at their perihelion and aphelion.

Type of body	Body	Distance from Sun at perihelion	Distance from Sun at aphelion	difference (%)	insolation difference (%)
Planet	Mercury	46,001,009 km (28,583,702 mi)	69,817,445 km (43,382,549 mi)	34%	57%
	Venus	107,476,170 km (66,782,600 mi)	108,942,780 km (67,693,910 mi)	1.3%	2.8%
	Earth	147,098,291 km (91,402,640 mi)	152,098,233 km (94,509,460 mi)	3.3%	6.5%
	Mars	206,655,215 km (128,409,597 mi)	249,232,432 km (154,865,853 mi)	17%	31%
	Jupiter	740,679,835 km (460,237,112 mi)	816,001,807 km (507,040,016 mi)	9.2%	18%
	Saturn	1,349,823,615 km (838,741,509 mi)	1,503,509,229 km (934,237,322 mi)	10%	19%
	Uranus	2,734,998,229 km (1.699449110×109 mi)	3,006,318,143 km (1.868039489×109 mi)	9.0%	17%
	Neptune	4,459,753,056 km (2.771162073×109 mi)	4,537,039,826 km (2.819185846×109 mi)	1.7%	3.4%
Dwarf planet	Ceres	380,951,528 km (236,712,305 mi)	446,428,973 km (277,398,103 mi)	15%	27%
	Pluto	4,436,756,954 km (2.756872958×109 mi)	7,376,124,302 km (4.583311152×109 mi)	40%	64%
	Haumea	5,157,623,774 km (3.204798834×109 mi)	7,706,399,149 km (4.788534427×109 mi)	33%	55%
	Makemake	5,671,928,586 km (3.524373028×109 mi)	7,894,762,625 km (4.905578065×109 mi)	28%	48%
	Eris	5,765,732,799 km (3.582660263×109 mi)	14,594,512,904 km (9.068609883×109 mi)	60%	84%

Mathematical formulae

These formulae characterize the pericenter and apocenter of an orbit:

Pericenter

Maximum speed, $v_{\text{per}}=\sqrt{\frac{(1+e)\mu }{(1-e)a}}$, at minimum (pericenter) distance, $r_{\text{per}}=(1-e)a$.

Apocenter

Minimum speed, $v_{\text{ap}}=\sqrt{\frac{(1-e)\mu }{(1+e)a}}$, at maximum (apocenter) distance, $r_{\text{ap}}=(1+e)a$.

While, in accordance with Kepler's laws of planetary motion (based on the conservation of angular momentum) and the conservation of energy, these two quantities are constant for a given orbit:

Specific relative angular momentum

${\displaystyle h=\sqrt{\left(1-e^2\right)\mu a}}$

Specific orbital energy

${\displaystyle \epsilon =-\frac{\mu }{2a}}$

where:

• $r_{\text{ap}}$ is the distance from the apocenter to the primary focus
• $r_{\text{per}}$ is the distance from the pericenter to the primary focus
• a is the semi-major axis:

  ${\displaystyle a=\frac{r_{\text{per}}+r_{\text{ap}}}{2}}$

• μ is the standard gravitational parameter
• e is the eccentricity, defined as

  ${\displaystyle e=\frac{r_{\text{ap}}-r_{\text{per}}}{r_{\text{ap}}+r_{\text{per}}}=1-\frac{2}{\frac{r_{\text{ap}}}{r_{\text{per}}}+1}}$

Note that for conversion from heights above the surface to distances between an orbit and its primary, the radius of the central body has to be added, and conversely.

The arithmetic mean of the two limiting distances is the length of the semi-major axis a. The geometric mean of the two distances is the length of the semi-minor axis b.

The geometric mean of the two limiting speeds is

${\displaystyle \sqrt{-2\epsilon }=\sqrt{\frac{\mu }{a}}}$

which is the speed of a body in a circular orbit whose radius is ${\displaystyle a}$.

Time of perihelion

Orbital elements such as the time of perihelion passage are defined at the epoch chosen using an unperturbed two-body solution that does not account for the n-body problem. To get an accurate time of perihelion passage you need to use an epoch close to the perihelion passage. For example, using an epoch of 1996, Comet Hale–Bopp shows perihelion on 1 April 1997. Using an epoch of 2008 shows a less accurate perihelion date of 30 March 1997. Short-period comets can be even more sensitive to the epoch selected. Using an epoch of 2005 shows 101P/Chernykh coming to perihelion on 25 December 2005, but using an epoch of 2012 produces a less accurate unperturbed perihelion date of 20 January 2006.

Numerical integration shows dwarf planet Eris will come to perihelion around December 2257. Using an epoch of 2021, which is 236 years early, less accurately shows Eris coming to perihelion in 2260.

4 Vesta came to perihelion on 26 December 2021, but using a two-body solution at an epoch of July 2021 less accurately shows Vesta came to perihelion on 25 December 2021.

Short arcs

Trans-Neptunian objects discovered when 80+ AU from the Sun need dozens of observations over multiple years to well constrain their orbits because they move very slowly against the background stars. Due to statistics of small numbers, trans-Neptunian objects such as 2015 TH367 when it had only 8 observations over an observation arc of 1 year that have not or will not come to perihelion for roughly 100 years can have a 1-sigma uncertainty of 77.3 years (28,220 days) in the perihelion date.