Apsis


Apsis denotes either of the two extreme points in the orbit of a planetary body about its primary body. The plural term, "apsides," usually implies both apsis points ; apsides can also refer to the distance of the extreme range of an object orbiting a host body. For example, the apsides of Earth's orbit of the Sun are two: the apsis for Earth's farthest point from the Sun, dubbed the aphelion; and the apsis for Earth's nearest point, the perihelion..
There are two apsides in any elliptic orbit. Each is named by selecting the appropriate prefix: ap-, apo-, or peri- —then joining it to the reference suffix of the "host" body being orbited.
According to Newton's laws of motion all periodic orbits are ellipses, including: 1) the single orbital ellipse, where the primary body is fixed at one focus point and the planetary body orbits around that focus the two-body system of interacting elliptic orbits: both bodies orbit their joint center of mass, which is located at a focus point that is common to both ellipses,. For such a two-body system, when one mass is sufficiently larger than the other, the smaller ellipse around the barycenter comprises one of the orbital elements of the larger ellipse.
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, 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. 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.
In orbital mechanics, the apsides technically refer to the distance measured between the barycenters of the central body and 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.

Terminology

The words "pericenter" and "apocenter" are often seen, although periapsis/apoapsis are preferred in technical usage.
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- and apo-, affixed to the Greek word for the sun,.
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. Regarding black holes, the terms perimelasma and apomelasma were used by physicist and science-fiction author Geoffrey A. Landis in a 1998 story; which occurred before perinigricon and aponigricon appeared in the scientific literature in 2002, and before peribothron in 2015.

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/ system. However, only for the Earth and Sun systems are the unique suffixes commonly used. Typically, for other host systems the generic suffix, -apsis, is used instead.
Astronomical host objectSunMercuryVenusEarthMoonMarsCeresJupiterSaturn
Suffix‑helion‑hermion‑cythe‑gee‑lune
‑cynthion
‑selene		‑areion‑demeter‑jove‑chron
‑kronos
‑saturnium
‑krone
Origin
of the name		HeliosHermesCythereanGaiaLuna
Cynthia
Selene		AresDemeterZeus
Jupiter		Cronos
Saturn

Astronomical host
object		StarGalaxyBarycenterBlack hole
Suffix‑astron‑galacticon‑center
‑focus
‑apsis		‑melasma
‑bothron
‑nigricon
Origin
of the name		Lat: astra; starsGr: galaxias; galaxyGr: melos; black
Gr: bothros; hole
Lat: niger; black

Perihelion and aphelion

The perihelion and aphelion 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 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 image below-left features the inner planets: their orbits, orbital nodes, and the points of perihelion and aphelion, as seen from above Earth's northern pole and Earth's ecliptic plane, which is coplanar with Earth's orbital plane. From this orientation, the planets are situated outward from the Sun as Mercury, Venus, Earth, and Mars, with all planets travelling their orbits counterclockwise around the Sun. The reference Earth-orbit is colored yellow and represents the orbital plane of reference. For Mercury, Venus, and Mars, the section of orbit tilted above the plane of reference is here shaded blue; the section below the plane is shaded violet/pink.
The image below-right shows the outer planets: the orbits, orbital nodes, and the points of perihelion and aphelion of Jupiter, Saturn, Uranus, and Neptune—as seen from above the reference orbital plane, all travelling their orbits counterclockwise. For each planet the section of orbit tilted above the reference orbital plane is colored blue; the section below the plane is violet/pink.
The two orbital nodes are the two end points of the "line of nodes" where a tilted orbit intersects the plane of reference; here they may be 'seen' where the blue section of an orbit becomes violet/pink.
The two images below show the positions of perihelion and the aphelion in the orbits of the planets of the Solar System.

Lines of apsides

The chart shows the extreme range—from the closest approach to farthest point —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 are the lines of apsides of the orbits of various objects around a host body.

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 astronomical units or 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 or.
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 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.
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 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, summers in the northern hemisphere last longer than summers in the southern hemisphere by 92 days versus 89.
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. For the orbit of the Earth, this is called the longitude of perihelion, and in 2000 it was about 282.895°; by the year 2010, this had advanced by a small fraction of a degree to about 283.067°.
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.
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 that is called the apsidal precession. The dates and times of the perihelions and aphelions for several past and future years are listed in the following table:

Other planets

The following table shows the distances of the planets and dwarf planets from the Sun at their perihelion and aphelion.
Type of bodyBodyDistance from Sun at perihelionDistance from Sun at aphelion
PlanetMercury
PlanetVenus
PlanetEarth
PlanetMars
PlanetJupiter
PlanetSaturn
PlanetUranus
PlanetNeptune
Dwarf planetCeres
Dwarf planetPluto
Dwarf planetHaumea
Dwarf planetMakemake
Dwarf planetEris

Mathematical formulae

These formulae characterize the pericenter and apocenter of an orbit:
; Pericenter: Maximum speed,, at minimum distance,.
; Apocenter: Minimum speed,, at maximum distance,.
While, in accordance with Kepler's laws of planetary motion and the conservation of energy, these two quantities are constant for a given orbit:
; Specific relative angular momentum:
; Specific orbital energy:
where:
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
which is the speed of a body in a circular orbit whose radius is.

Time of perihelion

such as the time of perihelion passage are defined at the epoch chosen using an unperturbed two-body solution. 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 2011 produces a less accurate unperturbed perihelion date of 10 January 2006.