Apsis

perigeeperihelionapogeeaphelionperiapsisperiastronapoapsisapastronapsesperihelia
* The line of apsides is the line connecting positions 1 and 2.wikipedia
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Orbit

orbitsorbital motionplanetary motion
Apsis (ἁψίς; plural apsides, Greek: ἁψῖδες; "orbit") denotes either of the two extreme points (i.e., the farthest or nearest point) in the orbit of a planetary body about its primary body (or simply, "the primary"). The perihelion (q) and aphelion (Q) are the nearest and farthest points respectively of a body's direct orbit around the Sun.
As two objects orbit each other, the periapsis is that point at which the two objects are closest to each other and the apoapsis is that point at which they are the farthest.

Sun

solarSolThe Sun
The perihelion (q) and aphelion (Q) are the nearest and farthest points respectively of a body's direct orbit around the Sun.
The mean distance of the Sun's center to Earth's center is approximately 1 AU, though the distance varies as Earth moves from perihelion in January to aphelion in July.

Orbital elements

orbital parametersorbital elementKeplerian elements
For such a two-body system, when one mass is sufficiently larger than the other, the smaller ellipse (of the larger body) around the barycenter comprises one of the orbital elements of the larger ellipse (of the smaller body).
It can be converted into the true anomaly ν, which does represent the real geometric angle in the plane of the ellipse, between periapsis (closest approach to the central body) and the position of the orbiting object at any given time.

Lunar orbit

SelenocentricSelenocentric orbitlunar orbit insertion
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.
The altitude at apoapsis (point farthest from the surface) for a lunar orbit is known as apolune, apocynthion, or aposelene, while the periapsis (point closest to the surface) is known as perilune, pericynthion, or periselene, from names or epithets of the moon goddess.

Orbital mechanics

astrodynamicsastrodynamicistorbital dynamics
In orbital mechanics, the apsides technically refer to the distance measured between the barycenters of the central body and orbiting body.

Astronomical unit

AUastronomical unitsAUs
The reference average distance from Sun to Earth is defined as one astronomical unit, AU.
However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year.

Elliptic orbit

elliptical orbitellipticalelliptic
There are two apsides in any elliptic orbit. 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 (see top figure); and 2) the two-body system of interacting elliptic orbits: both bodies orbit their joint center of mass (or barycenter), which is located at a focus point that is common to both ellipses, (see second figure).
The following chart of the perihelion and aphelion of the planets, dwarf planets and Halley's Comet demonstrates the variation of the eccentricity of their elliptical orbits.

Barycenter

barycentrebarycentricbarycentric coordinates
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 (see top figure); and 2) the two-body system of interacting elliptic orbits: both bodies orbit their joint center of mass (or barycenter), which is located at a focus point that is common to both ellipses, (see second figure).
But all celestial orbits are elliptical, and the distance between the bodies varies between the apses, depending on the eccentricity, e.

Moon

lunarthe MoonLuna
The distance between the Moon and Earth varies from around 356,400 km to 406,700 km at perigee (closest) and apogee (farthest), respectively.

Johannes Kepler

KeplerDioptriceJohan Kepler
The words perihelion and aphelion were coined by Johannes Kepler to describe the orbital motions of the planets around the Sun.
Based on measurements of the aphelion and perihelion of the Earth and Mars, he created a formula in which a planet's rate of motion is inversely proportional to its distance from the Sun.

Earth

Earth's surfaceterrestrialworld
In modern times, Earth's perihelion occurs around 3 January, and its aphelion around 4 July.

Halley's Comet

Comet Halley1P/HalleyHalley
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.
While he had personally observed the comet around perihelion in September 1682, Halley died in 1742 before he could observe its predicted return.

Jupiter

JovianGioveplanet Jupiter
Because the eccentricity of its orbit is 0.048, Jupiter's distance from the Sun varies by 75 million km between its nearest approach (perihelion) and furthest distance (aphelion).

Solar System

outer Solar Systeminner Solar Systemouter planets
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.
A body's closest approach to the Sun is called its perihelion, whereas its most distant point from the Sun is called its aphelion.

Apsidal precession

perihelion precessionprecessionorbital precession
There is a corresponding movement of the position of the stars as seen from Earth that is called the apsidal precession.
.]] In celestial mechanics, apsidal precession is the precession (gradual rotation) of the line connecting the apsides (line of apsides) of an astronomical body's orbit.

Saturn

Atmosphere of SaturnOrbit of SaturnPhainon
The perihelion and aphelion distances are, respectively, 9.195 and 9.957 AU, on average.

Mercury (planet)

MercuryMercurioplanet Mercury
Its orbital eccentricity is the largest of all known planets in the Solar System; at perihelion, Mercury's distance from the Sun is only about two-thirds (or 66%) of its distance at aphelion.

Season

seasonsseasonalfour seasons
However, this fluctuation does not account for the seasons, as it is summer in the northern hemisphere when it is winter in the southern hemisphere and vice versa. Instead, seasons result from the tilt of Earth's axis, which is 23.4 degrees away from perpendicular to the plane of Earth's orbit around the sun.
In fact, Earth reaches perihelion (the point in its orbit closest to the Sun) in January, and it reaches aphelion (the point farthest from the Sun) in July, so the slight contribution of orbital eccentricity opposes the temperature trends of the seasons in the Northern Hemisphere.

Ceres (dwarf planet)

Ceres1 CeresAtmosphere of Ceres
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 top right is a close-up demonstrating the locations of the perihelia (q) and aphelia (Q) of Ceres and Mars.

Longitude of the periapsis

longitude of perihelionlongitude of periastronLongitude of pericenter
Astronomers commonly express the timing of perihelion relative to the vernal equinox 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).
In celestial mechanics, the longitude of the periapsis, also called longitude of the pericenter, of an orbiting body is the longitude (measured from the point of the vernal equinox) at which the periapsis (closest approach to the central body) would occur if the body's orbit inclination were zero.

Milankovitch cycles

Milankovitch cycleMilankovitch theoryMilankovich cycle
Dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles.
The relative increase in solar irradiation at closest approach to the Sun (perihelion) compared to the irradiation at the furthest distance (aphelion) is slightly larger than four times the eccentricity.

Orbital eccentricity

eccentricityeccentriceccentricities
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.
The eccentricity of an elliptical orbit can also be used to obtain the ratio of the periapsis to the apoapsis:

Haumea

136108 HaumeaHaumea (dwarf planet)0.7
Haumea has an orbital period of 284 Earth years, a perihelion of 35 AU, and an orbital inclination of 28°.

Kepler's laws of planetary motion

Kepler's third lawKepler's lawslaws of planetary motion
The calculation is correct when perihelion, the date the Earth is closest to the Sun, falls on a solstice.

Mean anomaly

mean
In celestial mechanics, the mean anomaly is the fraction of an elliptical orbit's period that has elapsed since the orbiting body passed periapsis, expressed as an angle which can be used in calculating the position of that body in the classical two-body problem.