Apsis
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.
The words perihelion and aphelion were coined by Johannes Kepler[8] to describe the orbital motions of the planets around the Sun.
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 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,[12] thus appearing before perinigricon and aponigricon (from Latin) in the scientific literature in 2002.
[13] 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.
[14][failed verification] The perihelion (q) and aphelion (Q) are the nearest and farthest points respectively of a body's direct orbit around the Sun.
The first image (below-left) features the inner planets, situated outward from the Sun as Mercury, Venus, Earth, and Mars.
These extreme distances (between perihelion and aphelion) are the lines of apsides of the orbits of various objects around a host body.
In contrast, the Earth reaches aphelion currently in early July, approximately 14 days after the June solstice.
The dates of perihelion and aphelion change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles.
[20] 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).
[23] 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.
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°,[25] i.e. a mean increase of 62" per year.
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.
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.
[36] 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.
∗Periapsis and apoapsis as distances: the smallest and largest distances between the orbiter and its host body.
