Apsis

An apsis is a special point in the path, or orbit, that an object in space follows as it moves around another object. The word comes from an ancient word meaning “arch” or “vault.” Every time an object, like a moon or a planet, travels around something bigger, like a planet or the Sun, it has two important points in its path: the farthest point and the closest point. These two points are called apsides.

When we talk about objects orbiting the Earth, we use special names for these points. The farthest point is called the apogee, and the closest point is called the perigee. For example, when the Moon is farthest from Earth, it is at its apogee, and when it is closest, it is at its perigee.

For objects orbiting the Sun, like planets, comets, and asteroids, we use different names. The farthest point in the orbit around the Sun is called the aphelion, and the closest point is called the perihelion. Earth, Jupiter, and all the other planets have both an aphelion and a perihelion in their yearly journey around the Sun.

General description

There are two special points in the path of any object moving around another object in an oval shape, called an elliptic orbit. The point farthest from the main object is called the apoapsis, and the point closest to the main object is called the periapsis.

The names for these points change depending on which object is being orbited. When orbiting Earth, the farthest point is called an apogee and the closest point is called a perigee. When orbiting the Sun, the farthest point is called an aphelion and the closest point is called a perihelion. These points help us understand how objects move in space.

Terminology

The words "pericenter" and "apocenter" are often used, but "periapsis" and "apoapsis" are preferred in technical situations.

• For general orbits where the main body isn’t named, pericenter and apocenter are used for the closest and farthest points. Periapsis and apoapsis mean the same thing and can also refer to the distances.
• For a body orbiting the Sun, the closest point is called the perihelion, and the farthest point is the aphelion; around other stars, these are called periastron and apastron.
• For a satellite orbiting Earth, including the Moon, the closest point is the perigee, and the farthest point is the apogee.
• For objects orbiting the Moon, the closest point is called the pericynthion and the farthest point the apocynthion. Other terms like perilune and apolune are also used. Since the Moon has no natural satellites, these terms apply only to human-made objects.

Etymology

The words perihelion and aphelion were created by Johannes Kepler to describe how planets move around the Sun. They come from the Greek words peri- (near) and apo- (away from) combined with the Greek word for the Sun, hēlíos.

Different terms are used for other celestial objects. The endings -gee, -helion, -astron, and -galacticon are often used in astronomy for Earth, the Sun, stars, and the Galactic Center, respectively. The ending -jove is sometimes used for Jupiter, but -saturnium has rarely been used for Saturn in the last 50 years. The -gee form can also be a general term for the closest approach to any planet.

During the Apollo program, the terms pericynthion and apocynthion were used for orbiting the Moon; they refer to Cynthia, another name for the Greek Moon goddess Artemis. More recently, in the Artemis program, the terms perilune and apolune have been used.

For black holes, the term peribothron was first used in 1976. The terms perimelasma and apomelasma were used by a scientist and writer in a story in 1998.

Terminology summary

The endings shown below can be added to the beginnings peri- or apo- to create unique names for the closest and farthest points in orbits around different main bodies. However, only for the Earth, Moon, and Sun systems are these special endings commonly used. Studies of planets outside our solar system often use -astron, but for most other systems, the general ending -apsis is used instead.^{[failed verification]}

Perihelion and aphelion

The perihelion and aphelion are the closest and farthest points in a planet's orbit around the Sun. Perihelion is when the planet is nearest to the Sun, and aphelion is when it is farthest.

The Earth reaches perihelion in early January and aphelion in early July. Even though the Earth is farthest from the Sun during northern hemisphere summer, summers are still warmer there because it has more land, which heats up easily. The timing of these points changes slowly over many years due to the influence of other objects in space.

Mathematical formulae

These special math rules help us understand the closest point (called pericenter) and the farthest point (called apocenter) in the path of a planet or object moving around another body.

There are special numbers that stay the same for any given path, no matter where you look along it. These numbers help scientists predict how fast the object will move and how far it will go.

The middle point between the closest and farthest points is called the semi-major axis. This helps us measure the size of the path. There are also other important numbers that help us understand how stretched out the path is and how strong the pull of gravity is between the objects.

Time of perihelion

Orbital elements like the time of perihelion passage are based on a specific point in time, called an epoch, using a simple two-body solution. To find the exact time of perihelion, the epoch should be close to the actual perihelion passage. For example, using an epoch of 1996, Comet Hale–Bopp reached perihelion on April 1, 1997. Using an epoch of 2022 gives a less accurate date of March 29, 1997. Short-period comets can be even more affected by the chosen epoch.

Numerical integration shows that the dwarf planet Eris will reach perihelion around December 2257. Using an epoch of 2025 gives a less accurate date of August 2257. 4 Vesta reached perihelion on December 26, 2021, but using a two-body solution at an epoch of July 2021 shows it came on December 25, 2021.

Short observation periods

Trans-Neptunian objects found far from the Sun, more than 80 AU away, are hard to study. Because they move very slowly, astronomers need many observations over several years to understand their paths correctly.

With limited data, such as only 8 observations of object 2015 TH367 over one year, the uncertainty can be huge. For objects that won’t reach their closest point to the Sun for about 100 years, this limited data can create big mistakes. For 2015 TH367, scientists first thought the perihelion date could be off by about 77.3 years (28,220 days). This shows why watching these distant objects needs patience and long-term efforts to learn their true paths.