Apsis

In the vastness of space, most objects do not move in perfect circles. Instead, they follow paths called ellipses, which are shaped like stretched-out circles. Because of this shape, an orbiting body is constantly changing its distance from the object it orbits. The points where the body is at its absolute closest and absolute farthest are known as the apsides.

Understanding these points is essential for orbital mechanics. It allows scientists to predict exactly where a planet, a moon, or a spacecraft will be at any given moment. The line that connects these two extreme points is called the line of apsides.

To understand how this works, we must look at the physics of a two-body system. When two objects orbit one another, they actually both move around a shared center of mass. This shared center is called the barycenter. In many cases, the larger body is so massive that the barycenter stays inside it. For example, the barycenter of the Earth and the Moon is located about 75% of the way from the Earth's center to its surface.

Technically, an apsis is the distance measured between this barycenter and the center of mass of the orbiting body. However, when engineers talk about spacecraft, they often use the term to describe the altitude above the surface of the central body.

Astronomers use a specific naming system to identify these points. The names are built using prefixes and suffixes. The prefix "peri-" means near, while the prefix "apo-" means away from. The suffix changes depending on what the object is orbiting. For instance, the suffix "-helion" refers to the Sun. This gives us the terms perihelion for the closest point and aphelion for the farthest point.

If an object orbits the Earth, we use the suffix "-gee" to create perigee and apogee. When orbiting a star, the suffix "-astron" is used, resulting in periastron and apastron. This logical system allows scientists to communicate clearly about any object in the universe.

These terms were not always used in this way. The famous scientist Johannes Kepler coined the terms perihelion and aphelion. He used them to describe the complex motions of the planets as they moved around the Sun. As space exploration grew, new names were needed for different missions. During the Apollo program, scientists used the terms pericynthion and apocynthion for orbits around the Moon. These names were inspired by Cynthia, an alternative name for the Greek Moon goddess. Today, the Artemis program uses the terms perilune and apolune. Even for extreme objects like black holes, scientists have proposed names like peribothron, using the Greek word for a pit.

Earth provides a perfect example of these orbital mechanics in action. Earth reaches perihelion, its closest approach to the Sun, in early January. At this moment, the distance between the centers of the Earth and Sun is approximately 147 million kilometers. Conversely, Earth reaches aphelion in early July. At this farthest point, the distance is about 152 million kilometers.

Interestingly, being closer to the Sun does not cause summer. Our seasons are actually caused by the 23.4° tilt of the Earth's axis. In fact, at aphelion in July, it is summer in the Northern Hemisphere.

The timing of these points is not perfectly fixed every year. The dates can shift by up to three days due to the gravitational influence of the Moon. While the Earth-Moon barycenter moves on a stable path, the Earth itself can shift slightly. This affects the exact timing of the closest approach. Over much longer periods, these dates change due to Milankovitch cycles, which are long-term shifts in Earth's orbit. For example, the next time we will experience a July 3 aphelion is in the year 2060. The next January 2 perihelion will occur in 2089.

This system of apsides applies to almost everything in our solar system. It describes the paths of the inner planets like Mercury and Venus, as well as the outer giants like Jupiter and Neptune. It even applies to smaller objects like asteroids, comets, and dwarf planets such as Ceres. Even the radiation levels change based on these distances. At aphelion, only about 93.55% of the solar radiation reaches a specific area of Earth's surface compared to what it receives at perihelion. By studying these points, we gain a complete map of the movement of our entire celestial neighborhood.