Apsis

Space is full of objects moving in oval paths called ellipses. Because these paths are not perfect circles, objects do not stay the same distance from the thing they orbit.

Instead, they move closer and then they move farther away. An apsis is a name for one of these extreme points. There are always two apsides in any elliptical orbit. One is the nearest point, called the periapsis. The other is the farthest point, called the apoapsis. The line connecting these two points is called the line of apsides.

Scientists use different names for these points depending on what is being orbited. They use prefixes to show how close or far an object is. The prefix peri- means near, and the prefix apo- means away from. The end of the word changes based on the host body. For example, the suffix -helion is used for the Sun. This gives us the names perihelion and aphelion.

If an object orbits the Earth, we use the suffix -gee. This creates the terms perigee and apogee. These names help astronomers keep track of many different moving parts in space.

Johannes Kepler was the scientist who created the terms perihelion and aphelion. He used them to describe how planets move around the Sun. He wanted to explain the specific motions of these distant worlds.

Later, other scientists found ways to name orbits for different objects. During the Apollo program, people used the terms pericynthion and apocynthion for the Moon. These names came from Cynthia, a name for the Greek Moon goddess. Today, the Artemis program uses the terms perilune and apolune instead.

Earth has its own specific schedule for these points. Earth reaches perihelion, its closest point to the Sun, in early January. At this time, the center of the Earth is about 147 million kilometers from the Sun. Earth reaches aphelion, its farthest point, in early July. At aphelion, the distance is about 152 million kilometers. These dates can change by a few days each year. This happens because the Moon pulls on the Earth as it moves. Even though we are farther away in July, it is summer in the northern hemisphere. This is because of the tilt of the Earth's axis.

Understanding these points helps us understand how the whole solar system works. We can use these same rules for planets like Jupiter or Mars. We even use them for small things like asteroids and comets. If we look at stars far away, we call the points periastron and apastron. Even scientists studying black holes have special names for these distances. It is all part of the same big map of how things move. By knowing the apsides, we can predict exactly where a spacecraft or a planet will be.