Periapsis

Given two objects, A and B (two planets for example), if object B orbits around object A; periapsis is the point in the orbit of B which is closest to A.

If object A is Earth, it's also known as perigee.
If object A is the Sun, it's also known as perihelion.
If object A is the Moon, it's also known as perilune.

According to Kepler's laws of planetary motion, as a planet travels in an elliptical orbit, around a star; as the planet approaches the periapsis; it will increase in velocity.

The point of farthest approach is the apoapsis.

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Periapsis