For an object moving in an elliptical orbit about another celestial body, the point of greatest separation is called the apoapsis (from the Greek apo = away from). At this point in the orbit, the object is travelling at its slowest speed (Kepler’s Second Law).

The apoapsis is equivalent to the:

• Aphelion: for a celestial body orbiting the Sun,
• Apogee: for a celestial body (in particular the Moon or artificial satellites) orbiting the Earth,
• Apastron: for a celestial body orbiting a star (e.g. in a binary star system).

A planet at aphelion in its elliptical orbit around the Sun.

The Moon at apogee in its elliptical orbit around the Earth.

The point of closest approach in the orbit is called the periapsis.