The **apoaster** , **apoapsis** , **apoápsid** , or **apocenter** (from the Greek ἀπό *apó* 'far from', ἀστήρ *aster* 'astro') is the point of an elliptical orbit farthest from its gravitational center.

In the orbits there is always a body of greater mass called primary around which another body called secondary revolves. The apoaster or apoapsis is the point in the orbit where the secondary is at the maximum distance from the primary.

If it is about the Sun it is called aphelion , if it is about Earth it is called apogee , if it is about the Moon and in a way that can be considered inappropriate aposelenium , but in all other cases it is called **apoastro** .

It is represented by *Q* . If *a* is the mean distance and *e* the eccentricity :

It only makes sense in elliptical orbits and, as established by the second of Kepler's laws , the translational speed of the planet is minimal in apoapsis and the semi-major axis can be calculated once of the orbit, the eccentricity , the mass of the primary and the universal constant of gravitation by the expression: ^{[ 1 ]}

The minimum distance between primary and secondary is called periapsis or **periastrum** and exists in all types of orbits, (whether elliptical, parabolic or hyperbolic).

## References

- ↑ The Physics website. "Calculation of velocity in elliptical orbits" . Retrieved September 13, 2017 .