A'psis. n.s. apsides, plural. [άψίς.]
Is applied, in astronomy, to two points in the orbits of planets, in which they are at the greatest, and the least distance from the sun or earth. The higher apsis is more particularly denominated aphelion, or apogee; the lower, perihelion, or perigee. Chambers.
If bodies revolve in orbits that are pretty near circles, and the apsides of these orbits be fixed, then the centripetal forces of those bodies will be reciprocally as the squares of the distances. Cheyne's Philosophical Principles.