**Ap"sis** (#), n.; pl. Apsides (#). See Apse. [L. *apsis*, *absis*, Gr. , , a tying, fastening, the hoop of a wheel, the wheel, a bow, arch, vault, fr. to fasten.]

**1.** Astron.

One of the two points of an orbit, as of a planet or satellite, which are at the greatest and least distance from the central body, corresponding to the aphelion and perihelion of a planet, or to the apogee and perigee of the moon. The more distant is called the *higher apsis*; the other, the *lower apsis*; and the line joining them, the *line of apsides*.

**2.** Math.

In a curve referred to polar coordinates, any point for which the radius vector is a maximum or minimum.

**3.** Arch.

Same as Apse.

© Webster 1913.