Apsides

astronomy. either of two points in an eccentric orbit, one (higher apsis) farthest from the center of attraction, the other (lower apsis) nearest to the center of attraction.
architecture. an apse.
historical examples

the two are joined by a diameter called the line of the apsides.
the sailor’s word-book william henry smyth

the line of apsides of a large proportion of comets will be approximately coincident with the solar orbit.
comets and meteors daniel kirkwood

the line of the apsides of the moon’s orbit revolves from west to east, through her whole course, in about nine years.
letters on astronomy denison olmsted

the “line of apsides” is that which joins them, forming the major axis of the orbit.
encyclopaedia britannica, 11th edition, volume 2, slice 3 various

this secular displacement of the line of the apsides brings a sixth complication to the motion of our abiding-place.
urania camille flammarion

this point was called “the excentric,” and the line from the excentric to the earth was called “the line of apsides.”
the science of the stars e. walter maunder

noun (pl) apsides (æpˈsaɪdiːz; ˈæpsɪˌdiːz)
also called apse. either of two points lying at the extremities of an eccentric orbit of a planet, satellite, etc, such as the aphelion and perihelion of a planet or the apogee and perigee of the moon. the line of apsides connects two such points and is the princ-p-l axis of the orbit
another name for apse (sense 1)
n.

“perigree of the moon, perihelion of a planet” (plural apsides), 1650s, from latin apsis “arch, vault” (see apse).
apsis
(āp’sĭs)
plural apsides (āp’sĭ-dēz’)
in the path of an orbiting body, either of the two points at which it is closest to or farthest away from the body it is orbiting. see also apoapsis, periapsis.