See under apsis.
noun, plural ap·si·des [ap-si-deez] /ˈæp sɪˌdiz/.
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.
Origin of apsis
1595–1605; < Latin < Greek hapsís (felloe of) a wheel, arch, vault, orig., fastening, equivalent to háp(tein) to fasten + -sis -sis
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
noun plural 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 principal axis of the orbit
another name for apse (def. 1)
Word Origin for apsis
C17: via Latin from Greek; see apse
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
"perigree of the moon, perihelion of a planet" (plural apsides), 1650s, from Latin apsis "arch, vault" (see apse).
Online Etymology Dictionary, © 2010 Douglas Harper
Plural apsides (ăp′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.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.