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[ap-sis] /ˈæp sɪs/
noun, plural apsides
[ap-si-deez] /ˈæp sɪˌdiz/ (Show IPA)
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 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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Examples from the Web for apsides
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 apsides” is that which joins them, forming the major axis of the orbit.

  • 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
  • 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
  • This secular displacement of the line of the apsides brings a sixth complication to the motion of our abiding-place.

    Urania Camille Flammarion
British Dictionary definitions for apsides


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 principal axis of the orbit
another name for apse (sense 1)
Derived Forms
apsidal (æpˈsaɪdəl; ˈæpsɪdəl) adjective
Word Origin
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
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Word Origin and History for apsides



"perigree of the moon, perihelion of a planet" (plural apsides), 1650s, from Latin apsis "arch, vault" (see apse).

Online Etymology Dictionary, © 2010 Douglas Harper
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apsides in Science
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.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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