celestial sphere سپهر ِ آسمانی، کرهی ِ ~ sepehr-e âsmân (#), kore-ye ~ (#)
*Fr.: sphère céleste*
An imaginary sphere, of large but indefinite dimension, used as a
basis to define the position coordinates of celestial bodies. The center can
be the Earth, the observer, or any other point which plays the role of origin
for a given system of coordinates. Seen from the Earth, the celestial sphere rotates
around the → *celestial axis* every 23h 56m 04s
(the → *sidereal day*), as a
result of the Earth's rotation. Two important circles on the celestial
sphere are the → *celestial equator* and the
→ *ecliptic*. The angle between them, about 23.40 degrees, is known
as the → *obliquity of the ecliptic*. The celestial equator
and the ecliptic intersect at two points, → *vernal equinox*
and → *autumnal equinox*. The positions of the
→ *celestial pole*s and therefore that of the
→ *celestial equator* move gradually on the celestial
sphere, due to → *precession*. → *celestial*; → *sphere*. |