The Celestial Sphere
All celestial object spear to be attached to an invisible sphere of infinite size centered on the Earth. Known as the celestial sphere, this seems to spin around the Earth once o day, although it is actually the Earth that is rotating. The portion of the celestial sphere seen from the Earth depends on the observer’s latitude, the time of night, and the time of year.
There are various important points and lines in the celestial sphere, similar to those on the earth’s globe. Directly over the Earth’s poles lie the celestial poles, around which the sphere appears to turn each day. The celestial equator is a circle on the celestial equator sphere above the Earth’s the celestial sphere above the Earth’s equator. Another circle, known as the ecliptic, represents the Sun’s apparent path around the sphere each year. Of course, Sun’s motion is actually due to the earth orbiting the Sun; hence the ecliptic is really the plane of the Earth’s orbit projected on the celestial sphere. Because the Earth’ axis is tiled at 23.5 degrees, the celestial equator is tilted at the same angle to the ecliptic.
How much of the celestial sphere we can see depends on our latitude. From either pole, only half the sky can ever be seen; on any night, objects circle the celestial pole but do not rise or set (they are circumpolar). At the equator, the whole celestial sphere can be seen in a year; the celestial poles lie on the horizons to the north and south, and all objects rise and set. At multitudes, all of one celestial hemisphere is visible over a year, plus part of the other one. Only some objects are circumpolar.
TIME IF NIGHT AND TIME OF YEAR
As the earth spins, the stars seem to move across they sky; hence our view if the sky changes as the night progresses. The celestial sphere turns once in the time it takes the Earth to spin on its axis relative it the stars-23 hours 56 minutes. But the time between successive noon’s (a mean-time day) is longer, 24 hours, since the Earth is also orbiting
Earlier each night, as measured in mean time. The Earth’s orbit of the Sun also means that a stat near the celestial equator that is in the night sky (and hence visible) at one time of year will be in the daytime sky (and hence invisible) six months later.
During one year, the Sun passes in front of band of constellations along the ecliptic. Collectively known as the zodiac, they from an area in which the planets are always to be seen. The dates when the Sun is in each constellation no longer accord with the date attached to the astrological house of the same name.