A celestial sphere is an arbitrary sphere assumed by humans on Earth when they started observing the stars. It is assumed that the center of the celestial sphere is the Earth, such that the poles and equator of both the Earth and the celestial sphere aligns with each other. The celestial sphere or more generally, the celestial sky is used as a reference point for navigation in the earlier times. Just like our local star, Sun, the stars in the celestial sky, appear to set in the west and rise in the east due to Earth's rotation. Any star can be located on the celestial sky with two coordinates declination and right ascension, which are analogous to the Earth's latitude and longitude.
Frequently Asked Questionsedit
Are all stars fixed in the celestial sphere?edit
The position of the stars, nebulae and galaxies are assumed to be fixed in the celestial sky. Their positions do change, but the relative distances between them seldom change, when observed from Earth. As the distance between the Earth and the stars are seemingly long, they make a negligible movement due to minuscule angles that are almost zero. Using small-angle approximation, the positions of stars are simplified and assumed to be fixed.
Is the position of the Sun fixed?edit
As Earth rotates around the Sun, in fixed Earth and celestial sphere sense, it would appear that the Sun follows its own path around the Earth throughout one Earth year relative to the stars in the celestial sphere. This path is known as the ecliptic, which marks the Tropic of Cancer and Tropic of Capricorn on Earth.