Celestial Sphere

The celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere.

Many early Indian astronomers distinguished the 'Khagola' (sphere of the sky, celestial sphere) from the Bhagola (sphere of the asterisms, or stars). For example, Lalla in his description of an armillary sphere instrument.

Features
The early Indian astronomers created names for the features of the celestial sphere.

The Sphere along the ecliptic was in earlier times divided into 27 asterisms, called Nakṣatras. Later on the 12 signs of the Zodiac were adopted, and given the name of the rāśis.

The prime meridian (longitude 0) went through the real city of Ujjain, the mythical city of Laṅkā, and a number of other cities as well.

The astronomer Lalla, in his, gives a discussion of matters regarding the Celestial Sphere. He discusses how it appears to the residents of the South Pole (demons) and the residents of the North Pole (gods), what would happen if they walked towards the equator, and how they see the sun. He also discusses calculations of the latitudes. Many other astronomers gave similar pictures of how the heavens might appear the different inhabitants of the Earth at different times of the day or year.

Pṛthūdaka-svāmi, Makkibhaṭṭa, and Āryabhaṭa I postulated a rotating Earth causing the apparent movement of the stars. This notion was rejected by others because the Vedas had described the Earth as fixed.

Kamalākara wrote that the Pole Star was not fixed.

Arrangement of the planets
Many astronomers gave an ordering of the heavens. Āryabhaṭa I's,, and all wrote of the order of the heavens from highest to lowest, as follows, from highest to lowest:


 * Stars, Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, Earth

Āryabhaṭa says the Earth is at the end, "like a hitching peg".

Nīlakaṇṭha wrote a great deal on the arrangement of the heavens in his Golasāra. The Earth he viewed as 'down' from everywhere. The atmosphere went up so far, and then the Pravaha winds blew the stars around the Earth, 21,600 arcminutes around each day. The ecliptic was divided into 30° sections called rāśis. He also discusses several great circles of the sphere, including the Ghaṭikā-maṇḍala, the Unmaṇḍala, and the Samamaṇḍala. Also described are the Lagna, epicycles, the equation of center, higher apsis, etc. He also mentions the planets' Śīgha circles and Manda circles.

Details of motion can be found in the article on the planets.

Earth centered vs sun centered
The Celestial Sphere model of the solar system, being geocentric, differs greatly from the modern sun-centered model. However, many of the ideas and methods apply to both systems. For example, the ecliptic plane in the sun-centered model is the orbit of the Earth around the sun. In the Earth-centered model, it is the orbit of the Sun around the Earth. Both of these orbits define the exact same plane, regardless of the viewpoint used as the center, or what name the plane is called by.