Note 3 — JOSHI

The universe may be conceived for the purpose of understanding the sun’s path among the stars as if it were a huge ball, of which looking from the earth’s surface we see part of the inside with the stars marked on it, as on the inside of a dome. This imaginary inside of a ball is called the celestial sphere, and the ancients believed that it actually existed, and also, in order to account for the varying distances of the stars, supposed that there were several of them, one inside the other, and each with a number of stars fixed to it. The sun and earth may be conceived as smaller solid balls suspended inside this large one. Then looking from the surface of the earth we see the sun outlined against the inner surface of the imaginary celestial sphere. And as the earth travels round the sun in its orbit, the appearance to us is that the sun moves over the surface of the celestial sphere. The following figure will make this clear.3