In the night sky, the stars seem to lie on a spherical vault. For this reason, the Ancients believed the stars were inlaid in a crystal sphere with the Earth at the center. It was therefore decided that the best way to represent the constellations would be to make a model of the starry sphere. The resulting instrument was the celestial globe.

The main circles of the celestial sphere are drawn on the globe's surface. The equator is divided into 360 degrees. The ecliptic, which represents the Sun's annual path across the constellations of the zodiac, is also subdivided into 360 degrees. Two great circles, the solstitial colure and the equinoctial colure, intersect at the celestial poles.

The globes show all or some of the 1,025 stars that Claudius Ptolemy catalogued in the Almagest. The stars are enclosed in the outlines of the constellations to which they belong. Rotating the globe, we can recognize Sagittarius, Scorpio, Leo, and Orion. These constellations are depicted not as observed when we look up to the sky, but in specular form, that is, as they would appear to someone observing the celestial sphere from the outside.

The globe rests on a stand formed by two rings. The first represents the observer's horizon. The second represents the local meridian.

The globe revolves on pivots embedded in the meridian ring at the points corresponding to the celestial poles. It can be tilted to make the altitude of the pole over the horizon coincide with the altitude that can be measured in the specific locality of observation.

The globe is a versatile calculation instrument. To find out the hour when the star Sirius rises in Alexandria of Egypt, tilt the globe to the latitude of the city. Note the Sun's position on the ecliptic on the day selected for the computation. Then, rotate the globe until Sirius touches the horizon circle. The angle between the Sun and the meridian on the celestial equator, measured against the 24 hours of the celestial sphere's apparent rotation around the Earth, gives the exact hour at which the star will rise.