The Celestial Sphere

Two Minute Astronomy, 2

Last time, we saw how stars and other celestial objects outside of the Solar System can be considered as stationary for the purposes of observing them, as they are much too far away for them to actually show any movement discernible to the naked eye over a single human lifetime. We thus laid foundation for the concept of the celestial sphere.

Though we now know that the Earth is not at the center of the universe (as the ancients once believed), such a model is useful when we are observing celestial objects. Since stars appear to be stationary as explained previously, we can treat them as being fixed onto a huge sphere that has the Earth at its center. This is called the celestial sphere. As the Earth rotates from west to east (and the celestial sphere remains stationary), it appears as though stars on the celestial sphere rise in the east and set in the west while seen from the Earth, much like the Sun’s motion across our sky.

A simple diagram illustrating the celestial sphere.

This image shows a conceptual diagram of the celestial sphere, with the basic elements labelled. The Earth’s axis of rotation (which is inclined at an angle of 23.5°)is shown in red, and as you can see, it passes through the Poles of the Earth as well as those of the celestial sphere. In other words, the Celestial North Pole is directly above the Earth’s North Pole, and the same applies for the South Pole as well. Moreover, the celestial equator (in yellow) is concentric to the Earth’s own Equator (shown as a faint line across the blue globe of the Earth). In fact, the celestial sphere has a full-fledged coordinate system, not just poles and an equator. We’ll come back to this in a couple of articles.

A drawing of the celestial globe with constellations marked. Image by DarkmoonArt_de from Pixabay

So far we’ve discussed about celestial objects outside the Solar System, but what about the Sun, Moon and the planets? We know that while they too are far away, they are not far enough to be considered as stationary and fixed onto the celestial sphere. Then how exactly do they fit into our concept of the celestial sphere? Wait till the next episode to find out!