Celestial Sphere


CELESTIAL SPHERE

If you go out at night (where the sky is dark) and look up at the sky, it will appear as if you are standing on a stationary Earth situated at the center of a large hemispherical dome (which can be completed by the hemispherical dome you can't see because it is blocked by the ground). The stars are seemingly fixed, while the Sun, planets, comets, and asteroids move through the stars on this great sphere. This conceptualization serves as a very nice representation of the objects in the sky. This great Celestial sphere was first postulated by the ancients and called the Celestial Sphere.

To make this picture useful we must figure out a way to describe where objects can be found and how they move on the sky. The constellations were first defined as a way to locate objects on the Celestial Sphere. For example, a set of 12 constellations, the so-called Zodiac constellations, were defined to mark the path the Sun followed through the stars on the Celestial Sphere, the Ecliptic. There, however, are better more precise ways to describe how objects can be located on the Celestial Sphere.

In order to describe unambiguously where objects are located on the Celestial Sphere , we define a system analogous to longitude and latitude used for locating places on the surface of the Earth. For the stars and other celestial objects, we define coordinates known as Right Ascension and Declination.

For locations on the Earth, we measure latitude as north or south from the equator and longitude as how far we are east and west of Greenwich England. For locations on the Celestial Sphere, we also measure positions relative to some fixed places. Analogous to the Earth, we define:

  • North Celestial Pole (NCP) --the point above the North Pole where the continuation of the Earth's rotation axis strikes the Celestial Sphere.
  • South Celestial Pole (SCP) --the point above the South Pole where the continuation of the Earth's rotation axis strikes the Celestial Sphere.
  • Celestial Equator --The circle where the continuation of the Earth's equatorial plane strikes the Celesital Sphere.

More on this later, but we also define a position on the Celestial Equator, known as the Vernal Equinox, from which we measure east-west locations.

We define Declination as the location north or south of the Celestial Equator and Right Ascension as the distance east or west of the Vernal Equinox.


The Celestial Sphere was (and remains) a useful way in which to represent the sky and, in fact, it is still the way astronomers represent the observable sky; the Celestial Sphere gives us a natural way in which to understand the daily motions (diurnal motions) of the objects that we observe in the heavens. However, it fares less well when it is applied to the long-term motions of planets, comets, and asteroids. (The daily motion of planets, comets, and asteroids are generally similar to those of a star, that is they also roughly circle about the NCP or SCP.) The long-term motions are, however, more complex (retrograde motion) which led to contrived scenarios as explanations for their motions. More on this point later.


Is the Celestial Sphere a Physical Model?


Although the Celestial Sphere remains a useful model today, we understand that, physically, it does not make a lot of sense because we know that: (i) the Earth is not stationary and that it does not sit at the center of the Universe; (ii) the stars (and other celestial objects) are not stationary, attached to the surface of a great crystalline sphere; and (iii) the Sun does not orbit about the Earth. So,

Why did the Greeks consider the Celestial Sphere to be a viable physical description of the Universe?

To answer this question we must think about the kinds of effects produced by the motions of the Earth, the Sun, and the stars and how the motions manifest themselves.

Consequences of the Motions of Celestial Bodies