A short intro
Stars appear to us as tiny points of light attached to the surface of a huge sphere surrounding us, an infinitely large sphere, with our planet Earth in the center. Stars and planets appear to be moving on the surface of this sphere. This is the model depicted by the ancients to describe the sky with its mysterious moving objects and to predict their motions. Many of them considered these stars as Gods who had great deal of influence on rain, plant, and even social life!
Even though we now know that this picture is merely an illusion, we still use it to describe the apparent locations of celestial objects as it turns out that it is a very convenient and useful model. But before we delve into the elements of this celestial sphere, we need to start with our very own location, a point on the surface of yet another sphere – a very tiny one compared to the sky – our planet Earth!
Equator, Horizon, North/South Poles
As figure 1 show, Earth is considered to be a sphere – although we know that it is not a perfect sphere at all. The rotation axis of Earth shown defines two poles: a north pole NP and a south pole SP. The great circle (one that has its radius equal to the radius of the sphere) perpendicular to the rotation axis is what we know as the equator. The equator splits Earth into two hemispheres: north and south. Another important concept is the Horizon; it is the surface tangent to Earth at the point P. Note that each observer has his own Horizon.
Locations on the Earth surface
To be able to determine the location of points on the surface of earth, a number of circles are drawn on the surface of the planet to form a grid that allow us to assign two coordinates to each point – these are the two known numbers called latitude and longitude. Latitude is defined by circles drawn parallel to the equator at angles starts from zero at the equator and increases to +90 at the North Pole and -90 at the South Pole. So, Equator is the reference circle for latitudes, and the latitude of any point on the equator will be zero by definition.
Longitude is determined by great circles perpendicular to the equator and – naturally – passing through the two poles of the planet. The reference circle with zero longitude happens to be this great circle that pass through Greenwich city in London for merely historical reasons. Figure 2 shows Earth, equator, and the longitude/latitude of a point P.
The Celestial sphere
Now, let’s draw a much larger sphere around Earth to represent the sky. Since the sky is enormously larger than Earth, if we draw this sphere – known as the celestial sphere – to any small scale and no matter how large we make it, Earth will be just a point at the center. But, how the equator, horizon, and other elements we mentioned earlier will look like? The answer is simple; we’ll just extend them to cover the celestial sphere. As you can see in Figure 3, The Celestial equator coincides with Earth equator. The North and South Celestial poles result from extending Earth rotation axis until it hits the celestial sphere. So basic elements of the celestial sphere can be summarized as follows: