The Celestial Sphere

         You must know that the stars are huge balls of extremely hot gas which are very, very far away from us. Anyone who has seriously studied the solar system also knows that the planets, including our planet Earth, orbit the Sun which itself is just one of countless stars in the universe.

However, that is not the way it looks to us.

   The sky above us seems to be a hemisphere with us at the center. In order to understand how objects in the sky appear to move, it is worth studying the concept of the celestial sphere.
    First, it is important to recognize that every individual observer on Earth has a different sky above them. Not everyone sees the same stars at the same time in the same place.
    If you have ever been so fortunate as to spend a long, quiet evening under a crystal clear night sky, your impression is that the sky is a dome over our heads. We can imagine that the Earth is a relatively small, spherical world which is at the exact center of it all. We are such small creatures on this little globe that the small part of the Earth we are able to see looks pretty much flat. We are clever enough to know that, even though we can only see the sky above the horizon, there is another half of the sky which is always below our feet. The result of all this is that the stars and other celestial bodies look as if they are stuck on a spherical sky which turns around once each sidereal day. Look at an illustration of the celestial sphere which seems to surround the Earth. Notice the little observer at the center. That is you. Is this not the way the sky at least looks to you? Well, maybe there are a lot more trees and buildings where you are. If you look overhead you find the point called the zenith. Stretching from the north point on the horizon through the zenith then down to the south point is half a circle called the upper celestial meridian. Every celestial body must cross our meridian at some time or another. When the Sun crosses our meridian, we call it noon. Note carefully that the Sun is not directly overhead at noon, unless it is that one special day of the year and you live within twenty-three degrees of Earth's equator.

As mentioned earlier, this sky turns around, or at least, seems to. In our everyday conversations we speak of the Sun rising and setting. Have you ever seen a beautiful Full Moon rising in the east at sunset? We talk about these two objects as if they really did move around the Earth. The illusion of a spinning sphere around us must be very strong indeed. How does this sphere seem to spin? You do know, no doubt, that there is a single star which is close enough to the North Celestial Pole so as to not seem to move throughout the night. This star is Polaris, more commonly known as the North Star. It is by no means the brightest star in the night sky. Its only claim to fame is the fact that it just happens to be in line with the rotation axis of the Earth. This will not always be the case. The Earth does not maintain a steady spin direction over time. In a few thousand years or so, Polaris will no longer be a useful north star and will diminish in rank to just one of dozens of stars of similar brightness.

    Observers who are south of the equator of Earth do not have a convenient bright star near the southern pole of the sky. However, in the south, one can find the south celestial pole using the constellation of the Southern Cross. There is even a way to tell time using the Southern Cross.

    If the sky seems to have a north and south pole, it must also have an equator. Just as Earth's equator is halfway between the north and south poles of Earth, the celestial equator is halfway between the north and south poles of the sky.

Things get a little difficult here because the Earth does not rotate around an axis which is well aligned with its orbit. Our rotation axis is tilted by about twenty three and one half degrees. The result of this is that the other objects in our solar system, including the Sun, will not be found to be above our equator, out on the celestial equator, but rather along a tilted circle called the ecliptic. For many this concept of the ecliptic is difficult to grasp at first. Understanding it requires that you be able to think three dimensionally. It is sufficient for studying the celestial sphere to understand the basic definition of the ecliptic.


   Are you ready to quiz yourself?