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We can not perceive distance while looking at the sky. All the stars seem to be pasted on the dome of the sky, at roughly the same distance from us. While driving down the highway, a driver uses two different techniques to determine the distance the other cars and his own. The first distance – determining technique is called parallax. Each of the driver’s eyes provides a slightly different viewpoint, which his brain interprets as a sense of depth. When you shut one of your eyes, you partially lose the feeling of three dimensions and find it slightly difficult to pick up objects across the table. We also use size to help us determine distance. The farther away an object is, the smaller it will appear to use. If a driver sees a car in his own lane grow increasingly larger, he better do something about it. Neither of these techniques helps us when looking at the stars. The nearest star is so far away that both our eyes see exactly the same image so parallax does not give us any sense of depth. Even through the largest telescope, a star appears as a mere point of light. Stars have different brightness, but all have the same size when observed from the Earth.
So, since all stars appear equidistant from us in the sky, let us treat them as if they were actually the same distance from us. In other words, let us imagine that all the stars are fixed on an invisible giant celestial sphere. In reality, the stars vary enormously in their distance from the Earth. Furthermore, they are not fixed. Most stars are moving through space at a speed of tens of kilometers per second away or toward other stars. But the distances between stars are so vast that their locations appear fixed. Tens of thousands of years will pass until the sky appears significantly different that it does today.
At first, talking about a giant invisible celestial sphere seems like a step backwards to the days of Ptolemy. But a giant sphere in the sky is no different than a giant grid around the Earth. The equator and all the other lines of latitude and longitude are merely concepts that people have invented to more easily specify locations on the Earth. The celestial sphere does the same thing for the sky.
Tonight, go outside and look towards the eastern horizon. Pick out any group of stars that make a distinctive pattern and remember where the pattern is located in relation to some earthbound object such as a tree or a building. Now wait half an hour and enjoy the view. (If it’s a cold night, go back into the house, make yourself a hot cup of coffee, return outside and then enjoy the view.) Your stars have by this time moved a bit. While watching the eastern horizon you will see “new” stars rise, and you can observe other stars setting on the western horizon. If you are north of the equator and look above the northern horizon, the stars will be circling counterclockwise around a particular point in the sky. The patterns of the stars remain the same. The pattern you selected may not be in the same place tomorrow or it may not appear above the horizon at a particular time, but it will still exist tomorrow, next week, next month or a hundred years from now.
I apologize to those “down under” for the Northern Hemisphere bias in this book. The sky over the Southern Hemisphere works the same way as its northern counterpart except that its stars circle clockwise around a point over the southern horizon.
All these movements of stars are caused by the rotation of the Earth around its axis. As the Earth rotates from west to east, we see the stars appear to move in the opposite direction from east to west. I have claimed that some stars rise and set while other circle counterclockwise but they are all attacked to the same fixed heavenly sphere. All the stars are circling counterclockwise around a point over the northern horizon. Some of the stars that you see towards the north remain above the horizon all the time. We do not see them during the daytime, but they are still there. For those in the Northern Hemisphere, these are the north circumpolar stars. The south circumpolar stars are those that never rise above the horizon, so you can never see them unless you travel south. The stars that are rising from the eastern horizon and setting down over the west are also moving in a counterclockwise circle, but the Earth is blocking part of the circle.
Whether a star is circumpolar or whether it rises and sets is determined by your location north or south of the equator -- your latitude. Everyone standing on the same line of latitude on a particular night sees the stars at the same positions, although at different times. For instance, if you were watching your star pattern rise from the eastern horizon of Minneapolis, Minnesota, a stargazer would have seen the same sky at Belgrade a little over seven and a half hours before you did at Minneapolis. During that seven and a half hours, Minneapolis was gradually rotating to Belgrade’s former position under the celestial sphere. To understand how our latitude affects how the sky appears, it will help to imagine stargazing from different latitude. But first, let us take a closer look at the celestial sphere.