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Now that we have familiarized ourselves with the celestial sphere, let us take a look at it from different points on the Earth. The first observation point is the North Pole. Directly overhead, at the sky’s zenith, hovers Polaris. As the Earth slowly rotates around its axis, we see the stars revolve counterclockwise around Polaris. Each star traces a concentric circle around the central point exactly overhead.
What about six months from now when the Earth has revolved to the other side of the Sun? Would the movement that Earth makes every half a year in respect to the Sun cause us to see a jump in any star's location. The Earth's jump is about 300,000,000 kilometers or 186,000,000 miles, but the nearest star to the Earth, Proxima Centauri, is over 130,000 times as far away from us as that semiannual jump. To notice this would be like detecting that the State of Liberty has sidestepped an inch while standing two miles away from her. The distances to the stars are so vast that our eyes are unable to detect any change in position as we revolve around the Sun. By the way, accurate instruments are able to detect a very small change in position and this is how scientists determine the distance of the nearest stars.
Let us return to the North Pole. Since Polaris is in the exact “center” of the sky and all the other stars are revolving around it, none of the stars are rising or setting. All the stars, as observed from the North Pole, are circumpolar. If a star is exactly on the horizon (and let us assume that the horizon is perfectly flat), then the star will continually roll along the northern horizon, to the eastern horizon, to the southern horizon, to the western horizon and back to the northern horizon and so on, without ever changing its elevation in the sky.
While at the North Pole we are unable to see any stars below the celestial equator. So, from the North Pole, we see exactly the northern half of the celestial sphere and all its stars are circumpolar.
The time has come to travel southward. As we move away from the North Pole, Polaris sinks towards the horizon. For every degree of latitude southward we travel, Polaris sinks exactly one degree. In fact, the elevation of Polaris always equals the latitude of the observer. If you look at Polaris from Bangoor, Maine, whose latitude is 44 degrees and 49 minutes, it will appear halfway up the sky from the horizon. But if you continued travelling south to Miami Beach, Florida at latitude North 25 degrees and 47 minutes, Polaris would have sunk to the bottom third of the sky.
While we were at the North Pole, with Polaris at the very center of the sky, no star rose or set. Once we travel a bit south, the stars continue to travel in concentric circles around Polaris, but their paths start to dip towards the horizon. Those stars whose paths dip below the horizon are no longer circumpolar. Let us stay at Bangor, Maine for a while. One of the brighter stars near Polaris is Dubhe, which is part of the Ursa Major (Big Dipper) constellation. Dubhe traces a circle around Polaris whether its is being observed from the North Pole or from Bangor, Maine. The circle over Bangor is much lower in the northern sky, but Dubhe remains above the horizon and is circumpolar from both places.
Let us now look at one of the brightest stars in the sky, Betelgeuse, in the constellation Orion which is just a bit north of the celstial equator. Because at Bangor, Polaris is only halfway up the sky and Betelgeuse circles at such a great angle from it , Betelgeuse dips below the horizon is not circumpolar. As you travel further south until the equator, Polaris approaches close and close to the horizon and more stars dip below the horizon.
Until now, it seems that those in the northern latitudes have an advantage over the rest of us. Anybody willing to sit in the frozen tundra can watch most of the northern celestial sphere spin around and around and almost none of its stars will sink out of sight. Fortunately, there is still the southern celestial sphere. To a northern hemisphere observer, no star in the southern celestial sphere remains above the horizon all the time, but the further south he travels, the more stars of the southern celestial sphere the observer can see.
No stars are circumpolar from any point on the equator. On the other hand, every point on the celestial sphere can be seen rising and setting.
For the southern hemisphere, everything is reversed except that there is no convenient marker like Polaris at the South Celestial Pole. At the South Pole itself you can observe all the stars of the southern celestial sphere and they are all circumpolar.