The fact that celestial objects rise in the east and set in the west (or, for circumpolar stars, circle around the north or south celestial pole) each day led the ancient Greeks to assume that the entire celestial sphere rotates around us once each day, going from east to west as shown in Figure 2.9. Of course, today we know that the celestial sphere is just an illusion, and that the reason it appears to rotate around us is because Earth is rotating in the opposite direction — from west to east — once each day. Still, for purposes of understanding what we see in the local sky, it’s useful to use the model of a celestial sphere rotating around us.
As you’ll see, this daily circling of the sky explains all the daily motions we see for celestial objects in the sky, including why some objects just follow daily circles without rising or setting, and why all other objects rise in the east and set in the west.
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The Sky at the North Pole
We’ll start with the local sky at the North Pole, which is probably the easiest local sky to understand.
If you add a little stick figure person standing at the North Pole to Figure 2.9, the person would be oriented as shown in Figure 2.10. Imagine that you are standing at that position. Earth itself would be blocking your view of the southern half of the celestial sphere, so your sky would consist only of the northern half, which would be making its daily circles as shown.
Notice that, as viewed from the North Pole, the daily paths of all celestial objects are parallel to the horizon. Therefore, you would see all stars making daily circles around your sky, never rising or setting. In other words, for the sky at the North Pole, all stars are circumpolar. Moreover, your sky would always consist only of the northern half of the celestial sphere, so that you could never see any stars or other objects that are in the southern half of the sphere.
Latitude and Longitude
Stating both a latitude and a longitude therefore pinpoints a location on Earth. For example, as Figure 2.11 shows, Miami lies at about 26° N latitude and 80° W longitude.
The Sky at Other Latitudes
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Look again at Figure 2.10, and notice that the horizon at the North Pole cuts the celestial sphere in half. In fact, the horizon must always cut the celestial sphere in half — that is why the local sky always looks like a dome (which is a half-sphere, or hemisphere). The only thing that changes for other latitudes is the angle at which your horizon cuts through the celestial sphere.
Figure 2.12 shows the idea, using a latitude of 40°N (the latitude of cities including Denver, Philadelphia, and Beijing) as an example. Figure 2.12a shows a person standing at latitude 40°N, so you can see where “up” and the horizon are located for that person. If you rotate that diagram so that the person’s “up” is also “up” on your screen, and extend the horizon to look more like a local landscape, then you get the view shown in Figure 2.12b.
Notice that the celestial sphere is still doing the same thing it always does, which is appearing to turn around us once each day. However, because the horizon cuts through the sphere at an angle, the paths of stars you would see in your local sky at latitude 40° N — or any other latitude — are a little more complex than they were at the North Pole. To help you understand these paths, try to answer each question below, discussing with classmates if you wish. Then click to see the answers.
1. Do any of the circles in Figure 2.12 represent stars that are always above the horizon (circumpolar stars)? If so, which one(s)?
2. For the latitude shown in Figure 2.12, are there any stars on the celestial sphere that would never be seen?
3. Which circle(s) in Figure 2.12 represent stars that rise in the east and set in the west each day?
4. Would it ever be possible that we could discover a star that rises in the west and sets in the east (rather than rising in the east and setting in the west) each day?
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Figure 2.13 shows how this procedure allows us to understand the local sky at the equator (latitude = 0°) and Figure 2.14 does the same for the local sky at latitude 30°S (which cuts through Chile, Argentina, South Africa, and Australia). Study these diagrams and compare them with the ones you already studied for the North Pole (Figure 2.10) and latitude 40°N (Figure 2.12), then you should be ready to answer the quick quiz questions that follow.
Finding Direction by the Stars
If you study the diagrams showing how the local sky at different latitudes cuts through the celestial sphere, and use a little geometry, you’ll notice these interesting facts:
- For anyone living in the Northern Hemisphere: The north celestial pole is located in your local sky due north at an altitude that is equal to your latitude. For example, if you are at the North Pole, which is latitude 90°N, the altitude in your local sky of the north celestial pole is 90° (the zenith). If you are at latitude 40°N, the north celestial pole is located in your local sky due north at altitude 40°.
- For anyone living in the Southern Hemisphere: The south celestial pole is located in your local sky due south at an altitude that is equal to your latitude. For example, if you are at latitude 30°S, the south celestial pole is located in your local sky due south at altitude 30°.
These facts are very useful for navigation, because it means you can determine your latitude simply by finding the north or south celestial pole in your night sky. Figure 2.15 shows how you can use the Big Dipper to help you find the north celestial pole (with the North Star, Polaris, located very close to it) or the Southern Cross to help you find the south celestial pole.
Effects of Longitude
Note that although the sky varies with latitude, it does not vary with longitude, because longitude has no effect on the angle at which your horizon cuts through the celestial sphere. For example, Charleston (South Carolina) and San Diego (California) are at about the same latitude, so people in both cities see the same set of constellations at night.
However, longitude effects the time at which you see particular celestial objects. You can see why by looking again at Figure 2.12. As Earth rotates, making the stars appear to circle around us, different stars will be on the meridian at any given time for different longitudes. For example, when a particular star is directly overhead in Charleston, the same star will still be rising up for people in San Diego, and it won’t reach the horizon until about 3 hours later — by which time it will be on its way downward in the Charleston sky.
The same is true for the Sun, which is why we need different time zones for different longitudes. For example, when it is noon in Charleston (meaning the Sun is near its highest point in the sky), it is about 3 hours earlier (9 a.m.) in San Diego.
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