Today, we know that it is not the celestial sphere that turns as night and day proceed, but rather the planet on which we live. It is because Earth turns on this axis every 24 hours that we see the Sun, Moon, and stars rise and set with clockwork regularity. Today, we know that these celestial objects are not really on a dome, but at greatly varying distances from us in space. Nevertheless, it is sometimes still convenient to talk about the celestial dome or sphere to help us keep track of objects in the sky.
There is even a special theater, called a planetarium , in which we project a simulation of the stars and planets onto a white dome. As the celestial sphere rotates, the objects on it maintain their positions with respect to one another.
A grouping of stars such as the Big Dipper has the same shape during the course of the night, although it turns with the sky. During a single night, even objects we know to have significant motions of their own, such as the nearby planets, seem fixed relative to the stars. This is because they are not stars at all. We can use the fact that the entire celestial sphere seems to turn together to help us set up systems for keeping track of what things are visible in the sky and where they happen to be at a given time.
Figure 3: Circling the South Celestial Pole. This long-exposure photo shows trails left by stars as a result of the apparent rotation of the celestial sphere around the south celestial pole. In reality, it is Earth that rotates. Imagine a line going through Earth, connecting the North and South Poles. If we extend this imaginary line outward from Earth, the points where this line intersects the celestial sphere are called the north celestial pole and the south celestial pole.
As Earth rotates about its axis, the sky appears to turn in the opposite direction around those celestial poles Figure 3. The apparent motion of the celestial sphere depends on your latitude position north or south of the equator. If you stood at the North Pole of Earth, for example, you would see the north celestial pole overhead, at your zenith.
As you watched the stars during the course of the night, they would all circle around the celestial pole, with none rising or setting. Only that half of the sky north of the celestial equator is ever visible to an observer at the North Pole. Similarly, an observer at the South Pole would see only the southern half of the sky.
As the sky turns, all stars rise and set; they move straight up from the east side of the horizon and set straight down on the west side. During a hour period, all stars are above the horizon exactly half the time. Of course, during some of those hours, the Sun is too bright for us to see them. What would an observer in the latitudes of the United States or Europe see? For those in the continental United States and Europe, the north celestial pole is neither overhead nor on the horizon, but in between.
As Earth turns, the whole sky seems to pivot about the north celestial pole. They are always above the horizon, day and night. This part of the sky is called the north circumpolar zone. For observers in the continental United States, the Big Dipper, Little Dipper, and Cassiopeia are examples of star groups in the north circumpolar zone. That part of the sky is the south circumpolar zone. To most U. It is called Polaris , the pole star, and has the distinction of being the star that moves the least amount as the northern sky turns each day.
Astronomers measure how far apart objects appear in the sky by using angles. To give you a sense of how big a degree is, the full Moon is about half a degree across. We described the movement of stars in the night sky, but what about during the daytime? The stars continue to circle during the day, but the brilliance of the Sun makes them difficult to see. The Moon can often be seen in the daylight, however. On any given day, we can think of the Sun as being located at some position on the hypothetical celestial sphere.
When the Sun rises—that is, when the rotation of Earth carries the Sun above the horizon—sunlight is scattered by the molecules of our atmosphere, filling our sky with light and hiding the stars above the horizon. For thousands of years, astronomers have been aware that the Sun does more than just rise and set. Very reasonably, the ancients thought this meant the Sun was slowly moving around Earth, taking a period of time we call 1 year to make a full circle. We have a similar experience when we walk around a campfire at night; we see the flames appear in front of each person seated about the fire in turn.
The path the Sun appears to take around the celestial sphere each year is called the ecliptic Figure 4. Because of its motion on the ecliptic, the Sun rises about 4 minutes later each day with respect to the stars. Earth must make just a bit more than one complete rotation with respect to the stars to bring the Sun up again. For locations further south you will see in the figures below that the stars will rise up and then set down at steeper angles as you get closer to the equator.
During daylight, the meridian separates the morning and afternoon positions of the Sun. At local noon the Sun is right on the meridian the reason why this may not correspond to on your clock is discussed a little later in this chapter. At local noon the Sun is due south for northern hemisphere observers and due north for southern hemisphere observers though observers near the Earth's equator can see the local noon Sun due north or due south at different times of the year for reasons given in the next section.
For each degree you move south with Santa in his sleigh, the North Celestial Pole NCP from here on moves 1 degree away from the zenith toward the north and the highest point of the celestial equator's curved path in the sky moves up one degree from the southern horizon. This effect has nothing to do with the distance between a celestial object or marker and you at different points on the Earth remember that the celestial sphere has a practically infinite radius.
In fact, observers on a spherical world only ten miles across would see the same effect! The picture above shows the celestial sphere for the far northern city of Fairbanks in Alaska. By the time you reach your hometown, the NCP has moved away from the zenith so it is now a number of degrees above the horizon equal to your latitude on the Earth.
Remember that your position on the Earth is specified by a latitude and a longitude coordinate. The latitude is the number of degrees north or south of the Earth's equator. On a map or globe, lines of latitude run horizontally, parallel to the equator.
On a map or globe, lines of longitude run vertically, perpendicular to the equator. The NCP is therefore 34 degrees above the north horizon. Notice that finding the angle of the NCP above the horizon provides a very easy way of determining your latitude on the Earth a fact used by navigators even today! But your hands and fingers are a remarkably accurate and convenient measuring tool.
Not everybody's hands are the same size and thus there would be inaccuracies in using this method for anything other than quickly finding obejects. There is a way to minimize the errors but "calibrating" your hands. Using this picture, you can gauge where to hold your hand in front of you to get the same results.
The symbol for arcminutes is a single apostrophe '. So the full Moon, for example, is about 31' thirty one arcminutes across.
Coincidentally, so is the Sun. This is why the moon covers the sun almost perfectly during a solar eclipse.
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