Celestial Objects

Celestial objects​are objects that can be seen in the sky that are not associated with Earth's atmosphere. The most numerous of the celestial objects are the stars. To an observer on Earth, stars are simply points of light that vary in size, brightness, and color. The Sun, the Moon, the planets, and comets are also examples of celestial objects. Clouds, rainbows, halos, and other phenomena seen in the sky that are part of, or occur in, Earth's atmosphere are ​not​considered celestial objects.

Celestial Motion

If you observe celestial objects for even a short while, it is clear that they change position int he sky over time. You have probably noticed that the Sun is in different places int he sky at different times of the day. Thus, it seems that the Sun is "moving." Similarly, if you observer the Moon and stars carefully you find that they, too, are seen in different places in the sky at different times of the night.

Try going out on a moonlit night and noting the Moon's position at 7:00 p.m. If you go out again at 10:00 p.m., you'll notice that the Moon has changed position in the sky. The same is true of stars. When you observe the sky you find that every celestial object changes position over time, or is in motion.

Photograph showing the crescent Moon and Venus setting. Exposures were made every 8 minutes, showing the changes in the position of these celestial objects over time. Note the motion of the Moon relative to Venus.

Polaris is almost directly above the North Pole. As a result, locating the North Star can only be performed in the Northern Hemisphere ­ you won't see it otherwise. If you are in the Northern half of the planet, though, locating it is best done by first finding the constellation known as the Big Dipper.

If you keep track of this motion, you discover something curious. The motion of celestial objects is not random. They don't all move in different directions at different speeds. Instead, with very few exceptions, every single one of these thousands of objects appears to move in the same general direction ­ ​from east to west.​And if you measure the rates at which all of these celestial objects are moving, you discover something even more curious. With few exceptions (such as the Moon), ​they appear to move at the same rate!

Careful records of this motion reveal that all celestial objects appear to move across the sky from east to west along a path that is an arc, or part of a circle. Since celestial objects appear to follow a circular path at a constant rate of 15 degrees per hour, or one complete circle every day (24 hours per day x 15​oper hour = 360​oper day), this motion is called ​apparent daily motion.

In the Northern Hemisphere, all the circles formed by completing the arcs along which celestial objects move are centered very near the star Polaris. The apparent circular motion of celestial objects causes them to come into view from below the eastern horizon and to sink from view beneath the western horizon (that is, to rise in the east and set in the west).

Early observers noted that the positions of celestial objects change in a daily and yearly cyclic pattern. They discovered that understanding these patterns of motion was very useful. Since the positions of the celestial objects change with time and location, such changes can be use to determine time​and to ​find one's position on Earth​.Since the distribution of stars is random, these observers devised ​constellations​,imaginary patterns of stars, to help them keep track of the changing positions of celestial objects.

Constellations are imaginary patterns of stars.

"Apparent" Versus "Real" Motion

So far, we have used the word ​apparent​when referring to celestial motions because the motion of an object is always judged with respect to some other object or point. The idea of absolute motion or rest is misleading because there are several possible reasons why an object may ​appear​to an observer to be moving. One possibility is that the observer is standing still and the ​object​is moving. Another possibility is that the object is standing still and ​the observer ​ismoving. A third possibility is that ​both the observer and the object​are moving, but one is moving faster, or in a different direction, than the other. This is the case when you are in a car speeding down a highway; as you look out of the car window, trees along the side of the road seem to whiz by. Of course, your brain tells you that the trees are rooted to the ground and that they only seem to whiz by because you are riding in a car. But to your eyes alone, you are not moving; the image of the trees is moving from one side of your window to the other. Now think about driving past a person walking along the sidewalk. The

person is moving, but also seems to whiz by your window. Now think of a person sitting next to you in your car. To you, would that person look as though he or she was moving?

By now you should realize that the problem of determining which of the two is moving, the object or the observer, is not always easy to solve. If the signs that tell the body it is moving are removed, an observer may not realize that he or she is in motion. (Do you really feel as if you are moving at 400 miles per hour when watching a movie in an airplane cruising in level flight at that speed?) Without signs telling the observer's body that he or she is moving, any object seen changing position will be interpreted as a moving object by the observer.