Mimicking the sun, stars also rise and set


The distances between the Earth and the stars are so great that the positions of the stars, relative to each other, will not significantly change in your lifetime.

However, if you watch the night sky for a few hours, you will see that all the stars move as a group from east to west. As you watch, stars will slowly rise in the east while others will set in the west.

This movement of the stars from east to west is called “diurnal motion.”

It is the Earth spinning on its axis that causes the stars to appear to move. This is the same rotation that causes the sun to rise in the east and set in the west, creating day and night.

Let’s take a closer look at diurnal motion.

From our viewpoint, it appears our Earth is at the center of a very large sphere that contains the stars. This imaginary ball, with all the stars the same distance from Earth, was the ancient view of the night sky.

They imagined that the Earth was fixed at the center and did not rotate. Instead, the entire celestial sphere rotated once a day around the Earth.

We now know that this view is not correct. The stars are at different distances from the Earth, and it is the rotation of the Earth, not of the celestial sphere, that causes diurnal motion.

Although today we understand that the celestial sphere is an imaginary object, we can use this idea to create a useful model for describing the directions and motions of the sky. For example, if we project the Earth’s axis of rotation onto the celestial sphere, the point of intersection is called the north celestial pole. This is the point in the sky directly over the earth’s North Pole.

Because the diurnal motion is caused by the rotation of the Earth, there are two points on the celestial sphere that the stars rotate around. The other point is the south celestial pole, over the Earth’s South Pole.

The north celestial pole is directly overhead, 90 degrees from the horizon, when you are at the North Pole. As you move south, the north celestial pole moves lower in the sky, closer to the horizon. When you reach the equator, both celestial poles are on the horizon.

In Walla Walla, we are about halfway between the North Pole and the equator, so the north celestial pole is about 45 degrees above the northern horizon. The stars in the sky appear to rotate about this point during the night.

You can use the stars to find this point in the sky. The star Polaris is very close to the north celestial pole. Polaris is the star at the end of the handle of the Little Dipper in the constellation of Ursa Minor, the Little Bear.

The easiest way to find Polaris is to look north and find the Big Dipper in the constellation of Ursa Major. The two stars that form the bowl that are farthest from the handle are called the pointer stars.

Use them to create an imaginary line that extends toward the Little Dipper. Polaris is the bright star in the Little Dipper on this imaginary line. Because it is so close to the north celestial pole, it will not appear to move with the other stars.

The stars move slowly, and you will not be able to see their motion. One way you can detect it is to find the Big Dipper, make note of its location, and then go inside for an hour.

When you come back out, you will see that it has moved a little. In 12 hours, it will be 180 degrees from where it started.

Another way to see the motion is with a camera. With a film camera you can take a 5- to 10-minute exposure. The stars will appear as arcs.

The farther a star is from the north celestial pole, the longer its arc will be. If you use a digital camera, you will have to continually take images over 5 to 10 minutes and then stack the images to see the arcs.

The Earth’s axis of rotation wobbles, so the position of the north celestial pole moves in relation to the stars. The path of the pole forms a circle, taking 26,000 years to return to the same position.

For the next 100 years, the north celestial pole will draw closer to Polaris. After 2114, they will begin to move farther apart. Polaris will not always be the star closest to the north celestial pole.

In Egyptian times — 3,000 BC — the pole star was Thuban, in the constellation Draco. In the year 14,000, it will be Vega in the constellation Lyra. In another 26,000 years it will again be Polaris.

Your assignment: On the next clear night, see if you can find the north celestial pole. After you locate the Big Dipper and Polaris, keep checking back to see the rotation of the night sky.

Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at marty.scott@wallawalla.edu.


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