Motion in the Day and Night Sky

There are few natural sights as creative and beautiful as the magnificence of the sun setting in the west or as it rises in the east. If you were to stretch out in your backyard with a blanket, and stay there for several hours gazing at the sky, it would appear that objects in the sky would be moving past you over time. Have you ever wondered why?

We aren't aware of the earth's movement, but the earth is in constant motion. Our earth is rotating on an axis and revolving around the sun.

We experience night and day due to the rotation of the Earth on its axis, actually tilted 23.5° on its axis. It takes the earth 23 hours and 56 minutes to make a complete rotation.

In March 2011 Japan suffered a massive earthquake. Since then, NASA estimates the tilt of the Earth has slightly changed. Richard Gross, of NASA's Jet Propulsion Laboratory, calculates that the earthquake off Japan's coast (2011) may have shortened the length of Earth's days by about 1.26 microseconds (that's 1.26 millionth of a second) and shifted its axis by about 6.5 inches. This change will likely not be noticed by average citizens. However, astronomers must be aware of this change to be sure they are making accurate calculations for space flight and satellite launches.

Orderly Movement

You have already learned that the study of Earth Science includes understanding the systems on Earth. We must also understand that the Universe has systems. These systems are wonderful, powerful processes that come together giving order to the universe. Systems run in cycles. Ancient astronomers tracked the movement of celestial bodies. Fortunately, over time astronomers have continued to track these movements and expanded upon the tracking by using modern technology. Obviously there is a series of cycles, and these cycles all have different periods to them. But why? The earth's rotation on its axis is one of the many celestial cycles we see in the universe in constant motion.

The Celestial Sphere

Watch the following animation by clicking on the TV to understand more about the celestial sphere. There are no words with the animation. Imagine you are the person standing outside looking up at the sky. Think about the sounds you would hear.

This animation is a simple model of the apparent motion of the stars in the night sky. Ancient people described the motions as if the stars were all attached to a vast globe centered about the earth. This globe, which we refer to as the celestial sphere, rotates about once a day on an axis that is lined up with Polaris.

The daily motions, we see in the sky are caused by the rotation of the Earth on its axis. When viewed from above Earth, the North Pole, the earth spins in a counterclockwise direction, eastward (from west to east),. It makes a complete rotation of 360 degrees in a little less than 24 hours (more precisely 23.9 hours). For this reason, when viewed from the earth's surface, the sun, moon, and stars appear to rise in the east and set in the west, making a complete rotation of 360 degrees in a little less than 24 hours. So here we are on the earth, it's a big, spinning ball.

Why Does Earth Have Day and Night

We have day and night because the Earth rotates. It spins on its axis, which is an imaginary line passing through the North and South Poles.

The Earth spins slowly all the time, but we don't feel any movement because it turns smoothly and at the same speed.

How long does it take the Earth to turn around?

The Earth takes a whole day (24 hours) to make a complete turn.

Movement in the Day/Night Sky Self Assessment

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