Wikibooks: High School Earth Science/Earth's Motions

• Format: Book
• Language: English
• Subjects: North Pole; South pole
• Online Access: https://en.wikibooks.org/wiki/High_School_Earth_Science/Earth%27s_Motions

TOC right Imagine a line passing through the center of Earth that goes through both the North Pole and the South Pole. This imaginary line is called an axis . Earth spins around its axis just as a top spins around its spindle. This spinning movement is called Earth s rotation. At the same time that the Earth spins on its axis it also orbits or revolves around the Sun. This movement is called revolution. =Lesson Objectives= Describe Earth s rotation on its axis. Describe Earth s revolution around the Sun. =Earth s Rotation= In 1851 a French scientist named Léon Foucault took an iron sphere and swung it from a wire. He pulled the sphere to one side and then released it letting it swing back and forth in a straight line. A ball swinging back and forth on a string is called a pendulum. A pendulum set in motion will not change its motion so it will not change the direction of the swinging. However Foucault observed that his pendulum did seem to change direction. He knew that the pendulum itself could not change its motion so he concluded that the Earth underneath the pendulum was moving. Figure 24.9 shows how this might look. It takes 23 hours 59 minutes and 4 seconds for the Earth to make one complete rotation on its axis if we watch Earth spin from out in space. Because Earth is moving around the Sun at the same time that it is rotating Earth has to turn just a little bit more to reach the same place relative to the Sun so we experience each day on Earth as 24 hours. At the equator the Earth rotates at a speed of about 1 700 kilometers per hour. Thankfully we do not notice this movement because it would certainly make us dizzy. =Earth s Revolution= Earth s revolution around the Sun takes much longer than its rotation on its axis. One complete revolution takes 365.25 days or one year. The Earth revolves around the Sun because gravity keeps it in a roughly circular orbit around the Sun. The Earth s orbital path is not a perfect circle but rather an ellipse which means that it is like a slight oval in shape (Figure ...