Pendulum helped prove how Earth rotates around sun
"How does the world rotate?" asked Esmeralda Arizmendi, 10, a fifth-grader at O'Plaine School in Gurnee.
Once each day, the Earth makes a full turn called a rotation. When we are facing the sun, it's daytime. Night falls when our part of the Earth has rotated away from the sun.
"The Earth rotates once every 23 hours 56 minutes," said Adler Planetarium Director of Astronomy Geza Gyuk.
At the same time, our planet travels in a huge arc around the sun. For Earth, this process takes place over 365 days -- one year. Because we are moving around the sun as we rotate, the position of the sun in our sky changes a little each day.
It takes 24 hours for the sun to be back in the same place. That's why our day has a full 24 hours.
How did the Earth start rotating?
"We don't really know why the Earth rotates at the rate it does, but it probably has something to do with the impact that formed the moon," Gyuk said.
"Immediately after this impact, the Earth was probably spinning a lot faster than it is now. Over billions of years, some of that spin has been transferred to the moon, which is now much farther away than it was when it was first created."
Scientists were not able to prove that the Earth rotates around the sun until about 150 years ago. In 1851, French scientist Leon Foucault used a pendulum to show that it's the Earth that rotates around the sun, not the stars and sun turning around the Earth.
He set up a very long, weighted wire and forced it to swing in a north-south direction. After the initial push, nothing exerted force on the wire. Over the course of a day, however, the direction of the pendulum changed, proving that the Earth was rotating underneath it.
For more information
The Warren Newport Public Library in Gurnee suggests these titles on the Earth's rotation:
• "Day and Night: As the Earth Turns" by Lynn M. Stone • "On Earth" by G. Brian Karas • "The Solar System: Earth" by Stephen Feinstein • "Sun Up, Sun Down: The Story of Day and Night" by Jacqui Bailey • "What Makes Day and Night?" by Franklyn M. Branley
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