# Rotation of Earth

## Earth rotates on its axis, which creates the day-night cycle.

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Rotation of Earth

#### What would you do if you were in Paris?

Take a view from the top of the Eiffel Tower? March up the stairs to eye the gargoyles at Notre Dame? Nibble on coffee and croissants in a sidewalk cafe? Visit Foucault's Pendulum in the Pantheon? Yes! When in Paris, don't forget to go to the Pantheon and visit this testament to Earth's rotation.

### Foucault's Pendulum

Foucault's Pendulum is at the Pantheon in Paris, France.

In 1851, a French scientist named Léon Foucault took an iron sphere and hung it from a wire. He pulled the sphere to one side and then released it, as a pendulum. Although a pendulum set in motion should not change its motion, Foucault observed that his pendulum did seem to change direction relative to the circle below. Foucault concluded that Earth was moving underneath the pendulum. People at that time already knew that Earth rotated on its axis, but Foucault's experiment was nice confirmation.

### Earth’s Rotation

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.

An observer in space will see that Earth requires 23 hours, 59 minutes, and 4 seconds to make one complete rotation on its axis. But because Earth moves around the Sun at the same time that it is rotating, the planet must turn just a little bit more to reach the same place relative to the Sun. Hence the length of a day on Earth is actually 24 hours.

At the Equator, the Earth rotates at a speed of about 1,700 km per hour, but at the poles the movement speed is nearly nothing.

### Day-Night Cycle

Earth rotates once on its axis about every 24 hours. To an observer looking down at the North Pole, the rotation appears counterclockwise. From nearly all points on Earth, the Sun appears to move across the sky from east to west each day. Of course, the Sun is not moving from east to west at all; Earth is rotating. The Moon and stars also seem to rise in the east and set in the west.

Earth’s rotation means that there is a cycle of daylight and darkness approximately every 24 hours, the length of a day. Different places experience sunset and sunrise at different times and the amount of daylight and darkness also differs by location.

Shadows are areas where an object obstructs a light source so that darkness takes on the form of the object. On Earth, a shadow can be cast by the Sun, Moon, or (rarely) Mercury or Venus.

### Summary

• Foucalt's pendulum shows that Earth moves beneath a swinging pendulum.
• Earth rotates on its axis every 24 hours.
• Earth rotates so that the Sun, Moon, and stars appear to travel from east to west each day.

### Review

1. How does Foucalt's pendulum show that Earth rotates on its axis?
2. Why do the Sun, Moon, and stars appear to rise in the east and set in the west each day?
3. Why does a point on the Equator travel at a speed of 1,700 km per hour and a point at the poles not travel at all?

### Explore More

Use these resources to answer the questions that follow.

1. What is the tilt of Earth's axis of rotation?
2. How often does Earth rotate on its axis?
3. Under what circumstances is it summer in the Northern Hemisphere?
4. As Earth revolves around the Sun over 6 months, how much does the tilt of the axis of rotation change?
5. Under what circumstances is it summer in the Southern Hemisphere? At this time, what is the season in the Northern Hemisphere?
6. What is the wobble effect? How long is one cycle of this effect?
7. What will happen to the seasons in 13,000 years and why?

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### Vocabulary Language: English Spanish

TermDefinition
axis An imaginary line that runs from the North Pole to South Pole, and it includes the center of Earth.
rotation The motion of the Earth spinning on its axis.