Lesson Objectives

• Recognize that Earth is a modified sphere (oblate spheroid), and describe the evidence for this conclusion.
• Explain what causes Earth’s magnetism and the effects that magnetism has on the Earth.
• Describe Earth’s rotation on its axis.
• Describe Earth’s revolution around the Sun.

Vocabulary

• axis
• ellipse
• hemisphere
• revolution
• rotation

Introduction

This book so far has been almost entirely about Earth. This chapter is concerned with Earth as a planetary body, a member of the Earth-Moon pair that orbit each other and the Sun.

Earth as a Planetary Body

Earth is an inner planet in the solar system and it is very much like the other inner planets, at least in its size, shape, and composition. But many features make Earth very different from the planets and any other planet that we know of so far.

Earth’s Shape

Earth is a sphere or, more correctly, an oblate spheroid, which is a sphere that is a bit squished down at the poles and bulges a bit at the equator. Or to be more technical, the minor axis (the diameter through the poles) is smaller than the major axis (the diameter through the equator). Half of the sphere is a hemisphere. North of the equator is the northern hemisphere and south of the equator is the southern hemisphere. Eastern and western hemispheres are also designated.

What evidence is there that Earth is spherical? What evidence was there before spaceships and satellites?

Try to design an experiment involving a ship and the ocean to show Earth is round. If you are standing on the shore and a ship is going out to sea, the ship gets smaller as it moves further away from you but the ship’s bottom also starts to disappear as the vessel goes around the arc of the planet (Figure below). There are many other ways that early scientists and mariners knew that Earth was not flat.

Even the ancient Greeks knew that Earth was round by observing the arc shape of the shadow on the Moon during a lunar eclipse. NASA has an animation of a lunar eclipse here: http://science.nasa.gov/media/medialibrary/2003/10/29/04nov_lunareclipse2_resources/reddy1_big.gif.

The Sun and the other planets of the solar system are also spherical. Larger satellites, those that have enough mass for their gravitational attraction to have made them round, are as well.

Earth’s Magnetism

Earth has a magnetic field (Figure below) that behaves as if the planet had a gigantic bar magnet inside of it. Earth’s magnetic field also has a north and south pole and a magnetic field that surrounds it. The magnetic field arises from the convection of molten iron and nickel metal in Earth’s outer liquid iron core.

Earth’s magnetic field extends several thousand kilometers into space. The magnetic field shields the planet from harmful radiation from the Sun (Figure below).

Earth’s Motions

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.

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 (Figure below). A ball swinging back and forth on a string is called a pendulum.

A pendulum set in motion will not change its motion, and so the direction of its swinging should not change. However, Foucault observed that his pendulum did seem to change direction. Since he knew that the pendulum could not change its motion, he concluded that the Earth, underneath the pendulum was moving. Figure below shows how this might look.

An observer in space will see that Earth requires 23 hours, 56 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.

A Turn of the Earth

In this video, MIT students demonstrate how a Foucault Pendulum is used to prove that the Earth is rotating. See the video at https://www.youtube.com/watch?v=_pECtfYa2Us.

Earth’s Revolution

For Earth to make one complete revolution around the Sun takes 365.24 days. This amount of time is the definition of one year. The gravitational pull of the Sun keeps Earth and the other planets in orbit around the star. Like the other planets, Earth’s orbital path is an ellipse (Figure below) so the planet is sometimes farther away from the Sun than at other times. The closest Earth gets to the Sun each year is at perihelion (147 million km) on about January 3rd and the furthest is at aphelion (152 million km) on July 4th. Earth’s elliptical orbit has nothing to do with Earth’s seasons.

During one revolution around the Sun, Earth travels at an average distance of about 150 million km. Earth revolves around the Sun at an average speed of about 27 km (17 mi) per second, but the speed is not constant. The planet moves slower when it is at aphelion and faster when it is at perihelion.

The reason the Earth (or any planet) has seasons is that Earth is tilted 23 1/2^o on its axis. During the Northern Hemisphere summer the North Pole points toward the Sun, and in the Northern Hemisphere winter the North Pole is tilted away from the Sun (Figure below).

Lesson Summary

• Earth rotates or spins on its axis approximately once each day and revolves around the Sun approximately once a year.
• Earth’s orbit around the Sun is elliptical; the planet is closer at perihelion and farther at aphelion.
• The tilt of Earth’s axis produces seasons.
• The Earth and other planets in our solar system are rotating spheres.
• Earth has a magnetic field created by the convection of molten liquid in the outer core.
• The magnetic field shields Earth from harmful solar radiation.

Review Questions

1. When you watch a tall ship sail over the horizon of the Earth, you see the bottom part of it disappear faster than the top part. Why does this happen?

2. Why are we able to use magnets to determine north-south directions on Earth?

3. Describe the difference between Earth’s rotation and its revolution.

4. What is the force that keeps the Earth and other planets in their orbital paths?

5. In its elliptical orbit around the Sun, the Earth is closest to the Sun in January. If Earth is closes to the Sun in January, why is January winter in the Northern Hemisphere?

6. Where on Earth would Foucault’s pendulum appear to not be moving? Where would it appear to be moving the most?

7. The planet Jupiter is about 778,570,000 kilometers from the Sun; Earth is about 150,000,000 kilometers from the Sun. Does Jupiter take more or less time to make one revolution around the sun? Explain your answer.

Points to Consider

• What type of experiment could you create to prove that the Earth is rotating on its axis?
• If you lived at the equator, would you experience any effects because of Earth’s tilted axis?
• If Earth suddenly increased in mass, what might happen to its orbit around the Sun?
• Would life on Earth be impacted if Earth lost its magnetic field?
• Why are the inner planets spherical?