High School Earth Science/Planet Earth

The Earth and Moon revolve around each other as they orbit the Sun. As planet Earth rotates and revolves, we experience cycles of day and night as well as seasons. Earth has a very large moon for an inner planet. We are the only inner planet that does, so how did the Moon form and what is the surface of the Moon like? We revolve around an average, ordinary star, the Sun. It does not look like any of the other stars we see in the sky. What can we discover about our amazing Sun?

Lesson Objectives

Recognize that Earth is a sphere, and describe the evidence for this conclusion.
Describe what gravity is, and how it affects Earth in the solar system.
Explain what causes Earth's magnetism, and the effects that magnetism has on the Earth.

Earth's Shape, Size, and Mass

Every day you walk across some part of Earth's surface, whether that surface is your yard or the sidewalks by your school. For most of us, a walk outside means walking on fairly flat ground. We don't usually stop and realize that the Earth is a sphere, an object similar in shape to a ball. How do we know that Earth is a sphere? How could you convince someone that even though the surfaces we walk on look flat, the Earth as a whole is round?

One of the most convincing pieces of evidence for a spherical Earth are the pictures we have of it from Space. Astronauts aboard the Apollo 17 shuttle took one of the most famous photographs in history, called "The Blue Marble" (Figure 24.1). This outstanding image shows Earth as it looks from about 29,000 kilometers (18,000 miles) away in Space. The picture shows us that Earth is spherical and looks like a giant blue and white ball. Hundreds of years before humans ever made it into space, we knew the Earth was round. What ways have you been able to see this for yourself?

The Sun and the other planets of our Solar System are also spheres. The Sun is found in the center of the solar system, and the planets travel around the Sun in regular paths called orbits. Earth is the third planet from the Sun, and its mass is approximately 6.0×10²⁴ kilograms. In contrast, the volume of planet Jupiter is about 1,000 times greater than Earth's volume, and the Sun's volume is about 1,000 times greater than Jupiter's (Figure 24.2).

While the outer planets in the Solar System are giant balls of swirling gas with very low densities, Earth is an inner planet. The inner planets are relatively small, denser, rockier planets than the outer planets. Three-fourths of Earth's rocky surface is covered with water. As far as we know, Earth is also the only planet that carries liquid water, another important requirement for life. The entire planet is also surrounded by a thin layer of air called the atmosphere. Earth's atmosphere is unique in the solar system in that it contains just the right amount of oxygen to support animal life. Therefore, Earth is the only planet in the solar system on which life is found. The Solar System chapter will discuss the features of other planets in more detail.

When describing Earth, it's useful to name and define the many components of our planet. Since Earth is a sphere, the layers that make it up are also referred to as spheres (Figure 24.3).

**Figure 24.3**: Earth has a hydrosphere, lithosphere, atmosphere, and biosphere.

They are:

Atmosphere—the thin layer of air that surrounds the Earth.
Hydrosphere—the part of Earth's surface that consists of water.
Biosphere—the part of the Earth that supports life. The biosphere includes all the areas where life is found.
Lithosphere—the solid part of the Earth. The lithosphere consists of mountains, valleys, continents and all of the land beneath the oceans. Only one-fourth of Earth's surface is land, but solid rock makes up more than 99% of Earth's total mass.

Earth's layers all come into contact with each other and interact. Therefore, Earth’s surface is constantly undergoing change.

Earth's Gravity

We know that the Earth orbits the Sun in a regular path (Figure 24.4). The Earth's Moon also orbits the Earth in a regular path. Gravity is the force of attraction between all objects. Gravity keeps the Earth and Moon in their orbits. Isaac Newton was one of the first scientists to explore the idea of gravity. He understood that the Moon can only circle the Earth because some force is pulling the Moon toward Earth's center. Otherwise, the Moon would continue moving in a straight line off into space. Newton also came to understand that the same force that keeps the Moon in its orbit is the same force that causes objects on Earth to fall to the ground.

Newton defined the Universal Law of Gravitation, which states that a force of attraction, called gravity, exists between all objects in the universe (Figure 24.5). The strength of the gravitational force depends on how much mass the objects have and how far apart they are from each other. The greater the objects' mass, the greater the force of attraction; in addition, the greater the distance between the objects, the smaller the force of attraction.

**Figure 24.5**: The force of gravity exists between all objects in the universe; the strength of the force depends on the mass of the objects and the distance between them.

Earth's Magnetism

Earth has a magnetic field (Figure 24.6). It may be helpful to imagine that the Earth has a gigantic bar magnet inside of it. A bar magnet has a north and south pole and a magnetic field that extends around it. Earth's magnetic field also has a north and south pole and a magnetic field that surrounds it. Scientists believe Earth's magnetic field arises from the movements of molten metals deep inside Earth’s outer liquid iron core. Iron and nickel flow within the Earth's core, and their movement generates Earth's magnetic field. Earth's magnetic field extends several thousand kilometers into space.

Earth's magnetic field serves an important role. It shields the planet from harmful types of radiation from the Sun (Figure 24.7). If you have a large bar magnet, you can tie a string to it, hang it from the string, and then watch as it aligns itself in a north-south direction, in response to Earth's magnetic field. This concept allows a compass to work, so that people can navigate by finding magnetic north (Figure 24.8).

Lesson Summary

The Earth and other planets in our solar system are rotating spheres, that also revolve around the Sun in fixed paths called orbits.
The inner four planets are small, dense rocky planets like Earth. The next four planets are large, gaseous planets like Jupiter.
The balance between gravity and our motion around the Sun, keep the planets in orbit at fixed distances from the Sun.
Earth has a magnetic field, created by motion within Earth’s outer, liquid iron core that shields us from harmful radiation.

Review Questions

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. With what you have learned about the shape of the Earth, describe why this happens.
What are two reasons that Earth is able to support life?
The planet Jupiter is gaseous and lacks a solid surface. How does this compare to Earth?
Give one example of how Earth's lithosphere and hydrosphere can interact, and can exchange material.
How do mass and distance influence the force of gravity?
Why are we able to use magnets to determine north-south directions on Earth?

Vocabulary

atmosphere: The thin layer of air that surrounds the Earth.
biosphere: All parts of the Earth that supports life.
gravity: An attractive force that exists between all objects in the universe; gravity is responsible for planets orbiting the Sun, and for moons orbiting those planets.
hydrosphere: The layer of Earth consisting of water.
lithosphere: The solid layer of Earth, made of rock.
magnetic field: A region of space surrounding an object, in which an attractive magnetic force can be detected.
orbit: The path of an object, such as a planet or moon, around a larger object such as the Sun.
sphere: An object similar in shape to a ball.

Points to Consider

What would other planets need to have if they were able to support life?
Would life on Earth be impacted if Earth lost its magnetic field?
Could a large gas planet like Jupiter or Saturn support life?

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