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Earth, Moon and the Sun; An Introduction
Earth’s Size and Shape
Like most people, you are aware that Earth is round like a ball. But can you prove that
this is true? If you jump up, you know that you’ll come back down, but why is this so? What is the
force that brings you down? You may have used a compass to tell directions, but do you know how
a compass works? You will learn the answers to these questions and also about many physical
characteristics of Earth in this section.
Ancient Measurements
Earth’s shape is similar to a sphere. A sphere is a round, three-dimensional object, the
surface of which is the same distance from the center in all directions. Even ancient astronomers
knew that Earth is spherical in shape. We have pictures of Earth from space that show us that it is
spherical, but how could astronomers from long ago have learned this? They used evidence from
observations. Aristotle was one of these early astronomers. He made three different observations
that indicated that Earth’s shape is spherical. First, as shown in Figure 1, no matter where you are
on Earth, objects fall straight down to the surface, as if they are falling toward the center of a
sphere. Second, Earth’s shadow on the Moon during a lunar eclipse is always curved. If Earth
weren’t spherical, this might not always be the case. For example, a flat disk casts a straight-edged
shadow sometimes. Finally, people in different parts of the world see different stars above their
horizons. More specifically, the pole star Polaris is lower in the sky at some locations on Earth than
at others.
Everyday Evidence of
Earth’s Shape
What have you seen, other than pictures from space, that indicates Earth’s shape? Think
about walking toward someone over a hill. First, you see the top of the person’s head, and then
you can see more and more of that person. Similarly, if you sail toward a lighthouse, you first see
the top of the lighthouse and then see more and more of it as you move over Earth’s curved
surface. You can see other evidence, too. Just like ancient astronomers, you can see for yourself
that objects always fall straight down. Today, however, we know more about gravity. Gravity is the
attractive force between two objects that depends on the masses of the objects and the distance
between them. Astronomers think Earth formed by the accumulation of infalling objects toward a
central mass. Energy released in the impacts kept the growing Earth molten. Gravity caused it to
form into the most stable shape, a sphere. In this shape, the pull of gravity toward the center of the
planet is the same in all directions. If a planet is massive enough, the pull of gravity could be so
strong that even tall mountains would collapse under their own weight.
Earth as a Planet
Earth is a planet, just as Venus, Mars, and Jupiter are planets. However, Earth is the only
planet whose characteristics make it possible for life as we know it to survive. Earth resembles
Venus more than any other planet. Earth and Venus are nearly the same size, and both have
atmospheres that contain carbon dioxide, although in greatly different amounts. Earth’s oceans,
shown in Figure 3, absorbed much of the carbon dioxide in Earth’s early atmosphere. Also,
Venus’s atmosphere is much denser than Earth’s, with pressures as high as those encountered by
submarines in Earth’s oceans at depth of 900 m. Another difference is the surface temperatures.
On Earth, you can walk outside and feel how cold or warm it is. However, temperature on Venus
is over 450°C. This high temperature is caused by the large amount of carbon dioxide in Venus’s
atmosphere, which traps heat energy and prevents it from escaping. Think about what life on Earth
Earth Orbits the Sun
Earth orbits the Sun at an average distance of 149,600,000 km. Its orbit, like those of all
the planets, moons, asteroids, and many comets, is shaped like an ellipse. An ellipse is an
elongated, closed curve with two foci. The Sun is not located at the center of the ellipse, but at one
of its two foci. This means the distance from Earth to the Sun varies during the year. Earth is
closest to the Sun—about 147 million km away— around January 3 and is farthest from the Sun—
about 152 million km away— around July 4 of each year
Analysis Questions:
1.) Identify two pieces of evidence that prove Earth’s spherical shape.
2.) Define the term gravity.
3.) What shape does the Earth orbit the moon?
4.) Describe how Venus and Earth are similar. How are they different?