Orbital Motion
Jean Brainard, Ph.D.
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Printed: January 14, 2013
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C ONCEPT
Concept 1. Orbital Motion
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Orbital Motion
• Define orbital motion and explain why it occurs.
• Describe the orbits of Earth around the sun and the moon around Earth.
As you can see in this NASA photo, Earth is tiny compared with the massive sun. The sun’s gravity is relatively
strong because the force of gravity between two objects is directly proportional to their masses. Gravity between the
sun and Earth pulls Earth toward the sun, but Earth never falls into the sun. Instead, it constantly revolves around
the sun, making one complete revolution every 365 days.
Q: Why doesn’t the sun’s gravity pull Earth down to the surface of the sun?
A: Earth has enough forward velocity to partly counter the force of the sun’s gravity.
What Is Orbital Motion?
Earth and many other bodies—including asteroids, comets, and the other planets—move around the sun in curved
paths called orbits. Generally, the orbits are elliptical, or oval, in shape. You can see the shape of Earth’s orbit in the
Figure 1.1. Because of the sun’s relatively strong gravity, Earth and the other bodies constantly fall toward the sun,
but they stay far enough away from the sun because of their forward velocity to fall around the sun instead of into
it. As a result, they keep orbiting the sun and never crash to its surface. The motion of Earth and the other bodies
around the sun is called orbital motion. Orbital motion occurs whenever an object is moving forward and at the
same time is pulled by gravity toward another object. You can explore orbital motion and gravity in depth with the
animation at this URL:
http://phet.colorado.edu/en/simulation/gravity-and-orbits
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FIGURE 1.1
Orbital Motion of the Moon
Just as Earth orbits the sun, the moon also orbits Earth. The moon is affected by Earth’s gravity more than it is by
the gravity of the sun because the moon is much closer to Earth. The gravity between Earth and the moon pulls
the moon toward Earth. At the same time, the moon has forward velocity that partly counters the force of Earth’s
gravity. So the moon orbits Earth instead of falling down to the surface of the planet.
The Figure 1.2 shows the forces involved in the moon’s orbital motion around Earth. In the diagram, v represents
the forward velocity of the moon, and a represents the acceleration due to gravity between Earth and the moon. The
line encircling Earth shows the moon’s actual orbit, which results from the combination of v and a. You can see an
animated version of this diagram at:
http://en.wikipedia.org/wiki/File:Orbital_motion.gif
FIGURE 1.2
Summary
• Orbital motion occurs whenever an object is moving forward and at the same time is pulled by gravity toward
another object.
• The forward velocity of the object combines with acceleration due to gravity toward the other object. The
result is a circular or oval path called an orbit, in which one object keeps moving around the other.
• Because of the relatively great gravity of the sun, Earth orbits the sun. The moon orbits Earth rather than the
sun because it is much closer to Earth.
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Concept 1. Orbital Motion
Vocabulary
• orbit: Path of one object around another, such as the path of the moon around Earth or of Earth around the
sun.
• orbital motion
Practice
At the following URL, experiment with the gravity-and-orbit simulator. Set different initial velocities for the cannon
ball and observe whether the cannon ball crashes, escapes from Earth, or is set into orbit. Then answer the questions
below.
http://home.snc.edu/takamasatakahashi/login/AstroSp10/Animations/Active_Figures/10/
1. Which range of initial velocities for the cannon ball puts it into orbit?
2. What happens to the cannon ball if its initial velocity is less than this range?
3. What happens to the cannon ball if its initial velocity is greater than this range?
Review
1. Define orbit and orbital motion, and explain why orbital motion occurs.
2. In addition to the moon, artificial satellites also orbit Earth. What factors do you think must be taken into
account to ensure that a satellite keeps orbiting Earth rather than falling back to Earth’s surface?
3. Earth is closer to nearby planets including Venus and Mars than it is to the sun. Why don’t these other planets
pull Earth toward them and cause it to veer off its orbit around the sun.
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