SECTION 13.4 • Kepler’s Laws and the Motion of Planets
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Thus, higher values of eccentricity correspond to longer and thinner ellipses. The
range of values of the eccentricity for an ellipse is 0 * e * 1.
Eccentricities for planetary orbits vary widely in the solar system. The eccentricity
of the Earth’s orbit is 0.017, which makes it nearly circular. On the other hand, the ec-
centricity of Pluto’s orbit is 0.25, the highest of all the nine planets. Figure 13.6a shows
an ellipse with the eccentricity of that of Pluto’s orbit. Notice that even this highest-
eccentricity orbit is difficult to distinguish from a circle. This is why Kepler’s first law is
an admirable accomplishment. The eccentricity of the orbit of Comet Halley is 0.97,
describing an orbit whose major axis is much longer than its minor axis, as shown in
Figure 13.6b. As a result, Comet Halley spends much of its 76-year period far from the
Sun and invisible from the Earth. It is only visible to the naked eye during a small part
of its orbit when it is near the Sun.
Now imagine a planet in an elliptical orbit such as that shown in Figure 13.5, with
the Sun at focus F
2
. When the planet is at the far left in the diagram, the distance
between the planet and the Sun is a & c. This point is called the aphelion, where the
planet is the farthest away from the Sun that it can be in the orbit. (For an object in or-
bit around the Earth, this point is called the apogee). Conversely, when the planet is at
the right end of the ellipse, the point is called the perihelion (for an Earth orbit, the
perigee), and the distance between the planet and the Sun is a " c.
Kepler’s first law is a direct result of the inverse square nature of the gravitational
force. We have discussed circular and elliptical orbits. These are the allowed shapes of
orbits for objects that are bound to the gravitational force center. These objects include
planets, asteroids, and comets that move repeatedly around the Sun, as well as moons
orbiting a planet. There could also be unbound objects, such as a meteoroid from deep
space that might pass by the Sun once and then never return. The gravitational force
between the Sun and these objects also varies as the inverse square of the separation
distance, and the allowed paths for these objects include parabolas (e ! 1) and hyper-
bolas (e + 1).
Kepler’s Second Law
Kepler’s second law can be shown to be a consequence of angular momentum conser-
vation as follows. Consider a planet of mass M
P
moving about the Sun in an elliptical
orbit (Fig. 13.7a). Let us consider the planet as a system. We will model the Sun to be
Sun
Center
Sun
Center
(a)
(b)
Orbit
of Pluto
Orbit of
Comet Halley
Figure 13.6 (a) The shape of the orbit of Pluto,
which has the highest eccentricity (e ! 0.25) among
the planets in the solar system. The Sun is located at
the large yellow dot, which is a focus of the ellipse.
There is nothing physical located at the center (the
small dot) or the other focus (the blue dot). (b) The
shape of the orbit of Comet Halley.
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PITFALL PREVENTION
13.2 Where is the Sun?
The Sun is located at one focus
of the elliptical orbit of a planet.
It is not located at the center of
the ellipse.