Download Page 190 7.3 Orbit and Motions of the Moon By watching the

Page 190
7.3 Orbit and Motions of the Moon
By watching the Moon for a few successive nights, you can see it move against the background stars as it
follows its orbit around the Earth. If you carefully measure the Moon's angular size, you will find that it
varies by about 14% during its orbit. This is a consequence of the Moon's elliptical orbit and its changing
distance (the relationship between angular size and distance is discussed in chapter 2). The Moon's orbit
has an average distance from Earth of 380,000 kilometers (about 250,000 miles), but it varies from about
360,000 to 405,000 kilometers during its orbital period of 27.3 days.* As a result, a full moon may be
significantly larger some months than others (fig. 7.11A).
Figure 7.11
(A) The Moon's distance from the Earth varies during its orbit, which can be seen in the changing angular
size of the Moon. (B) The precise distance from the Earth to the Moon can be determined by measuring
the time radar or laser signals take to reach the Moon and bounce back to Earth.
The Moon's distance can today be measured very precisely, as sketched in figure 7.11B, by bouncing
either a radar pulse or a laser beam off special reflectors that were placed on the Moon by the Apollo
astronauts. (Until recently, these reflectors were one of the most direct demonstrations that astronauts had
actually landed on the Moon, contrary to rather silly claims that they hadn't—see Extending Our Reach:
“The Moon Landing Hoax.”) Measuring the time it takes for a signal to travel to the Moon and back,
multiplied by the speed of light, gives the round­trip distance. Halving that value provides distances to an
accuracy of centimeters.
EXTENDING our reach
THE MOON LANDING HOAX
For years, a number of sensationalists claimed that the lunar landings were a hoax. Most of their
arguments were silly, as can be determined by anyone who spends a few hours examining the
voluminous image and video data freely available from NASA. These claims were profitable vehicles for
their authors, suggesting the real hoax.
Because the landers are too small to be seen by Earth­bound telescopes, it was difficult to refute these
claims directly. However, the Lunar Reconnaissance Orbiter has now imaged the Moon's surface with
unprecedented detail, showing the landing sites and even the tracks left by the astronauts (fig. 7.12).
Figure 7.12
An example of an LRO high­resolution image of one of the Apollo landing sites. In addition to the
lander, even the tracks left behind by astronauts remain visible.
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The Moon's Rotation
ANIMATION
The rotation of the Moon
As it orbits, the Moon keeps the same side facing the Earth, as you can see by watching it through a cycle
of its phases. You might think from this that the Moon does not rotate. Figure 7.13A shows, however,
that the Moon must slowly rotate to keep the same features facing the Earth. Thus, the Moon does turn on
its axis but with a rotation period exactly equal to its orbital period, a condition known as synchronous
rotation. The Earth's gravity causes this locking of the Moon's spin to its orbital motion, as we will
discuss in section 7.5. This is a common characteristic of satellites in the Solar System, which almost all
keep the same face toward their planet.
The Rotation of the Moon
Figure 7.13
(A) The Moon rotates once each time it orbits the Earth, as can be seen from the changing position of the
exaggerated lunar mountain. Notice that when the Moon is new (left side in the diagram), the lunar peak
faces to the right, while when it is new it faces to the left. Thus, from the Earth, we always see the same
side of the Moon even though it turns on its axis. (B) The Moon's orbit is nearly in the plane of the
Earth's orbit (the ecliptic), but is quite tilted with respect to the Earth's equator, which is quite unusual for
large satellites of planets. (Separation of Earth and Moon not to scale.)
Oddities of the Moon's Orbit
Unlike almost all other large moons, our Moon has an orbit with a large tilt with respect to its planet's
equator. In discussing eclipses in chapter 1, we noted that the Moon's orbit is tilted by a little more than
5° with respect to the Earth's orbit around the Sun. As a result, its orbit is tilted between 18° and 29° with
respect to the Earth's equator, as shown in Figure 7.13B.* This is unlike all of the major moons of Jupiter,
Saturn, and Uranus, which lie nearly exactly in their planet's equatorial plane. The only large satellite that
has a larger discrepancy than the Moon is Neptune's satellite Triton, which may have been an outer Solar
System body captured by Neptune, as we will discuss in chapter 10.
Our Moon is also exceptional in its size relative to its planet. Even the largest of the moons of Jupiter and
Saturn have masses less than 1/1000th that of their planet. But our Moon's mass is 1/81 that of the Earth.
These oddities suggest that our Moon formed differently from the moons of other planets, as we discuss
next.