92 Chapter 2 From the Solar System to the Milky Way
Size Approximately 0.27 times the size of Earth (equatorial radius of 1,080 miles,
or 1,738 km)
Mass Approximately 0.012 times the mass of Earth
Surface gravity Approximately 0.17 times the gravity of Earth
Orbital period Approximately 27.3 days
Rotation period Approximately 27.3 days
How Was the Moon Formed?
Earth and its satellite, the Moon, seem to have a parent-child relationship—the Moon is
composed of materials that are nearly identical to those of Earth’s mantle. As a result, one
hypothesis presented by astronomers in the past was that the centrifugal force of Earth’s
rotation caused a chunk of Earth to break off while it was being formed. This chunk formed
the Moon, and the hole that remained is the Pacific Ocean.
This hypothesis has a certain degree of persuasive power, but it also has several prob-
lems. The rotation speed of Earth today is not fast enough for such a strong centrifugal force
to have been produced—it is impossible for the amount of material that formed the Moon to
have been ejected suddenly while Earth rotated slowly enough to maintain its atmosphere
(air). There is also no evidence that the rotation of Earth slowed down at some point after
the chunk would have broken off.
If we delve into this a little deeper, we find that its greatest flaw is that the Moon is too
large. The diameter of the Moon (we will always mean the equatorial diameter throughout
the following discussion) is approximately 3,474 km (2,159 miles), which is approximately
1/4 the diameter of Earth. This is disproportionately large for a satellite, and as a result,
some astronomers claim that the Moon should be considered a double planet with Earth,
rather than a satellite of it.