The Moon Fast Facts about the Earths Moon in Orbit

The Moon's radius is 1,079.6 miles (1,737.4 km), about a quarter the size of the Earth's. The Moon would fit between New York City and Denver, Colorado, or between the east and west coasts of Australia. How did the Earth come to have such a huge moon, relative to its size? With a couple of exceptions, the other planets all have satellites much smaller than themselves, compared to the Earth and Moon (satellite means anything orbiting a planet; in this case the topic is natural satellites, also called moons).The table on page 118 lists the nine planets, the name of their largest natural satellite, and the ratio of the radius of the largest satellite to the radius of the planet. Only Charon is larger in relation to Pluto than the Moon is to Earth, but Pluto and Charon are better described as a two-body system rather than a planet-moon system, since they are each like the many Kuiper belt bodies that orbit near them.

Barring Pluto and Charon, the Moon is by far the largest natural satellite in comparison to its planet. The Moon's radius and other important physical parameters are listed in the table on page 121. The Moon's large size raises questions about how it was formed: Could the Moon have formed from the same material as the Earth, at the same time the Earth formed? Computer models show that the Moon could never have become so large, or to have its particular characteristics of spin, if the Earth and Moon had formed from the same material at the same time. The Moon is now thought to have formed from the debris left when a giant impactor, perhaps the size of Mars, struck the Earth

Many solar system objects have simple symbols; this is the symbol for the Moon.

PLANETS AND THEIR MOONS

Planetary radius

Satellite radius

Ratio of radii

Planet

(miles [km])

Largest satellite

(miles [km])

(moon/planet)

Mercury

(no satellites)

Venus

(no satellites)

Earth

3,986 (6,377)

Moon

1,079 (1,736)

0.27

Mars

2,110 (3,395)

Phobos

8.8 (14)

0.0004

Jupiter

44,423 (71,492)

Ganymede

1,649 (2,638)

0.04

Saturn

37,449 (60,268)

Titan

1,609 (2,575)

0.04

Uranus

15,882 (25,959)

Titania

494 (790)

0.03

Neptune

15,379 (24,750)

Triton

846 (1,353)

0.06

Pluto

747 (1,195)

Charon

371 (593)

0.50

when it was largely formed. The giant impactor theory is discussed in chapter 8.

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