THE MOON
The Moon is Earth’s sole natural satellite and nearest large celestial body. A moon is an object that orbits a
planet or something else that is not a star.
Distinctive Features
The Moon is a spherical rocky body, probably with a small metallic core, revolving around Earth in a slightly
eccentric orbit at a mean distance of about 384,000 km (238,600 miles).
Its equatorial radius is 1,738 km (1,080 miles), and its shape is slightly flattened in a such a way that it bulges a
little in the direction of Earth.
Its mass distribution is not uniform—the centre of mass is displaced about 2 km (1.2 miles) toward Earth
relative to the centre of the lunar sphere, and it also has surface mass concentrations, called mascons for short,
that cause the Moon’s gravitational field to increase over local areas.
The Moon has no global magnetic field like that of Earth, but some of its surface rocks have remanent
magnetism, which indicates one or more periods of magnetic activity in the past.
The Moon presently has very slight seismic activity and little heat flow from the interior, indications that most
internal activity ceased long ago.
Size and Distance
With a radius of about 1,080 miles (1,740 kilometers), the Moon is less than a third of the width of Earth. If
Earth were the size of a nickel, the Moon would be about as big as a coffee bean.
The Moon is an average of 238,855 miles (384,400 kilometers) away. That means 30 Earth-sized planets could
fit in between Earth and the Moon.
The Moon is slowly moving away from Earth, getting about an inch farther away each year.
Formation
The leading theory of the Moon's origin is that a Mars-sized body collided with Earth about 4.5 billion years
ago.
The resulting debris from both Earth and the impactor accumulated to form our natural satellite 239,000 miles
(384,000 kilometers) away.
The newly formed Moon was in a molten state, but within about 100 million years, most of the global "magma
ocean" had crystallized, with less-dense rocks floating upward and eventually forming the lunar crust.
Phases
New moon occurs when the Moon is between Earth and the Sun, and thus the side of the Moon that is in shadow
faces Earth.
Full moon occurs when the Moon is on the opposite side of Earth from the Sun, and thus the side of the Moon
that is illuminated faces Earth.
First and last quarter, in which half the Moon appears illuminated, occur when the Moon is at a right angle
with respect to the Sun when viewed from Earth.
The waxing and waning crescent phases occur when about one-fourth of the Moon is illuminated.
Waxing and waning gibbous phases occur when about three-fourths of the Moon is illuminated.
Orbit and Rotation
The Moon is rotating at the same rate that it revolves around Earth (called synchronous rotation), so the same
hemisphere faces Earth all the time.
The Moon makes a complete orbit around Earth in 27 Earth days and rotates or spins at that same rate, or in that
same amount of time.
Eclipses
From the perspective of a person on Earth, a solar eclipse happens when the Moon comes between the Sun and
Earth, and a lunar eclipse happens when the Moon moves into the shadow of Earth cast by the Sun.
Tides
High tides and low tides are caused by the moon.
The moon's gravitational pull generates something called the tidal force.
The tidal force causes Earth—and its water—to bulge out on the side closest to the moon and the side farthest
from the moon. These bulges of water are high tides.
