The Sun-Earth-Moon System
Earth’s Moon
Seeing the Moon
Why does the Moon shine? Why does its shape seem to
change? The Moon does not give off light. Unlike the Sun,
the Moon is a solid object. You see the Moon because it
reflects light from the Sun. Data about the Moon’s mass,
size, and distance from Earth are shown in the table below.
Moon Data
Mass
Diameter
Average
distance
from Earth
Time for
one
rotation
Time for
one
revolution
1.2% of
Earth’s
mass
27% of
Earth’s
diameter
384,000 km
27.3 days
27.3 days
The most widely accepted idea about how the Moon
formed is the giant impact hypothesis. The giant impact
hypothesis states that shortly after Earth formed, an object
about the size of the planet Mars crashed into Earth. The
impact caused a ring of vaporized rock to form around
Earth. Eventually, the material in the ring cooled and
clumped together and formed the Moon.
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The Moon’s Formation
The Moon’s Surface
Early in the Moon’s history, various features formed on
its surface. The main features on the Moon are craters, maria
(MAR ee uh), and highlands.
Craters The Moon’s craters formed when objects from space,
such as large rocks, crashed into the Moon. Light-colored
streaks called rays run outward from some craters.
Most of the Moon’s craters formed about 3.5 billion years
ago. This was long before dinosaurs lived on Earth. Earth
also was hit by many large objects during this time. Wind,
water, and plate tectonics erased these craters on Earth.
The Moon has no atmosphere, no water, and no plate
tectonics. Without these forces, Moon’s craters, formed
billions of years ago, have not changed much.
Maria The large, dark, flat areas on the Moon are called maria. The
maria (MAR ee uh) formed long after most impacts on the
Moon’s surface had ended. Maria formed when lava flowed
through the Moon’s crust and hardened. The hardened lava
covered large portions of the Moon’s surface, including
many of its craters and other features.
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Highlands The light-colored areas on the Moon are the
highlands. Highlands were not covered by the lava that
formed the maria because they were too high for the lava to
reach. Highlands are older than the maria and are covered
with craters.
The Moon’s Motion
As Earth revolves around the Sun, the Moon revolves
around Earth. The pull of Earth’s gravity causes the Moon to
move in an orbit around Earth. Recall from Lesson 1 that if
the pull of the Sun’s gravity ended, Earth would fly in a
straight line into space. The same would be true for the
Moon if the pull of Earth’s gravity ended. The Moon makes
one revolution around Earth every 27.3 days.
Like Earth, the Moon also rotates as it revolves. One
complete rotation of the Moon also takes 27.3 days. Notice
that this is the same amount of time it takes the Moon to
make one complete revolution around Earth.
The same side of the Moon always faces Earth because
the Moon takes the same amount of time to orbit Earth and
make one rotation. This side of the Moon that faces Earth is
called the near side. The side of the Moon that cannot be
seen from Earth is the far side of the Moon.
Phases of the Moon
The Sun is always shining on half the Moon, just as it is
always shining on half of Earth. As the Moon moves around
Earth, usually only one part of the Moon’s near side is lit.
The lit part of the Moon or a planet that can be seen from Earth is called
a phase.
The motion of the Moon around Earth causes the phase
of the Moon to change. The phases follow a regular pattern
that is called the lunar cycle. One lunar cycle takes 29.5 days,
or slightly more than four weeks, to complete.
Waxing Phases
The waxing phases occur during the first half of the lunar
cycle. During the waxing phases, more of the Moon’s near side is lit
each night.
Week 1—First Quarter The lunar cycle begins. A sliver of
light appears on the Moon’s western edge. Each night, the lit
part grows larger. By the end of the first week, the Moon
reaches its first quarter phase. The entire western half of the
Moon is now lit.
Week 2—Full Moon During the second week in the lunar
cycle, more and more of the near side of the Moon becomes
lit. By the end of the second week, the Moon’s near side is
completely lit. It is at its full moon phase.
After the Moon waxes, it is said to wane. During the waning
phases, less of the Moon’s near side is lit each night. As seen from
Earth, the lit part is now on the Moon’s eastern side.
Week 3—Third Quarter During the third week in the lunar
cycle, the lit part of the Moon becomes smaller. By the end
of the third week, only the eastern half of the Moon is lit.
This is the third quarter phase.
Week 4—New Moon During the fourth week in the lunar
cycle, less and less of the near side of the Moon is lit. When
the Moon’s near side is completely dark, it has reached the
new moon phase. The entire lunar cycle is summarized in
the figure on the next page.
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Waning Phases
Full moon
The Moon looks
like this in the sky.
Moon’s
orbit
Full moon
You don’t see the
shadowed part.
Third quarter
First quarter
You see the
sunlit part.
Third
quarter
First
quarter
Earth
The new moon phase is
not visible from Earth.
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New moon
Sunlight
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