DK Eyewitness - Astronomy

Phases of the moon
The phases of the Moon are caused
by the constantly changing series
of angles formed by the Sun and
the Moon as the Moon revolves
around Earth. When the Moon
and the Sun are on opposite sides
of Earth, the Sun shines directly
on the Moon’s surface, resulting in
a full moon. When the area of the
lit surface increases, the Moon is
said to be waxing; as it decreases, it
is said to be waning.
Waxing crescent
moon at 4 days
Full moon
at 14 days
Waning 19-day moon Moon at 21 days Moon at 24 days
Gearing
Meridian
circle
The surface of the moon
The features on the far side of the
Moon were a mystery until the late
1950s. This view of the terrain was
taken by the Apollo 11 lunar module in
1969. One of the primary purposes for
exploring the Moon was to bring back
samples of rock to study them and to
discover their origins. The Moon is
made up of similar but not identical
material to that found on Earth. There
is less iron on the Moon, but the major
minerals are silicates as they are on
Earth (p.43)—though they are slightly
different in composition. This discovery
supports the most popular theory
of the Moon’s origin. A small
planet, about the size of Mars, is
thought to have crashed into Earth
about 4.5 billion years ago. The
collision tore debris away from
both bodies and the Moon
formed from this material.
Investigating moon rock
Rocks from the Moon have been
investigated by geologists in the
same way as they study Earth rocks.
The rocks are ground down to thin
slices and then looked at under a
powerful microscope. The minerals,
chiefly feldspar and olivine, which are
abundant on Earth, are unweathered.
This is exceptional for geologists
because there are no Earth rocks that
are totally unweathered.
Cross-polarized
light in the
microscope gives
colors
Watery
clearness
shows no
weathering
Facts about the moon
Outer rocky crust
Hour circle
Earth
A moon globe
Selenography is the study of
the surface features of the Moon.
This selenograph, created by the
artist John Russell in 1797, is a Moon
globe. Only a little more than half of
the globe is filled with images because
at that time the features on the far side
of the Moon were unknown. Not until the
Russians received the earliest transmissions
from the Luna 3 probe in October 1959 was it possible
to see images of what was on the Moon’s far side.
Partially
molten region
Core (perhaps iron)
Dark rock mantle
• Interval between two new moons 29 days 12 hr 44 min
• Temperature at surface –245°F to 220°F (−155°C
to 105°C)
• Rotational period 27.3 Earth days
• Mean distance from Earth 239,000 miles/384,000 km
• Volume (Earth = 1) 0.02
• Mass (Earth = 1) 0.012
• Density (water = 1) 3.34
• Equatorial diameter 2,160 miles/3,476 km
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