32 PART 1^ |^ EXPLORING THE SKY
the moon’s near side, but you never see the far side of the
moon.
Th e changing shape of the moon as it passes through its
cycle of phases is produced by sunlight illuminating diff erent
parts of the side of the moon you can see.
Notice the diff erence between the orbital period of the
moon around Earth (sidereal period ) and the length of the
lunar phase cycle (synodic period ). Th at diff erence is a good
illustration of how your view from Earth is produced by the
combined motions of Earth and other heavenly bodies, such
as the sun and moon.You can fi gure out where the moon will be in the sky by making
a moon-phase dial from the middle diagram on page 34. Cover
the lower half of the moon’s orbit with a sheet of paper and align
the edge of the paper to pass through the word “Full” at the left
and the word “New” at the right. Push a pin through the edge of
the paper at Earth’s North Pole to make a pivot and, under the
word “Full,” write on the paper “Eastern Horizon.” Under the
word “New,” write “Western Horizon.” Th e paper now repre-
sents the horizon you see when you stand facing south. You can
set your moon-phase dial for a given time by rotating the dia-
gram behind the horizon-paper. For example, set the dial to
sunset by turning the diagram until the human fi gure labeled
“sunset” is standing at the top of the Earth globe; the dial shows,
for example, that the full moon at sunset would be at the eastern
horizon.
Th e phases of the moon are dramatic, and they have
attracted a number of peculiar ideas. You have probably heard
a number of Common Misconceptions about the
moon. Sometimes people are surprised to see the moon in the
daytime sky, and they think something has gone wrong! No,
the gibbous moon is often visible in the daytime, although
quarter moons and especially crescent moons can also be in the
daytime sky but are harder to see when the sun is above the
horizon. You may hear people mention “the dark side of the
moon,” but you will be able to assure them that this is a mis-
conception; there is no permanently dark side. Any location
on the moon is sunlit for two weeks and is in darkness for two
weeks as the moon rotates in sunlight. Finally, you have prob-
ably heard one of the strangest misconceptions about the
moon: that people tend to act up at full moon. Actual statisti-
cal studies of records from schools, prisons, hospitals, and so
on, show that it isn’t true. Th ere are always a few people who
misbehave; the moon has nothing to do with it.
For billions of years, the man in the moon has looked down
on Earth. Ancient civilizations saw the same cycle of phases that
you see (■ Figure 3-1), and even the dinosaurs may have noticed
the changing phases of the moon. Occasionally, however, the
moon displays more complicated moods when it turns copper-
red in a lunar eclipse.2
3
Even a man who is pure in heart and says
his prayers by nightMay become a wolf when the wolfbane
blooms and the moon shines full and bright.— PROVERB FROM OLD WOLFMAN MOVIESD
id anyone ever warn you, “Don’t stare at the moon—
you’ll go crazy”? For centuries, the superstitious have
associated the moon with insanity. Th e word lunatic
comes from a time when even doctors thought that the insane
were “moonstruck.” Of course, the moon does not cause mad-
ness, but it is so bright and moves so rapidly along the ecliptic
that people expect it to aff ect them. It is, in fact, one of the most
beautiful and dramatic objects in the sky.
The Changeable
MoonStarting this evening, begin looking for the moon in the sky.
As you watch for the moon on successive evenings, you will see
it following its orbit around Earth and cycling through its phases
as it has done for billions of years.
The Motion of the Moon
Just as the planets revolve counterclockwise around the sun as
seen from the direction of the celestial north pole, the moon
revolves counterclockwise around Earth. Because the moon’s
orbit is tipped a little over 5 degrees from the plane of Earth’s
orbit, the moon’s path takes it slightly north and then slightly
south, but it is always somewhere near the ecliptic.
Th e moon moves rapidly against the background of the
constellations. If you watch the moon for just an hour, you can
see it move eastward by slightly more than its angular diameter.
In the previous chapter, you learned that the moon is about 0.5°
in angular diameter, and it moves eastward a bit more than 0.5°
per hour. In 24 hours, it moves 13°. Th us, each night you see the
moon about 13° eastward of its location the night before.
As the moon orbits around Earth, its shape changes from
night to night in a month-long cycle.
The Cycle of Phases
Th e changing shape of the moon as it revolves around Earth is
one of the most easily observed phenomena in astronomy. Study
The Phases of the Moon on pages 34–35 and notice
three important points and two new terms:
Th e moon always keeps the same side facing Earth. “Th e
man in the moon” is produced by the familiar features on3-1