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3: Cycles of the Moon

Did anyone ever warn you, “Don’t stare at the moon—you’ll go crazy”? For centuries, the superstitious have associated the moon with insanity. The word lunatic comes from a time when even doctors thought that the insane were “moonstruck.” A “mooncalf” is someone who has been crazy since birth, and the word is probably related to the belief that moonlight can harm unborn children. Everyone “knows” that people act less rationally when the moon is bright, but everyone is wrong. Careful studies of hospital, school, and police records show that there is no real correlation between the moon and erratic behavior.

The moon is so bright and its cycles through the sky are so dramatic we expect it to influence us, and many people are disappointed to learn the moon does not influence us. Some simply refuse to believe the evidence.

The cycles of the moon are indeed dramatic, and the moon is a rich source of myths and traditions. Many different cultures around the world explain eclipses of the sun as invisible monsters devouring the sun (Figure 3-1). A Chinese story tells of two astronomers, Hsi and Ho, who were too drunk to predict the solar eclipse of October 22, 2137 BC, or perhaps failed to conduct the proper ceremonies to scare away the dragon snacking on the sun. When the emperor recovered from the terror of the eclipse, he had the two astronomers beheaded. The cycles of the moon provide a wealth of beauty in the sky and an important part of the cultural beliefs of Earth’s peoples.
This chapter discusses the lunar cycles of phases and eclipses. Studying these events will help you understand what you see in the night sky. They will also introduce you to some of the most basic concepts in astronomy.

You will discover how light and shadow move through space, how the size and distance of an object affect what you see, and how cyclic events can be analyzed by searching for their patterns. Most of all, this chapter helps you to begin thinking of your home as a world in space.


Looking Back
In the preceding chapter, you watched Earth rotating on its axis, which makes the sun rise and set. You watched Earth revolving along its orbit around the sun, which produces the cycle of the seasons. These two cycles so completely dominate your life on Earth that you hardly notice them. But there are other cycles in the sky, and now you are ready to study one of the most dramatic and beautiful actors on the celestial stage.

This Chapter
The moon is the brightest object in the night sky, and it moves rapidly against the background of stars, changing its shape and occasionally producing strange events called eclipses. You may feel you know the moon well, but you will probably find some surprises as you answer four essential questions about Earth's satellite:

Why does the moon go through phases?

What causes a lunar eclipse?

What causes a solar eclipse?

How can eclipses be predicted?

This chapter illustrates two powerful ways to analyze certain kinds of problems. Many problems in astronomy depend on seeing how light and shadow move through space in three dimensions. Phases and eclipses are good case studies. Also, if a process repeats, you can analyze it by searching for cycles. The seemingly complex motions of the sun and moon become simple and elegant when you see them as cycles.

Looking Ahead
Once you have a 21st-century understanding of your world and its motion, you will be ready to read the next chapter, where you will see how Renaissance astronomers analyzed what they saw in the sky and came to a revolutionary conclusion — that we live on a planet.

Chapter 3
Cycles of the Moon

Even a man who is
pure in heart
And says his prayers by night
May become a wolf when
the wolfbane blooms
And the moon shines
full and bright.


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