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3: Cycles of the Sky YOUR ALARM CLOCK is an astronomical instrument. So is your calendar. Your life is shaped by the cycles of the sky. The rotation of Earth on its axis defines the day, and its revolution around its orbit defines the year. The orbital motion of the moon across the sky defines the month and divides it into four weeks. Rotation is the turning of a body on its axis. Revolution means the motion of a body around a point outside the body. Consequently, astronomers are careful to say Earth rotates once a day on its axis and revolves once a year around the sun. Furthermore, the moon revolves once a month along its orbit around Earth. Using the right words is only the first step to thinking carefully about the motions you see in the sky. And that will reveal a great deal about the planet you live on.
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Looking Back
In the preceding chapter you watched Earth turning on its axis, making the sun rise and set. That daily cycle is probably the most important cycle in your life, but there are others, and now you are ready to watch the sun and moon move against the background of the stars.
This Chapter
The motions of the sun and moon have direct influences on your life and produce dramatic sights in the sky. As you explore these, you will find answers to five essential questions:
What causes the seasons?
Why does the moon go through phases?
What is a lunar eclipse?
What is a solar eclipse?
How do astronomical cycles affect Earth’s climate?
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 motions, 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 Sky
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.
PROVERB FROM OLD WOLFMAN MOVIES
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