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Chapter 3: Problem 21

What is the saros? How did ancient astronomers use it to predict eclipses?

Short Answer

Expert verified
The saros is approximately an 18-year cycle used by ancient astronomers to predict eclipses. Eclipses separated by one saros period occur at the same node and at similar distances from Earth, making it easier to predict when they will occur.

Step by step solution

01

Define 'Saros'

The saros is a period of approximately 223 synodic (lunar) months, or approximately 18 years, 11 days, 8 hours. After this period, the Sun, Earth, and Moon return to approximately the same relative positions, a phenomenon discovered by ancient Assyrians.
02

Understanding the Use of Saros

The ancient astronomers observed that after a saros period, not only did the Sun, Moon, and Earth roughly return to the same relative positions, but the two eclipses separated by one saros cycle occurred at the same node with the Moon at nearly the same distance from Earth and at the same time of year. Thus, ancient astronomers could predict when eclipses would occur.

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Most popular questions from this chapter

One definition of a "blue moon" is the second full moon within the same calendar month. There is usually only one full moon within a calendar month, so the phrase "once in a blue moon" means "hardly ever." Why are blue moons so rare? Are there any months of the year in which it would be impossible to have two full moons? Explain your answer.

The dividing line between the illuminated and unilluminated halves of the Moon is called the terminator. The terminator appears curved when there is a crescent or gibbous moon, but appears straight when there is a first quarter or third quarter moon (see Figure 3-2). Describe how you could use these facts to explain to a friend why lunar phases cannot be caused by the Earth's shadow falling on the Moon.

Use the Starry Night Enthusiast \({ }^{\mathrm{TM}}\) program to examine the Moon as seen from space. Select Solar System > Inner Solar System in the Favourites menu. Click the Stop button in the toolbar to stop time flow. Then, click on the Find tab and double-click on the entry for the Moon in the Find pane in order to center the view on the Moon. Close the Find pane and zoom in on the Moon by clicking and holding the mouse cursor on the Decrease current elevation button (the downward-pointing arrow to the left of the Home button in the toolbar) to approach the Moon until detail is visible on the lunar surface. You can now view the Moon from any angle by holding down the Shift key while holding down the mouse button (the left button on a two-button mouse) and dragging the mouse. This is equivalent to flying a spaceship around the Moon at a constant distance. (a) Use this technique to rotate the Moon and view it from different perspectives. How does the phase of the Moon change as you rotate it around? (Hint: Compare with Box 3-1.) (b) Rotate the Moon until you can also see the Sun and note particularly the Moon's phase when it is in front of the Sun. Explain how your observations show that the phases of the Moon cannot be caused by the Earth's shadow falling on the Moon.

Why doesn't a lunar eclipse occur at every full moon and a solar eclipse at every new moon?

Suppose the Earth moved a little faster around the Sun, so that it took a bit less than one year to make a complete orbit. If the speed of the Moon's orbit around the Earth were unchanged, would the length of the sidereal month be the same, longer, or shorter than it is now? What about the synodic month? Explain your answers.
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