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Exploring the Solar System Study Guide (page 3)

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Updated on Sep 25, 2011

Phases of the Moon

Though it was once thought that the moon might have condensed separately around the earth, the following scenario is now known to be true (from multiple lines of evidence). A few hundred million years after the formation of the earth, a rogue body about the size of Mars and having an odd orbit around the sun, smashed into the earth. Material from both the colliding body and the earth flew off and condensed around the earth to form the moon. The moon was much closer at that time and has been slowly moving away from the earth ever since.

The moon revolves around the earth in 29.5 days, from which we derive the calendar division called our month. The moon keeps its same face to the earth during this revolution. The side of the moon facing away from the earth, often referred to as the "dark" side of the moon, is not really dark. It's just that we never see it from Earth. Both "our" side of the moon and its "dark" side are lit by the sun and then go into shadow, as the moon goes through its phases.

Figure 2.4 shows the phases of the moon. It's great fun (I promise) to look at the moon in the sky and from its phase be able to sense where the moon is along its orbit, in relation to Earth and sun.

Figure 2.4 Phases of the Moon

The moon's gravity is largely responsible for the oceans' tides—the rise and fall of sea level that we notice when we visit a beach for several hours or kayak in a marine bay. (To a lesser degree, the sun contributes to the tides as well.) The sun is less important because even though it's huge compared to the moon, it's also very far away. Tides are high at places on the earth that are either closest to the moon or on the opposite side of the earth from the moon. Therefore, two high tides and two low tides happen each day, with the low tides coming between each of the two high tides. The time interval from high tide to low tide, or from low tide to high tide, is about 6 hours.

The shapes of the coastlines and the local topography of the ocean's depths affect the exact timing of the tides locally. That is why we need tide tables that are specific to each locality. Furthermore, tides are particularly high and low when the moon and sun are in alignment with the earth, which occurs during either new or full moon.

The moon is also responsible for the exciting astronomical events we call eclipses. Two kinds of eclipses happen (see Figure 2.5). In the solar eclipse, the moon blocks out either a portion of the sun (a partial solar eclipse) or all the sun (a total solar eclipse). That can occur only during new moon and, for any particular city on Earth, only once in a great while.

Figure 2.5

The second kind of eclipse is the lunar eclipse, which occurs only at full moon, when the earth's shadow is cast upon the moon. Lunar eclipses can also be full or partial. The moon doesn't disappear completely during a full lunar eclipse but turns a deep reddish-brown in the night sky. Because the earth's shadow is so large, lunar eclipses can be seen more often from any place on Earth, but they are still special astronomical events worth watching.

Practice problems of this concept can be found at: Exploring the Solar System Practice Questions

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