Understanding the Phases of the Moon

Monday 6 September 2004 Labor Day
Eastern Fairfax, Virginia
Last Quarter Moon
Sunrise 6:42 p.m. – Sunset 7:31 p.m.
Moonrise 11:44 p.m. – Moonset 2:35 p.m.

Understanding the Phases of the Moon

As I contemplated the fact that today is the last quarter moon, I began to wonder once again why the moon plays its monthly game of hide and seek with us. I know, of course, that the moon is the earth’s only natural satellite and that it circles our planet every month. However, I have always been curious as to the reason the flirtatious moon only shows part of her face at certain times of the month. I visited a site called Namaste Café (http://www.namastecafe.com/moonphase.htm) which offered interesting information on ceremonies, rituals, spells, and intentions in addition to a cursory astronomical explanation. One glance at the information made all the puzzle pieces fall into place for me. Here’s what I figured out:

The moon orbits the earth during the course of one month, with its near side always facing us. We never see the moon’s far side.

During the new moon, the moon is directly between the sun and the earth. This is why the new moon would appear (if we could see it) to rise in the east and set in the west with the sun. We can’t see the new moon, however, because the sun is illuminating the moon’s far side and leaving its near side in darkness. To visualize this, imagine the sun, moon, and earth lined up single file with the sun shining on the side of the moon we can’t see.

During the waxing crescent phase, the moon moves out of line to begin its monthly orbit around the earth. Sunlight falls on a tiny sliver of the near side, leaving the rest in the earth’s shadow. The inner curve of the crescent is the shadow of the curve of the earth.

As the earth turns on its axis at dawn, we see the sun first, then the moon. The moon appears to rise later and later each night, as more and more of its surface is illuminated by the sun. The thin crescent grows fatter and fatter until it becomes the half-circle known as the first quarter moon.

During the first quarter phase, the moon has completed the first quarter of its orbit around the earth. Half of its near side is reflecting the light of the sun and half is still in shadow. The first quarter moon usually appears to rise midday and set in the middle of the night.

The moon then continues toward the half way point in its orbit during the waxing gibbous phase. With more than half of the near side lit by the sun, it appears to grow rounder and rise later in the afternoon.

When the moon has progressed in its orbit to the other side of the earth, we see what is known as the full moon. As the earth turns away from the sun at dusk, it turns toward the moon, all of the moon’s near side reflecting the light of the sun. Hence, the moon appears to rise in the east at dusk and set in the west at dawn. If you are lucky enough to be awake at dawn on a clear morning, you’ll be able to see the sunrise and the moonset at the same time.

Next comes the waning gibbous phase, during which the moon appears to be rising later in the evening and growing smaller. Now that the moon is traveling around the other side of the earth in its orbit, less of its surface is reflecting the sun’s rays.

At the last or third quarter phase, the moon has traveled three quarters of the way around the earth. Again, only half of the near side is lit by the sun. It looks much the same as the first quarter, only it is visible during the opposite time of day, from the middle of the night to midday.

The waning crescent moon can be seen rising in the small hours before dawn. Once again, the sun shines on only a fraction of the near side. The moon has nearly completed its monthly journey around the earth.

For a delightfully down to earth demonstration of our view of the moon’s journey, you can try a moon dance. Light one lamp in a dim room. One person can be the earth and the other, the moon. The “earth” stands in the center of the room with lamp (the sun) on one side and the “moon” standing between the earth and the sun and facing the earth. The moon then begins to step around the earth, taking sideways steps and always facing the earth. The moon steps in a complete circle, always facing the earth, until coming back to the original spot between the earth and sun. The “moon” should be able to feel the increasing and decreasing amounts of lamp light (sunlight) on her face as she steps through the dance.

Copyright © 2004 by Allyson Denise Walker-Lawrence. All rights reserved. No part of this piece may be reproduced in any form, written or electronic, without the permission of the author.

Rise and set information calculated by the Clear Sky Institute, available on www.almanac.com/rise/index.php