Cumberland Times-News

Bob Doyle - Astronomy

June 21, 2014

Moon-watching easy when you know how

— Long before the first writing (scratches on clay tablets) appeared, our early ancestors noticed that the moon went through a regular cycle of shapes in about 30 days.

Archeologists have found bones with scratches lined up to show the changing shapes of the moon.

To best explain the moon’s lighted shape changes, take a peach (apple or orange will do) into a dark room. In the center of this room place a single bare light.

Stand in the corner of this room and hold the peach nearly in line with the central light. Most of the peach will be dark. (This matches the New Moon, the start of the lunar cycle.) Then rotate 90 degrees to left and notice that the peach has a right side illuminated while its left side is dark. This corresponds to the evening half-full moon (or first quarter moon).

Then turn another 90 degrees to the left, and the peach will be fully lit; this matches the full moon. Make another 90 degree turn to the left. The peach will appear half-full with the left side lit and the right side dark.

This matches the half-full morning moon (often called the third quarter moon). Make your last 90 degree turn to the left and you will back where you started, a mostly dark peach. Between each of the four main phases of our moon is about 7 1/3 days.

People long ago used the moon’s shape cycle as a time unit. Both Jews and Muslims start each of their months with the first sighting of the crescent moon (after New Moon) from their holiest city (Jerusalem for Jews and Mecca for Muslims).

African tribes note how the crescent moon is oriented to the horizon as an indicator of the start of their rainy season.

Can one use the moon to tell time at night? Let’s start with full moon.

The full moon is nearly opposite in direction to the sun. So as the sun sets, the full moon rises above the eastern horizon. In the middle of the night, the moon is due south and highest. This corresponds to 1 a.m. (for daylight time) or midnight (for standard time).

Just as the full moon rises at sunset, The full moon sets at sunrise. The half-full evening moon (first quarter) rises at midday, is highest in the South about sunset and sets about midnight. So this half-full moon is visible during the p.m. hours of the night. The half-full morning moon (third quarter) rises at midnight and sets about noon, visible in the a.m. hours.

What about the crescent moons? In the evenings after New Moon, a slender crescent appears low in the west. Each night, the crescent is broader and higher above the horizon.

The crescent moon’s width is proportional to the time it can be seen at night. A very slender crescent may be seen for less than an hour. A broader crescent is visible for several hours.

In general, the wider the illuminated part of the moon is, the longer it will visible at night. So the full moon can be seen all through the night. half-full moons are seen for half the night.

Are there any changes in the moon due to the seasons? In late spring/early summer, the full moon has a low sky path as it is the same zodiac position as the late fall/early winter sun. In late fall/early winter, the full moon has a high sky path, being in the same zodiac position as the late spring/early summer sun.

SKY SIGHTS AHEAD: This morning, the moon was a fat crescent in the predawn sky. On June 24 at dawn, the crescent moon will be near the brilliant planet Venus. On June 20, the moon will swing from the morning to the evening side of the sun (New Moon).

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.

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Bob Doyle - Astronomy
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