Cumberland Times-News

Bob Doyle - Astronomy

August 25, 2012

The ‘Blue Moon’ and other lunar fantasies

This Friday, Aug. 31, will feature August’s second full moon. (The moon will look very full on Thursday night as the actual time of full moon is 9:57 a.m. on Friday.)

This second full moon is called a “Blue Moon.” Two full moons tend to occur in the longer months of 31 days. The moon’s phase cycle (from full moon to full moon) is 29.5 days.

We had a full moon on Aug. 1, making the second full moon possible. We have a “Blue Moon” about every three years. Next month’s full moon will occur on very late in the evening of Sept. 29. This is the “Harvest Moon.”

There’s a fascinating book about the Moon by Bernd Brunner, covering both science and myth. “Moon – A Brief History” was published by Yale University Press in 2010 with ISBN 978-0-300-17769-5 (paperback).

Can the moon actually appear blue? As Brunner relates, after a volcanic eruption, there may be a large amount of tiny particles about one micron wide (millionth of a meter or 0.0000004 inches) injected into the stratosphere.

Since this particle size is close to the wavelength of red light (0.7 microns), more of the longer waves of moonlight (red, orange and yellow) are scattered away.

This would make the moonlight we see mostly the shorter color waves (green, blue). Such “blue moons” were seen for several years following the 1883 eruption of Krakatoa on an island of Southeast Asia.

The Chuckchee Shamans (medicine men) from northeast Siberia purposely exposed themselves to full moon, instilling them with magical powers. Some African tribes dance by the light of the full moon.

The Aztecs were fearful when the crescent moon disappeared in the glare of the sunrise, prompting them to pray for its return. So a few days later, when a narrow crescent moon was again seen in the western dusk, the Aztecs celebrated.

Eclipses by the moon (solar) and of the sun (lunar) were feared by ancient people. The Maasai of East Africa throw sand into the air during a solar eclipse. Some Native Americans shoot flaming arrows at the darkened moon during a lunar eclipse.

There is a charming story about the man in the moon. (When full, the light and dark areas of the moon form a crude pattern of a human face. Others claim to see a lady in the moon, while many see a rabbit on the moon.)

In a German myth, a man went to gather wood on Sunday, the biblical day of rest. The man never came back. His worried family saw his face, peering at them from the moon.

   Photometric (light sensing) measurements reveal that the full moon shines with only 1/400,000 of the intensity of the sun. The full moon is 25 times as bright as the half full moon (the evening moon that resembles a letter D).

At full moon, the sun is shining straight down on the moon and there are few shadows. At half full, there are a lot of shadows on the moon and the illuminated part of the moon we see is receiving sunlight at a low angle to the vertical.

EVENING SIGHTS THIS WEEK: Since moonlight dominates this week’s evening sky, it is best to focus on the brighter stars.

Nearly overhead is the bright white-blue star Vega. Vega is 25 light years away, meaning that its light we see tonight left that star twenty five years ago (in 1987).

The second brightest star is Arcturus, a bright golden star in the west. Follow the curve of the Big Dipper’s handle outward and in one Dipper’s length, you will come to Arcturus.

The light of this star has taken 36 years to reach us. So we see Arcturus as it was in 1976, our bicentennial year (200 years since the Declaration of Independence).

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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