Thursday, June 05, 2008

The Moon In The Evening Sky

Last Tuesday, June 3, was a new Moon. That means now the Moon will be in the evening sky, a thin crescent in the evening twilight.

Gradually the Moon will rise later and later in the evening, will become more and more full.

The numbers are that the Moon travels roughly 12 degrees in every twenty-four hour period. That means it rises just less than an hour later every night.

This is a great time to observe the Moon.

Many people new to astronomy think the best time to look at the Moon is when the Moon is full, but that’s not true. When the Sun illuminates the full Moon the light is so bright, so free of shadows that details are hard to see. And it’s easy to get a headache.

Most experienced lunar observers watch the light/shadow line on the Moon move across the waxing or waning crescent early and late in the lunar cycle and avoid the Moon when it is full. On the waxing Moon, the first major feature to get illuminated is the Sea of Serenity. On the waning Moon, the last major feature for the shadow line to engulf is the Sea of Clouds.

Early and late in the lunar cycle the Moon is not bright enough to obscure nearby stars with its light. So the Moon can be observed near interesting celestial companions or followed through the zodiac.

Over the next few days the lunar crescent will be passing close first to Mars, then Saturn.

Later, the Moon will pass near Spica in Virgo, then the two Z stars in Libra. By the time the Moon gets near Jupiter it will be very bright, making it difficult to see the constellations behind it.

Since each constellation is (very) roughly 30 degrees wide, the Moon spends about two nights in each constellation.

If the clouds cooperate, this is a cool time for Moon watching.

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