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Moon and Saturn

May 29, 2021

The Moon climbs into view after midnight tonight. The bright planet Saturn lines up to its left as they rise, and to the upper left at first light.

The Moon is in its “gibbous” phase — sunlight illuminates more than half of the lunar hemisphere that faces our way.

Many people think the Moon’s phases are caused by Earth’s shadow. The phases are indeed caused by a shadow, but not Earth’s. When the Moon is new or a thin crescent, Earth’s shadow is aiming almost directly away from it. Instead, the shadow is cast by the Moon itself.

The cycle begins at “new” Moon, when the Moon crosses the line between Earth and the Sun. The Moon is lost from view for a couple of days, but soon climbs into the evening sky as a thin crescent. The crescent is illuminated by sunlight, while the dark portion of the disk is immersed in the Moon’s own shadow. To put it another way, it’s nighttime on that portion of the Moon.

Over the following two weeks, the illuminated fraction increases until the Moon is full, when it lines up opposite the Sun. After that, the portion that’s in the sunlight gets smaller by the day. The Moon then once again disappears in the Sun’s glare, beginning a new cycle of phases.

Earth’s shadow does occasionally touch the Moon, but only two or three times a year, when the Moon’s orbit crosses the shadow at the time of full Moon. This shadow play is known as a lunar eclipse — an event that happened just four days ago.

Script by Damond Benningfield

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