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Shedding light on the moon’s dark side

Image via US Naval Observatory

Tonight – October 27, 2017 – the moon is at or near its first quarter phase, which means the moon’s disk is 50% illuminated by sunshine and 50% engulfed in the moon’s own shadow. First quarter moon arrives on October 27, at 22:22 UTC (or at U.S. time zones: 6:22 p.m. EDT, 5:22 p.m. CDT, 4:22 p.m. MDT and 3:22 p.m. PDT).

Half the moon is always illuminated in space. In other words, the moon has a day side and a night side, just as Earth does. Due to the angle between the sun, Earth and moon, we’re seeing about equal portions of its day side and night side tonight. Because the moon is now waxing, we’re bound to see more of its day side each evening until the night that the moon turns full on the night of November 3-4.

The part of the moon that isn’t in sunlight is often called the moon’s dark side. Just realize that – because of the moon’s motion around Earth – the portion of the dark side that we see from Earth constantly changes.

There is a permanent far side of the moon. But there is no permanent dark side of the moon, because any given lunar location experiences night for about two weeks, followed by about two weeks of daylight.

The moon does rotate on its axis. But billions of years of Earth’s strong gravitational pull have slowed it down such that today the moon takes as long to rotate as it does to orbit once around Earth. Astronomers would say that the moon is tidally locked with Earth. For that reason, one side of the moon always faces Earth, but it is not always dark – as you can see just by looking at the sky tonight.

Incidentally, the moon’s gravitational effects on Earth are much smaller, but – given billions of years of time – the Earth will slow down and keep one face always toward the moon.

Larry Sessions

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