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Shedding light on the moon’s dark side

Mark Gregory

Tonight for October 2, 2015

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Tonight – October 2, 2015 – if you live at mid-northern latitudes, you’ll see the moon rising in the east-northeast sky at mid-to-late evening. From the Southern Hemisphere, you’ll see the moon rising in the east-northeast, also, but at a later hour. It is now a waning gibbous moon, which means the moon’s disk is more than 50% illuminated by sunshine but less than 50% engulfed in the moon’s own shadow.

Once it rises, the moon will be out for rest of the night. In fact, if you look in your western sky after sunrise tomorrow, you’ll actually see the waning gibbous moon in the daylight sky.

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 more of its day side than night side tonight. Because the moon is now waning toward new moon, we’re bound to see more of its dark side each morning for the next one and one-half weeks.

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.

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