Looking Up: The crescent and “dark side” of the moon

Peter Becker
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This composite image of a total solar eclipse, taken by Fred Espenak, June 21, 2001 from Zambia, shows the Earthshine faintly illuminating the new moon, covering the sun. The sun's "atmosphere," the solar corona, is visible.

Ever see the dark side of the moon? When you see a nice lunar crescent in the evening or early morning sky, you often can detect the darker portion within the crescent shape, only dimly illuminated.

New moon was this Friday, Jan. 25. All this week, leading up to the first quarter phase on Feb. 1, there will be a waxing crescent adorning the southwestern sky as twilight fades into the dark of night.

Too often we hear the “dark side of the moon” referred always as the far side, the portion facing away from the Earth. Every part of the moon, even the portion looking our way, gets its turn as the “dark side.”

The dark side is the side away from the sun and is in shadow.

When the moon is bright and full as seen from Earth, the dark side is literally the far side.

The moon rotates once around, once every revolution around the Earth, which takes 27.32 days. This is called synchronous rotation; the moon’s spin is in “sync” with the rate of its revolution, tidally locked by gravitational forces.

That brings the moon’s rugged landscape into ever-advancing sunlight.

At new moon, our satellite is roughly right between the Earth and the sun. During a total solar eclipse, the entire side facing us is in its own shadow. That’s why total eclipse photos show the moon as an ink-black disc, surrounded by the light of the solar corona.

Since the plane of the moon’s orbit is titled slightly (by 5.1 degrees) from the plane of the Earth and sun, it usually misses the sun and there is no eclipse. At these times we still call it new moon, but actually there is still an extremely slight crescent of moon still lit by the sun. We don’t see it because it is lost in the glare of the sun.

So how come the dark side we see in the evening and morning crescent is not jet black? We are seeing it faintly illuminated by the reflection of sunshine from off the Earth. It’s called, of course, earthshine.

Guess what. Even in a total solar eclipse, the lunar disc is covered in earthshine, but thanks to the brightness of the sun’s corona and the fact we haven’t let our eyes fully adapt to darkness, we see the moon as black.

Some amateur astronomers have found great sport in trying to detect the thinnest possible crescent of the moon, only hours after new moon and only minutes after the sun has set. Of course, the thin arc of the crescent is very difficult to see, with the overwhelming light of early dusk (or dawn), and with the moon so very near the flat horizon where the atmosphere is the murkiest.

The line between day and night on the moon is called the terminator. The low sun creates stark shadows on the moon, making the craters and mountains stand out in even a small telescope. Binoculars will also show you how the lunar terminator reveals a rugged landscape.

This Tuesday, Jan. 28, the crescent moon will pass only about four degrees to the left (east) of Venus. This planet is extremely bright (magnitude -4.1) and white, dominating the southwestern evening sky this winter. The sight of the crescent moon, with the earthshine and Venus so close, should be glorious. Both will fit within the field of view of 7x or 10x binoculars, which makes it all the more stunning.

On the night of Jan. 27, by the way, if you have a small telescope, look at Venus with low magnification once the twilight is deepened. Close to Venus, you should see what appears to be a fainter star, which is actually the far-away planet Neptune. This also will be visible in binoculars. Neptune isn’t bright enough (magnitude +7.9) to be seen with unaided eyes.

While the moon is still an evening crescent, the night sky is still quite dark (except for what light pollution you may endure). Be sure to enjoy the bounty of stars spread across the winter sky. By the way, the red-orange star Betelgeuse in the upper left corner of the Orion constellation is still unusually dim this season, fainter than anyone can remember.

Keep looking up!

Peter Becker is managing editor at The News Eagle in Hawley, Pennsylvania. Notes are welcome at Please mention in what newspaper or website you read this column.