The old moon glows softly in the new moon’s arms

Because we are all earthbound skywatchers, we cannot directly see our planet floating in space. However, we can see it in an indirect manner — if we know how to interpret what we see and if we look closely enough. An insurmountable task? Hardly.

For a few evenings each month — especially April — the crescent moon gives an enchanting scene in the western sky when darkness falls. Look at the moon on April 12, 13, 14 and maybe 15.

The moon’s bright crescent shape is obvious, but its night side can be glimpsed as a subtle glow by the unaided eye. If you use binoculars, you can plainly see lunar maria and large craters revealed in the darkness of the lunar night. Why can we see features on the nighttime side of the moon?

When the moon is in a crescent phase, it makes a relatively narrow angle with the sun, which is why the crescent moon follows the sun, setting one to four hours after it. When sunlight reaches the Earth- moon system, the light travels past the moon and strikes Earth. Some of that light reflects off our planet and heads back to the moon. It then strikes the lunar surface, slightly illuminating it, and bounces to Earth, reaching skywatchers standing in the edge of night. This sunlight double bounce effect only partially explains why the lunar night side is visible.

From the moon’s point of view, Earth shows a gibbous phase — the phase that is almost full — when the moon presents a crescent phase as viewed from Earth. Our planet spans four times the diameter of the moon, which results in it covering 16 times the moon’s area in the sky. Therefore, there is a lot more surface area for Earth to reflect light than the moon has.

Because the moon’s surface is rock and because it has no atmosphere, it reflects only about 12 percent of the sunlight that strikes it, while Earth, with its vast oceans, polar ice caps and white cloud tops, reflects close to 37 percent of incident sunlight. (Astronomers say that Earth has an “albedo” of 0.37, while the moon has one of 0.12, the same as asphalt.)

This all results in Earth shining at least 50 times brighter in the lunar sky than the moon does in ours, easily providing enough “Earthlight” to illuminate the lunar landscape.

Back in Southwest Virginia, we see the gently glowing dark side of the moon and call it “Earthshine.” The last time that the current dark side was lit in full sunlight was a week earlier, before the moon was new and before it passed between Earth and the sun. Now as it glows in the west, it appears to early evening observers as “the old moon lying in the new moon’s arms.”

April is a particularly good month to witness this phenomenon because, as the crescent moon climbs steeply in the western sky each successive evening, it moves quickly out of the bright solar glow, causing the Earthshine to become more prominent. Topping it off this year, the crescent moon on April 13 lies between the delicate Pleiades star cluster and the coarser Hyades cluster. Gilding the lily, bright Jupiter shines near the two star clusters and just above the moon on April 14.

In an offbeat way, when the moon is full with Earthshine, you can see the effect of Earth’s reflection on the moon as it hangs in space. Make the effort to reward yourself at the end of the day with this magical and inspiring sight.

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