Can’t Get Enough Of That Mighty Fine Earthshine

Here’s how Earthshine works. Sunlight reflects off the blue and white Earth into space at the moon. The moon absorbs some and sends the rest back to us. We see this twice-reflected light as a dim illumination of the portion of the moon not lit by direct sunlight. Illustration, crescent and sun photos: Bob King; Earth image: NASA

Can’t get enough of that good old moonshine. Wait a second, I meant Earthshine. Our region’s been fortunate to have several clear nights in a row for watching this spooky light fill out the fattening crescent. I hope you’ve had a chance to see it too.

Last night during astronomy class, we talked about how late into the moon’s phase Earthshine might be visible. Years ago, I did an experiment using my 10-inch reflecting telescope and was able to see it when the moon was nine or ten days past new. More casually, I’ve seen it in binoculars when the moon was at half (first quarter phase).

This diagram shows the moon’s phase and the corresponding phase of the Earth as seen by someone on the moon. Notice that as the moon fills out, the Earth gets thinner. Illustration: Bob King; Earth and moon from Stellarium.

As the moon orbits the Earth, it marches upward from the sun while increasing in phase. As seen from the moon, the Earth also goes through phases that are complementary to the moon’s. When the moon’s a very thin crescent, an astronaut on its surface would look back toward our planet and see a nearly full Earth. A full Earth reflects a lot of sunlight back at the moon, so Earthshine is brightest when the crescent is thinnest.

As the moon’s phase increases toward first quarter and beyond, the Earth’s phase wanes, going from full to half to crescent. With less Earth to reflect sunlight, the Earthshine gets fainter and fainter. It also doesn’t help that the area for the Earth to illuminate shrinks as the sunlit portion of the moon grows ever larger night by night.

Our moon’s not the only one lit by its home planet. This is Saturn’s moon Iapetus photographed by the Cassini spacecraft. The right side is overexposed because it’s illuminated by the sun. The left side glows by light reflecting off Saturn’s clouds. Since sunlight is much fainter at Saturn’s distance than at Earth’s, Cassini had to make a long time exposure to capture this image. Credit: NASA

So now I’d like you to try a little experiment yourself. How many days after new moon can you still see the Earthshine — with your naked eye and also with binoculars? New moon was Friday April 24 which would make the moon five days old tonight (Weds.). Last night, the whole class saw the Earthshine easily without optical aid. As it becomes increasingly fainter in the coming nights, you’ll need to use averted vision to spot it. Look around and about the faint part of the moon instead of straight at it. Even better, hide the bright part of the moon behind a wall or power pole.

Drop me an e-mail and let me know how you fare. I’ll be out there drinking in my share of Earthshine too.

Lyle Anderson of Duluth sent this photo of the International Space Station he took from his home on Sunday at 4:42 a.m. (I added the annotations). The station’s been making regular pre-dawn passes over the U.S. this week. Check yesterday’s blog for times.

4 Responses

  1. John Mahan

    Very interesting! I am not a formal astronomy student but a medical doctor just fascinated with astronomy and this phenomenon. You explain very well how the phases of earth and moon change and how that impacts earthshine. My question is why when the earth is full during new moon is earthshine no longer visible. After all, that’s when the earth is at its brightest. Is it a function of optics? The new moon just can’t reflect back the earthshine at a straight angle during new moon? Thank you for indulging my question.

    1. astrobob

      You’re correct – earthshine is brightest at new moon, but when in new moon phase, the moon is lined up in the direction of the sun and not visible because it’s lost in the glare of day. New Moon earthshine can be photographed only during a total solar eclipse when the moon hides the sun. You can’t see it with your eye but properly exposed photos will show it.

  2. Robert

    Astro Bob,

    In the past I have noticed Earthshine on the unilluminated side of the moon, I often notice it when the moon is a thin crescent in the twilight sky after the sun has set, it is so beautiful against the darkening sky.

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