Earth’s Ghostly Light Touches The Crescent Moon Tonight

The 2-day-old crescent moon shines over a bog north of Duluth, Minn. last night. Light reflected from the Earth faintly illuminates the moon’s full outline. Details: 200mm f/2.8, ISO 800, 1/2″ exposure. Photo: Bob King

Wow, the moon sure looked lovely last night. At dusk it was a sharp crescent against the blue sky, but later the entire disk was visible thanks to the the Full Earth. Full Earth? Had you been able to stand on the moon and look back in this direction, you would have seen our planet hanging like a big, blue ornament in the velvety black lunar sky. When the moon’s a sickle, light reflected from Earth – called earthshine – lights up the part of our satellite still in shadow.

Earth and moon phases complement one another. The top strip shows the moon phases and the bottom the corresponding Earth phases seen by an astronaut standing on the moon’s surface looking back at Earth. One difference: Earth appears almost 4 times bigger than the moon. Illustration: Bob King

Moon and Earth phases are complementary. A thin crescent in our sky means a person standing on the moon sees a nearly full Earth. A half moon here means a half-Earth there, and around the time of full moon, our lunar astronaut sees a crescent Earth.

A ramble across the earthlit portion of the moon in binoculars will reveal large dark areas (lunar seas) and several bright blotches – the craters Tycho, Copernicus and Aristarchus. Photo: Bob King

Sunlight reflected from our blue, cloud-streaked globe gently illuminates the full outline of the moon. Since the light is reflected rather than direct sunlight, earthlight is faint and rather mysterious-looking. From the surface of the moon, it resembles twilight here on Earth. The crescent itself is lit directly by the sun and appears brilliant in comparison.

Earthshine is twice-reflected sunlight – one bounce off the Earth to the moon and then a bounce back from the moon to Earth. Both moon and Earth absorb much of the sun’s light, which is why earthshine appears faint compared to the sunlit crescent. Illustration: Bob King

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 waxes to half and beyond, the Earth’s phase wanes, going from full to half to crescent. With less Earth to reflect sunlight, 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 after night.

Jupiter (top left) and Venus (lower right) joined the crescent during twilight last night. Tonight the moon will be to the left of Jupiter. Photo: Bob King

Tonight’s crescent moon will be higher up in a darker sky, so the smoky earthlight should be even easier to see. When you step out for a look, you’ll also see a brilliant “star” a fist to the moon’s right. That’s Jupiter. If you have binoculars, take a minute to study the earthlit portion – you’ll see a surprising number of features there including several large dark areas (the lunar seas) and even a few craters, which look like bright spots.

15 Responses

  1. caralex

    Bob, is Earth three, or four times the size of the moon, as apparent from the lunar surface? I thought it was about four times, given that the diameter of Earth is about four times that of the moon. Am I wrong?

    The Earth wouldn’t really appear all that big, from the moon, given that it’s angular size would be only about 2 degrees, would it? The ‘we-never-went’ crowd always pounce on the ‘tiny’ size of the Earth in one of the Apollo photos to ‘prove’ that it was a hoax. They claim that the Earth should be ‘huge’ in the lunar sky, but I can’t see how that’s true. I don’t think they fully comprehend just how tiny the moon is, for example, in a photo taken by an ordinary camera without a zoom. You can barely see it! I would think that the Earth would fare only slightly better. Is that right?

    1. astrobob

      You know Carol, I’ve read that it was 1.5 degrees in diameter but upon calculation it’s actually 1.8 degrees across. Thank you for pointing that out.

  2. Edward M. Boll

    My beautiful sight was on May 11 with the Moon almost half way between Jupiter and Venus. While there is still the possibility of Comet ISON being brighter than the light of a Full Moon for a few hours, I am not as optimistic about it as a month or 2 ago as it seems to be lagging at still dimmer than magnitude 15. I am however a little more optimistic on Panstaars K1, now nearly a magnitude brighter than ISON. I would say that Panstaars has a potential +3 magnitude at brightest.

    1. astrobob

      Hi Edward,
      That was a nice scene wasn’t it – Jupiter, moon and Venus. I looked at L4 PANSTARRS again last night in the same field of view as Gamma Cephei. I still can’t get over this comet’s unique fan shaped appearance. In my 15-inch the anti-tail is now longer than the main dust tail (1 degree vs. 45′).

  3. Edward M. Boll

    There was a fan shaped tail the first time that I saw it. By recent pictures it has seemed to widen even more.

    1. astrobob

      It’s spectacular in a larger scope – two big, streaming tails that have slowly and continuously evolved.

  4. Giorgio Rizzarelli

    Very interesting topic Bob, the complementarity of Earth and Moon phases. It will become familiar when we’ll get pics from a manned Moon base (which, I suppose, will not be on the hidden face). So when we’ll have a Full Moon they will have a New Earth, and, if in particular we’ll have a total lunar eclipse, they’ll have a solar eclipse (or should I call it a Sun occultation?) – just had fun trying with Stellarium. Conversely when we have a solar eclipse they should be able to see the Moon’s shadow on the interested Earth region, isn’t it? (Stellarium doesn’t simulate this). I wonder if some lunar orbiter of Apollo mission already observed some such phenomenon.

    1. astrobob

      Some interesting thoughts here especially seeing the moon’s shadow on Earth. It should be possible to see a “dark spot” slowly travel across Earth at least in binoculars I’d think.

        1. astrobob

          Thanks Giorgio! I’ve got an old moon book that shows all the stages of such an eclipse. I remember just staring at those pages – so cool.

  5. JohnDean1999

    The waxing crescent moon is my favorite moon phase, as it is a great observing target to look at (with Earthshine) while astronomers wait for the sky to darken so they can stargaze, as the waxing crescent moon sets not too long after sunset.

    1. astrobob

      Yes, it’s the perfect combination – you get some moon observing plus an eventual dark sky to look at other things.

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