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.
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.
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.
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.
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.