American astronaut Ron Garan may be the single best promoter of space exploration we have today, if only because of his breathtaking photographs of the Earth from space. In July 2011, as he orbited our planet in the International Space Station, he took this gorgeous shot of the crescent Moon, setting over the Earth's silhouetted limb:
[Click to enlunenate.]
This shows no fewer than three of my favorite optical effects! First, the colors of the sunset are due to the Earth's air, which preferentially scatters away bluer light, leaving just the orange and red colors from the Sun to get through.
Second, the Moon is squashed! You can see it's not perfectly round as it usually looks; again, that's due to Earth's air. It acts like a lens, bending the Moon's light. Nearer the Earth's limb, you're looking through more air, so the effect is larger, making the moon look not just squashed, but squashed unevenly: the top part is rounder than the bottom. I explain in this in detail using a different picture of moonset from space.
And third, of course, is that you can see the dark part of the Moon, gently illuminated. This is called Earthshine, or, more poetically, "the old Moon in the new moon's arms" (soon to be a twilight sequel). When the Moon is new, it's more or less between us and the Sun. if you were standing on the Moon, you'd see the Sun on one side of the sky and the Earth on the other. To you, the Earth would be full! The full Moon is bright enough to cast shadows here on Earth, and in the Moon's sky The Earth is much brighter and bigger. It's actually bright enough to light up the part of the Moon not lit by the Sun, and that's what you're seeing.
Think about the journeys of all those photons that make up this picture: some came straight from the Sun, a trip of 150 million kilometers, only to be scattered away at the last moment; some redder ones made it all the way through; some hit the moon and came toward us but were bent a bit by Earth's air, distorting the Moon's face; and others still came from the Sun, hit us, went to the Moon, hit there, came back to us, finally making it into the lens of a camera held by a man hanging in the cupola of a football-field-sized metal can orbiting 350 kilometers above our planet at 30,000 kilometers per hour.
To some people, this might just be a pretty picture of the Moon. But when you look past that, you see it's also a portrait of the incredible subtlety and complexity of the way the Universe works. And that's one of the things I love about astronomy: what you get out of it depends on how much you want to know. There's always more.