On the origin of the Earth's moon and how our planet would be different if we didn't have a moon.
The Moon has been a stabilizing factor for the axis of rotation of the Earth. If you look at Mars, for instance, that planet has wobbled quite dramatically on its axis over time due to the gravitational influence of all the other planets in the solar system. Because of this obliquity change, the ice that is now at the poles on Mars would sometimes drift to the equator. But the Earth's moon has helped stabilize our planet so that its axis of rotation stays in the same direction. For this reason, we had much less climatic change than if the Earth had been alone. And this has changed the way life evolved on Earth, allowing for the emergence of more complex multi-cellular organisms compared to a planet where drastic climatic change would allow only small, robust organisms to survive.
Earthrise and earthset movies made by Kaguya, a Japanese spacecraft currently orbiting the moon. Also available here at a higher quality. I'm hoping these are available in HD at some point.
This New Yorker article on light pollution makes mention of the Bortle Dark-Sky Scale, which is a measure of how bright the stars are in a particular part of the sky.
In Galileo's time, nighttime skies all over the world would have merited the darkest Bortle ranking, Class 1. Today, the sky above New York City is Class 9, at the other extreme of the scale, and American suburban skies are typically Class 5, 6, or 7. The very darkest places in the continental United States today are almost never darker than Class 2, and are increasingly threatened. For someone standing on the North Rim of the Grand Canyon on a moonless night, the brightest feature of the sky is not the Milky Way but the glow of Las Vegas, a hundred and seventy-five miles away. To see skies truly comparable to those which Galileo knew, you would have to travel to such places as the Australian outback and the mountains of Peru.
The World Without Us looks like an interesting read: "If a virulent virus -- or even the Rapture -- depopulated Earth overnight, how long before all trace of humankind vanished? [...] Days after our disappearance, pumps keeping Manhattan's subways dry would fail, tunnels would flood, soil under streets would sluice away and the foundations of towering skyscrapers built to last for centuries would start to crumble." Here's a similar article from New Scientist from late 2006. (via mr)
Geological features called chevrons could be evidence of violent comet/asteroid impacts as recently as 1000 years ago. The chevrons are formed by massive tsunamis; scientists believe one such tsunami occurred in the Indian Ocean 4,800 years ago and was 600 feet high. These impact-caused tsunamis may also be responsible for the various flood myths found in world religions. (thx, matt)
Bob Holmes imagines an earth without people in New Scientist. "All things considered, it will only take a few tens of thousands of years at most before almost every trace of our present dominance has vanished completely. Alien visitors coming to Earth 100,000 years hence will find no obvious signs that an advanced civilisation ever lived here."
Update: Derek Miller notes that even after human traces have vanished from earth, evidence of our civilization would remain on the moon and in space.
And if you ever need to move the Earth, here's how you might accomplish that. "The Earth is very big, moving very fast, and therefore very difficult to stop or even slow down."
