The Moon and the planet Jupiter line up in the east as night falls this evening. Jupiter looks like a brilliant star to the right of the Moon.
Like everything else you see in the night sky, when you look at these two bodies, you see them as they existed in the past -- about a second ago for the Moon, and about a half-hour ago for Jupiter. That's because they're both a long way off, so it takes their light a while to reach us. In fact, Jupiter and its moons helped astronomers determine that light travels at a limited speed.
Until a few centuries ago, many thought that light crossed the universe instantly. But in the 1600s, Danish astronomer Ole Romer was keeping a careful eye on Jupiter and its moon Io, which periodically disappears in Jupiter's shadow. Careful timings would allow astronomers to predict these eclipses, which in turn could be used to help navigators determine their position here on Earth.
Romer noticed that as Earth and Jupiter drew closer to each other, the time between eclipses got shorter. And as they moved farther apart, the gap grew longer. From that, Romer deduced that light must travel at a limited speed. More than a century later, other astronomers used Io's eclipses to measure that speed: about 186,000 miles per second.
Watch Jupiter as it crosses the sky tonight, near our own Moon. And as you watch it, keep in mind that you're not only looking deep into space -- you're also looking back in time.
Script by Damond Benningfield, Copyright 2010