When you switch on a lamp, there’s no lag between the “click” and the light hitting your eyes. Or at least it seems that way. There’s really a lag of a few billionths of a second — an interval that’s far too short for human senses to perceive. But for some light sources, the lag is a lot longer. In fact, for three bright objects that huddle close together in this evening’s sky, the lag is measured in seconds, minutes, and years.
The Moon, the star Spica, and the planet Saturn are well up in the sky at nightfall. Spica is close to the upper left of the Moon, with Saturn above Spica.
We see these objects not as they look now, but as they looked in the past, because light doesn’t travel across space instantaneously.
Light moves through the vacuum of space at 186,000 miles every second — nothing in the universe is faster. Yet the universe is so vast that even at that pace, it takes a while for light to get around.
The Moon, for example, is only about a quarter-million miles away, so its light takes a bit more than a second to reach us. In other words, when you look at the Moon, you’re seeing it as it looked about a second ago.
Saturn is a good bit farther, so its light takes about 78 minutes to reach Earth. And Spica is farther still. Its light takes 250 years to reach us. So as you look at the star tonight, you’re actually seeing it as it looked in the year 1762 — at the start of the reign of Russia’s Catherine the Great.
Script by Damond Benningfield, Copyright 2012
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