We can't travel into the past, but we can get a glimpse of it. Every time we look at the Moon, for example, we see it as it was a little more than a second ago. That's because sunlight reflected from the Moon's surface takes a little more than a second to reach Earth. We see the Sun as it looked about eight minutes ago, and the other stars as they were a few years to a few centuries ago.
And then there's M31, the Andromeda galaxy -- the most distant object that's readily visible to human eyes. This great amalgamation of stars stands almost directly overhead late this evening. When viewed from a dark skywatching location, far from city lights, it looks like a faint, fuzzy blob. But that blob is the combined glow of hundreds of billions of stars -- seen as it looked more than two million years ago.
Andromeda is similar to our own Milky Way galaxy. It's a flat disk that spans more than a quarter-million light-years. Its brightest stars form spiral arms that make the galaxy look like a pinwheel. Yet the galaxy is so far away that its structure is visible only through telescopes.
The light from M31 has to travel about two and a half million light-years to reach us. That's about 15 billion billion miles -- the number 15 followed by 18 zeroes. Yet even across such an enormous gulf, the galaxy is so bright that we can see it -- faintly -- with our own eyes, crossing high overhead late tonight.
Script by Damond Benningfield, Copyright 2006, 2009
For more skywatching tips, astronomy news, and much more, read StarDate magazine.