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Sunday, July 26, 2009

Look Up And See Into The Past

There's a curious thing about looking up, especially if you happen to be looking up at a clear, dark night sky - you can literally see into the past! Now, this applies to things in the sky other than clouds and bats - things like stars, planets, and galaxies.

But before we get to the crux of the matter, you have to realize that light travels at about 186,000 miles (300,000 km) per second - that would be about 7 times around the Earth in one second. Now, that's pretty fast, but not infinetly so. And since the distances we are considering are significantly greater than 186,000 miles, you can see that it is going to take time for light to travel those immense distances.

The Moon is about 240,000 miles (384,00 km) away. Divide that by 186,000 and you get the appromately 1.3 seconds that light takes to travel from the Moon to Earth. So that means whenever you look a the Moon, you are seeing it as it was 1.3 seconds ago. Well, that's not so bad, you say? Harumpn, Big deal!

But it gets better, really! When you look at the Sun, you are seeing it as it was about 8 minutes ago. Do the math: 93,000,000 miles (distance from Earth to the Sun) divided by 186,000 (speed of light per second) = 500 seconds = about 8.3 minutes. Keep in mind that the light you see Now actually left the Sun about 8.3 minutes ago and is just now getting to your eyes. So, when you see the Sun rise, it actually rose 8.3 minutes ago. And if the Sun were to suddenly go supernova, we wouldn't know it until 8.3 minutes later because the light from the supernova event would take over 8 minutes to get here.

But we're not going to stop here,... because it just keeps getting sooo much better.

Take a look at Jupiter in the night sky. Currently, it is about 4 times farther away from us than the Sun. So the time it takes light to travel from Jupiter to Earth works out to be about 33.3 minutes. Okay, so now you are seeing Jupiter as it was over half an hour ago. Essentially you are looking into Jupiter's past.

Now, we'll take on the stars - this is where it really gets wild. So wild that we have to start thinking in terms of light years. A light year is the distance light travels in a whole year - about 6 trillion miles (10 trillion kilometers)! Remember, light is traveling at 186,000 miles (300,000 km) per second all during a whole year - 6,000,000,000,000 miles (10,000,000,000,000 km).

The closest star to us is Proxima Centauri, a small red dwarf that can't be seen with the naked eye, and it's about 4.2 light years away. But Alpha Centauri, a double star, can be seen with the unaided eye and it's 4.3 light years away. That means that you are looking 4.3 years into the past of Alpha Centauri - you see it as it was 4.3 years ago!

Sirius, the Dog Star, is a well known, very bright star. But it's about 8.6 light years away. The past is getting more past all the time, eh?

Polaris, the North Star, in the constellation of Ursa Minor (asterism Little Dipper), is about 430 light years away. Whoa - now that's going back in time!

And let's time travel a bit more. Betelgeuse, that famous, bright, red star in the constellation Orion, is about 640 light years away. Now that's some really elderly light that's hitting your eyes!

Finally, lets get serious about going back in time. The Andromeda galaxy is visible as a faint smudge on a clear, moonless, dark night. It's one of the closest galaxies to us and the only one that's visible to the naked eye. But even with a bit of light pollution, it can still be easily seen with binoculars. And the Andromeda galaxy is a whopping 2.5 million light years away. So, you look at Andromeda and you are looking 2.5 million years into the past, seeing it as it was 2.5 million years ago. Wow, hunh? Think about it, the light you see has been traveling through space for 2.5 million years!

Binoculars and telescopes, of course, let you see even older light - much, much older light. The Hubble Space Telescope, Spitzer Space Telescope, and a plethora of other space based and Earth based telescopes have allowed astronomers to see somewhere in the neighborhood of 13-14 BILLION years into the past! Now that's incredible, unimaginable, and downright cool - and it's true!

So, the next time you look up, think about what you are witnessing - The Past!

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