Gigantic Rulers for Gigantic Distances
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Distances in astronomy are, well, astronomical. It's not like we're measuring someone's shoe size here. We need rulers, gigantic rulers that can measure trillions of miles to get to even the next closest star. But even the biggest home improvement places can't supply that need.
So, when you read that Sirius is 8.6 light years away, or that the Andromeda galaxy is 2.6 million light years away, how do we really know?
There are a variety of mileage tricks up an astronomer's sleeve. Here we'll tackle just one, the one that helps us determine local distances. In future columns we'll take on the big scary long distance measurements.
You've been blessed with two eyes for a reason. Our eyes work in concert to give us a sense of depth. Each eye has a slightly different view of the world around us than the other. The brain can judge nearby distance pretty well using those views.
Put your thumb near your nose, but not in it! Look at it with just your right eye, then just your left. Notice each eye has a different thumby perspective.
Move your thumb slowly away, looking first with your right eye, then left, then right, then left. The thumb will appear to be moving back and forth less and less.
Imagine now you are Inspector Gadget and can extend your arm out another couple kilometers. Blink now and you'll notice no difference. Because our eyes are only about 6 cm from each other we have no sense of depth way out there. Our eyes are only good for a few hundred meters of depth perception at best.
To judge the distance of the thumb a couple kilometers away, you'd have to separate your eyes a couple meters from each other, which makes it tough to get through doorways.
But astronomers can separate their eyes by about 300 million kilometers and get a very good sense of depth out to amazing distances. How!?
Our eyes in astronomy are telescopes, and we can separate them so far because they move! Let's say we take an image of a nearby star in January. Then --- and here's the key --- six months later in July, when we are on the other side of the sun, we can take another image of the star to see how it has "moved" through the distant background stars.
Look at your thumb at arm's length. You right eye might represent Earth in January. Your left eye is Earth in July. (Your nose is the sun.) See how the starry thumb "moves" when you switch eyes? That is what nearby stars do from our point of view.
Nearby stars "move" a lot throughout the year. More distant star "move" less. This movement is called parallax. There is a simple equation that can change parallax to distance. But suffice it now to say that if one can measure the apparent angular movement of a star through the sky --- its parallax --- one can calculate distance.
Because of their enormous distances the parallaxes of stars are really, really tiny. Very few move more than an "arcsecond" annually. That's about the diameter of a Ping Pong ball 5 miles away!
That's why we need good scopes to do this hyperprecise measuring. One such telescope, the satellite Hipparcos, recently measured the parallaxes of over 120,000 nearby stars to thousandths of an arcsecond precision!
But our galaxy is over 100,000 light years across! Even our best equipment can measure star distances by parallax to only several hundred light years.
To measure farther we need something else…
Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.
Posted by Administrator at 2003.02.15 01:50 PM | Comments (0)
