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Parallax in action
Notice how the thumb jumps
relative to the galaxy picture depending on which side the camera was placed.
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Parallax
Astronomers have used parallax since the 1800s. Even today, it is the most
reliable and useful method for finding distances to nearby stars.
You have probably seen parallax before. Hold your thumb at arms
length. Close one eye and look at your thumb. Now switch which eye is closed.
You will notice your thumb appears to "jump" relative to the background.
Your thumb appears to jump because you are looking at it from a slightly
different angle. The distance between your eyes is called
the "baseline," and the angle that your thumb appears to jump is called the
parallax angle. The length of the baseline
determines the smallest parallax angle you can resolve; longer baselines can
resolve smaller angles.
Stars are extremely far away, so we need a very large baseline to
determine parallax angles. In fact, the baseline needs to be substantially
larger than the radius of the Earth. Astronomers use the
Earth's entire orbit to get a large enough baseline. Astronomers
observe a star on one night and then again about six months later, when
Earth has moved halfway around the sun. Using this technique,
astronomers can find parallax angles with a baseline of 186 million miles!
Even with such a large baseline, the parallax angles of stars are very small.
Proxima Centauri, the closest star, has a parallax angle of 0.772 arc seconds
(each degree is divided into 60 arcminutes and each arc minute is divided into
60 arcseconds. Therefore, 1 arcsecond is 1/3600 of a degree!) The parallax angle
of Proxima Centauri, 0.772 arcseconds, is about the same as the diameter of a dime
seen from a distance of 2.5 kilometers (1.5 miles).
Due to atmospheric blurring, you can measure parallax angles down to about 0.01
arcseconds from the surface of the Earth. Today, the best parallax measurements
are done from space, where the lack of atmospheric blurring makes smaller and
smaller parallax angles visible.
Click Next to find out how to calculate distance from parallax.
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