Why do astronomers try to find the distance to stars using parallax?

1 Answer
May 14, 2016

Because it is one of only a few ways of measuring distance in astronomy and the only direct method of measuring distance.


The earth orbits around the sun at a distance of 150 million kilometres, ( or 1 AU). This means that it's location changes 300 million kilometres ( or 2 AU) from January 1st to July 2nd (half a year). This change in location SLIGHTLY changes our perspective on thing like how walking though a room changes how the furniture looks, angles are different etc. The apparently location of a star, an angle, changes. We can use this angle shift, called parallax, the size of the earth's orbit and trigonometry to find the distance.

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The closest star, Proxima Centauri, has a parallax, angle shift of 0.770 arc-seconds (1/3600 of a degree), this corresponds to a distance of 4.246 lightyears or 1.3 parsec. A parsec = per second of arc and is found by taking one over the angle shift in arc-second, 1/0.77 ~= 1.3.

Because of the very small angles involve even with the best space telescope we can only measure distances directly up to 5,000 parsecs or 16300 lightyears. The galaxy is estimated to be over 100 000 lightyears in diameter.