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Use Big Dipper to find Mizar and Alcor

10mar09_430

Tonight for March 8, 2015

Tonight, find the Big Dipper – known as the Plough in the U.K. or, in Hindu astronomy Saptarishi, after the seven rishis. The chart at top shows the Big Dipper on an evening in March, ascending in the northeast during the evening hours. If you have a dark sky – and good vision – you might be able to see the faint star Alcor next to Mizar, the middle star in the Big Dipper’s Handle.

The two outer stars in the Bowl of the Big Dipper always point to Polaris.

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Still not sure the pattern you’ve found is the Dipper? Try to notice if the pattern you see matches the chart shown at right. That chart shows a well-known trick for finding the North Star, or Polaris. That is, the two outermost stars in the bowl of the Big Dipper point to Polaris. Those stars are Dubhe and Merak. They are well known among amateur astronomers as The Pointers. If you extend a line about five times the distance between Dubhe and Merak from the star Dubhe – and don’t find a medium-bright star (Polaris) – then the pattern you see is not the Big Dipper. Keep looking.

An ancient eye test for those wishing to join the Roman army involved spotting stars in the Handle of our modern-day Big Dipper. You can take this ancient eye test, too.

Go outside around 9 p.m. You should see the Big Dipper just off the northeast horizon. The middle star in the handle of the Big Dipper is Mizar. If you look for a couple seconds longer, you may see a little starry point right next to Mizar.

This second star is called Alcor. If you had lived in the time of the early Romans and could see Alcor, you would have been eligible to be an archer in the Roman army. If not, you likely would have served in another capacity for the Caesar. It’s said that sultans of the past also tested their soldiers’ eyesight in this way.

Mizar (brighter) and Alcor (fainter) as seen from Earth. Image Credit: ESO Online Digitized Sky Survey

Mizar and Alcor form a “visual double” star. At one time, astronomers doubted that Mizar and Alcor were gravitationally bound to one another. But Mizar is now thought to be four stars in one, and Alcor two stars in one. Given that Mizar and Alcor likely revolve around each other, that makes for six stars in an intricate gravitational dance.

Bottom line: As darkness falls, look for the Big Dipper and the star Mizar in the northeast sky. With either the unaided eye or binoculars, seek for Mizar’s nearby companion: Alcor.

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