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Dipper points to Polaris, plus see Mizar and Alcor


Tonight for August 9, 2015

The two outer stars in the bowl of the Big Dipper – Dubhe and Merak – always point to Polaris, the North Star. To find this Dipper at this time of year, look toward the northwest in the evening.

And once you’ve found it – after locating Polaris – look more carefully at the second star from the end of the Big Dipper’s handle. If your sky is dark enough, and your eyesight is good, you’ll see that this star, Mizar, has a nearby companion, called Alcor. But you probably won’t see Alcor tonight, because the bright waning gibbous moon will rise at early evening. Alcor should be visible through binoculars, however. In a few more days, the moon won’t be in the early evening sky, enabling you to see this faint star, and also the fainter stars of the Little Dipper with the unaided eye – that is, if you have a dark sky.

Big and Little Dippers: Noticeable in northern sky

Mizar and Alcor, famous double star, are really six stars

Arabian stargazers referred to Mizar and Alcor as the “horse and rider.” These stars are a good test of the night’s viewing conditions: if you can’t see Alcor, there might be thin clouds up there.

These two stars are what’s called “naked-eye double star,” appearing double from our earthly vantage point. But do they orbit each other? Astronomers aren’t sure. The distances to these stars (as to most stars) aren’t precisely known. If Mizar and Alcor make up a true binary star, it’s a very wide one. If they do lie at the same distance from Earth, their separation is 0.27 light-years … that’s in contrast to eight light-minutes for Earth’s distance from our sun … or several light-hours for the distance to our sun of the most distant worlds in our solar system. Still, it’s possible that Mizar and Alcor could be this far apart and still be orbiting one another, with a very long orbital period of three-quarters of a million years.

EarthSky’s meteor shower guide for 2013