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Tonight … can you find the Big Dipper?


Tonight for October 3, 2015

Tonight … can you find the Big Dipper at nightfall and early evening? As seen from the Northern Hemisphere, this most famous of star patterns – the Big Dipper – lurks low in the northwest after sunset and quickly sinks below the horizon for those at southerly latitudes. It’s tough (or impossible) to spot the Big Dipper over the horizon on autumn evenings from the southern half of the united States. But the pattern is visible all night from northerly latitudes, albeit low in the sky. And, before dawn around now, we’ll all find the Big Dipper ascending in the northeast.

To find the Big Dipper’s place in the sky, remember the phrase: spring up and fall down. That’s because the Big Dipper shines way high in the sky on spring evenings but close to the horizon in autumn.

The Big Dipper in the northwest after sunset on an autumn evening. As night passes – if you’re at a southerly latitude – the Dipper will be traveling below your northern horizon for much of the night during the autumn months. This beautiful photo from EarthSky Facebook friend John Michael Mizzi on the island of Gozo, south of Italy. Thank you, John Michael. Click here to expand image.

The distances of the stars in the Dipper reveal something interesting about them: five of these seven stars have a physical relationship in space. That’s not always true of patterns on our sky’s dome. Most star patterns are made up of unrelated stars at vastly different distances.

But Merak, Mizar, Alioth, Megrez and Phecda are part of a single star grouping. They probably were born together from a single cloud of gas and dust, and they’re still moving together as a family.

The other two stars in the Dipper – Dubhe and Alkaid – are unrelated to each other and to the other five. They are moving in an entirely different direction. Thus millions of years from now the Big Dipper will have lost its familiar dipper-like shape.

The Big Dipper makes up a part of the Ursa Major or Big Bear constellation. Image criedit: Old Book Art Image Gallery

Here are the star distances to the Dipper’s stars:

Alkaid 101 light-years
Mizar 78 light-years
Alioth 81 light-years
Megrez 81 light-years
Phecda 84 light-years
Dubhe 124 light-years
Merak 79 light-years

Bottom line: The Big Dipper is low in the northwest after sunset on autumn evenings. It’s below the northern horizon for southerly latitudes throughout much of the night in the autumn. Then, before dawn, the Big Dipper may be found ascending in the northeast.

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