It’s autumn here in the Northern Hemisphere. There’s a chill in the air, and nights are getting long. Maybe you’ve been standing outside on an autumn evening, looking for the Big Dipper? It’s perhaps the most famous of all star patterns, and – for those at latitudes 41 degrees north or farther north – it’s circumpolar, or always above the northern horizon. If you’re below that latitude, though, you won’t find the Big Dipper in the evening now. In autumn, the Big Dipper is below your horizon during the evening hours.
Want to see it? If you’re in the southern U.S. or a comparable latitude, you’ll want to wait until the hours before dawn. At this time of year, before dawn, you’ll easily see the Big Dipper ascending in the northeast.
To remember the best times to view the Big Dipper in the evening, remember the phrase: spring up and fall down. That’s because the Big Dipper shines way up high in the sky on spring evenings but close to the horizon on autumn evenings.
So you might or might not be able to see the Big Dipper now. But you can think about it. Did you know that 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.
Five of the Big Dipper’s stars – 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. 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
What’s more, Dubhe and Alkaid are moving in an entirely different direction from the other five stars.
And that’s why – millions of years from now – the Big Dipper will have lost its familiar dipper-like shape.
Bottom line: If you’re above 41 degrees north latitude, the Big Dipper star pattern is circumpolar; it stays in your sky always, circling around the around the sky’s pole star, Polaris. Below that latitude, the Dipper is below your horizon in the evening in autumn.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.