Eye Astronomy #9: Dip Into Star Hopping
by Dale E. Lehman
The best-known constellation isn’t a constellation at all. The Big Dipper is an asterism, a grouping of stars that form a picture. It’s part of the constellation Ursa Major, the Great Bear. The difference between a constellation and an asterism might seem insignificant. After all, aren’t constellations just pictures in the sky, too?
Originally, yes, but today constellations are well-defined regions of the sky. Every star in the sky lies within the boundaries of one of the 88 constellations, whether that star is part of the picture or not, whether that star is even visible to the naked eye or not.
As asterisms go, the Big Dipper is special. First, it’s big. Really big. From the tip of the handle to the rim of the bowl, it spans 25 degrees of sky. Second, it contains seven pretty bright stars. Even under suburban light pollution, you can see the whole thing. Third, it’s a circumpolar constellation, close to the north celestial pole, the point in the sky directly above Earth’s north pole. As seen from mid- to far-northern latitudes, the Big Dipper wheels around Polaris without ever setting.
If all that wasn’t special enough, the Big Dipper’s stars are aligned with several other important stars. This makes it a great starting point for “hopping” from star to star through the sky. Take a look at this:
Hopping from the Big Dipper to several key stars
This is the sky seen from my house at 9:00 PM on April 15th, tax day. (Sorry about that. It happened to be convenient.) The red arrows show how the stars of the Big Dipper lead to five other bright stars and the constellations they inhabit.
First, look at the arrow that runs downward to the left. The two stars at the end of the bowl of the Big Dipper (Merak and Dubhe) point almost directly to Polaris, the North Star. Merak and Dubhe are even called “the Pointers” because of this. Polaris is the end star in the handle of the Little Dipper, an asterism within Ursa Minor, the Little Bear.
Next, consider the curving pair of arrows running downward to the right. These arrows show that if you follow the arc of the Big Dipper’s handle, you’ll find the bright orange star Arcturus. (Arc…Arcturus. Convenient!) And if you keep going, you’ll reach Spica, the brightest star in Virgo. There’s even a mnemonic for this hop: “Follow the arc to Arcturus, then speed on to Spica.”
Back to the Dipper. Follow the arrow that runs upward to the right. The inner two stars of the Dipper’s bowl (Megrez and Phecda; don’t worry, they aren’t on the test) point roughly to Regulus, the brightest star in the constellation Leo. This line also crosses the back end of Leo Minor, the “Little Lion.” Nobody pays much attention to this cub. He’s small, dim, and contains little of interest to most amateur astronomers. But at least you know how to find him now.
The last arrow runs upward to the left. It uses the two stars at the top of the Big Dipper’s bowl (Megrez and Dubhe) to reach Capella in Auriga. Capella is the brightest yellow star in the sky. Although none of these alignments are perfect, this one is less perfect, but it’s close enough to get you there, given how bright Capella is.
This hopping from star to star (e.g., Merak to Dubhe to Polaris) is called star hopping. Really, it is. Star hopping is one of the key ways amateur astronomers find their way around the heavens. We even use it to point our telescopes sometimes. In the days before computerized “go-to” telescopes, it was the only way to find most objects of interest. Here’s one star hop we might do at this time of year:
Star hopping to the Leo Triplet
Find Regulus, the brightest star in Leo. (You can use the Big Dipper to get there, but Leo is an obvious constellation.
Hop along the bottom of Leo to Chertan. Although not so bright as Regulus, it shouldn’t be hard to find.
Drop “down” below the body of Leo (southeast) about 2.5 degrees. That’s a bit more than the width of your thumb when held at arm’s length. A trio of galaxies called the Leo Triplet lies right about there (inside the red circle in the graphic). While you can’t see the Leo Triplet with your eyes, this hop can be used to find them with a telescope.
Once you dip into star hopping, you’ll be on your way to locating stars, constellations, and hidden gems. So grab a chart, go outside on the next clear night, and have fun!
