Enjoying EarthSky? Subscribe.

126,442 subscribers and counting ...

Follow the arc of the Big Dipper to Arcturus

Use the Big Dipper to arc to Arcturus.

Tonight for April 1, 2015

Here’s a useful phrase for all of you stargazers. Scouts learn this phrase. Grandparents teach it to kids. It was one of the first sky tools I learned to use in astronomy. The phrase is: follow the arc to Arcturus and drive a spike to Spica. Arcturus and Spica are so bright that you can often see them on a moonlit night – like tonight. Follow the links below to learn more.

Follow the arc to Arcturus.

Drive a spike to Spica.

A planisphere is virtually indispensable for beginning stargazers. Order your EarthSky planisphere today.

Arc to Arcturus

View larger. | Photo taken by Janet Furlong on March 21, 2013. Thank you Janet! Big Dipper toward the center, the star Arcturus at lower right (in the trees) and the star Polaris at center left.

Follow the arc to Arcturus. Here’s how to locate the star Arcturus, using the Big Dipper as a guide. Find the Big Dipper asterism in the northeastern sky in the evening sky this month, maybe around 9 p.m. It’s very easy to see, a large noticeable dipper-shaped pattern in the northeast in the evening. Once you can see the Big Dipper, notice that it has two parts: a bowl and a handle. Then, with your mind’s eye, draw an imaginary line following the curve in the Dipper’s handle until you come to a bright orange star: follow the arc to Arcturus. Arcturus is the brightest star the constellation Bootes the Herdsman. This star is known in skylore as the Bear Guard.

Arcturus is a giant star with an estimated distance of 37 light-years. It’s special because it’s not moving with the general stream of stars in the flat disk of the Milky Way galaxy. Instead, Arcturus is cutting perpendicularly through the galaxy’s disk at a tremendous rate of speed . . . some 150 kilometers per second.

Millions of years from now this star will be lost from the view of any future inhabitants of Earth, or at least those who are earthbound and looking with the eye alone.

Drive a spike to Spica. Once you’ve followed the curve of the Big Dipper’s handle to the star Arcturus, you’re on your way to finding the star Spica. Just extend that same curve on the sky’s dome. You can read more about Spica here.

On springtime evenings in the Northern Hemisphere, extend the handle of the Big Dipper to arc to Arcturus, spike Spica and slide into the constellation Corvus the Crow. We sometimes call this extended arc the spring semicircle.

On springtime evenings in the Northern Hemisphere, extend the handle of the Big Dipper to arc to Arcturus, spike Spica and slide into the constellation Corvus the Crow. We sometimes call this extended arc the spring semicircle.

Bottom line: Use the curve in the handle of the Big Dipper to “follow the arc” to the star Arcturus. Then “drive a spike” to the star Spica. Have fun.

Get your kids interested in astronomy and the sky! Use EarthSky’s lunar calendar as a fun way to enjoy the moon phases throughout the year.