Tonight … learn and use the most useful star mnemonic you’ll ever encounter. It’s … follow the arc to Arcturus and drive a spike to Spica.
Scouts learn this phrase. Grandparents teach it to kids. It was one of the first sky tools I learned to use in astronomy. 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. 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.
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.