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Follow Big Dipper’s arc to Arcturus

Use the Big Dipper to arc to Arcturus.

Tonight for April 1, 2015

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.

Drive a spike to Spica.

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Arc to Arcturus

View larger. | Photo taken by Janet Furlong in March, 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.

From the western half of North America, the short-lived total lunar eclipse sits low in the west before sunrise on Saturday,  April 4. Be sure to find an unobstructed western horizon. Click for an eclipse calculator.

For western North America, the moon will be near the star Spica when it undergoes a total eclipse on Saturday morning, April 4. Read more about the April 4, 2015 eclipse. For Australia and Asia, this eclipse will be visible Saturday evening

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.

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