Arc to Arcturus, speed to Spica
Arc to Arcturus, and speed to Spica. Scouts learn this phrase. Grandparents teach it to kids. It’s one of the first sky tools many learn to use in astronomy.
What’s more, all of these stars are bright enough to be view from most suburbs and small cities. After all, Spica serves as a perfect example of a 1st-magnitude star, that is, one of our sky’s brightest stars, on a brightness scale used by the early astronomers Hipparchus (c.190 – c.120 BCE) and Ptolemy (c.100 – c.170 CE).
And the star Arcturus beams brighter yet, shining one magnitude (2 1/2 times) more brightly than Spica.
Arc to Arcturus.
Find the Big Dipper asterism in the northeastern sky in the evening this month, maybe around 9 p.m. It’s very easy to see, a large noticeable dipper-shaped pattern in the northeast. Once you see the Big Dipper, notice it has two parts: a bowl and a handle. Then, with your mind’s eye, extend the natural curve in the Dipper’s handle until you come to a bright orange star: Arc to Arcturus!
Arcturus is the brightest star in the constellation Boötes the Herdsman. This star is known in skylore as the Bear Guard.
Speed on 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 bright, blue-white star Spica. Just extend that same curve on the sky’s dome.
Spica is the brightest light in Virgo the Maiden, a large, rambling constellation. The star and its constellation were sometimes associated with the Greek goddess of the harvest, Ceres. Spica is from the Latin word for ear, and the general connotation is that this star name refers to an ear of wheat, held by the goddess. Less frequently, Spica is translated simply as spike, and it can also mean javelin, spear or dart.
Today we know Spica as a tight double star. The two stars are indistinguishable from a single point of light via ordinary telescopes. Their dual nature was revealed only by analysis of the light from this system via a spectroscope, or instrument that splits light into its component colors. Both stars in the Spica binary system are larger and hotter than our sun. Their diameters are estimated to be 7.8 and 4 times the sun’s diameter, and, taken together, they’re more than 2,000 times brighter than the sun!
Separated by just less than 11 million miles (18 million km), Spica’s two stars orbit a common center of gravity in only four days.
Bottom line: Remember … arc to Arcturus and drive a spike into Spica! Have fun.