The brilliant moon dominates our evening sky this week. Expect it to reach its full phase on the night of Tuesday, April 7, when you’ll see it rise in the east around sunset. Its orange disk will appear immense as it clears the horizon, but this is the result of a fascinating optical illusion known as the “moon illusion.”
As it ascends in the eastern sky, the moon’s light will obliterate all but the brightest of stars, giving us an opportunity to check out some of the brighter features of the springtime evening sky.
On that night, look to the north of the full moon and you’ll spot a bright, yellow-orange star named Arcturus. This is the fourth brightest in all the heavens, and it is also the most brilliant star north of the celestial equator. Arcturus is a red giant about 25 times larger, and about 180 times more powerful, than our sun.
Located near Ursa Major and Ursa Minor — the Great and Little Bears — Arcturus marks the constellation Boötes, the Herdsman or Bear-Driver, though most of its stars will be too faint to see under the full moon’s light.
During the 1933 Century of Progress Exposition in Chicago, astronomers at nearby Yerkes Observatory captured the light of Arcturus and used it to generate electricity that flipped a switch and illuminated the Exposition each night. Arcturus was selected because, at the time, it was believed to lie about 40 light-years from Earth, and its light would have left the star while Chicago’s Columbian Exposition of 1893 was still underway. Today, we think that Arcturus actually lies about 36.7 light-years away.
Just to the lower right of the full moon on that night, you will see another bright star; this is Spica, the brightest in the constellation Virgo, the maiden. Its name comes from the Latin word meaning “ear of grain” and, in many depictions of the constellation, the star represents a shaft of wheat.
Spica was an important star in ancient days. Around 3,200 B.C., the temple at Thebes was oriented to Spica and, in the second century B.C., Hipparchus used the star to discover the wobble of the Earth’s axis known today as precession.
We now know Spica as a blue-white star about 260 light-years from Earth; in fact, the light we see from Spica is actually the combined light from two stars that orbit one another every four days. Together, they produce about 2,200 times the luminosity of the sun.
So, how do we find these two stars when the full moon does not lie nearby? Well, that’s quite easy, because stargazers have long used the stars of the Big Dipper to point right at them.
On spring evenings, the Big Dipper stands on its handle in the northeastern sky not long after dark. To use it to locate these stars, simply follow the curve of the Dipper’s handle outward. We say, “Follow the arc to Arcturus, then speed on to Spica.”
The Dipper, Arcturus and Spica will appear higher in the sky each evening for the next few months, and will offer some beautiful sights throughout the summer months.
— Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at mammana@skyscapes.com and follow him on Twitter: @dennismammana. The opinions expressed are his own.