Friday, June 5, 2009

As the Crow Flies

More so than any other star pattern, when I see the defining asterism (recognizable star pattern) of Corvus the Crow flying high in the night sky after sunset, I know that it’s spring.

Corvus (KORR-vuss) is the Latin word for crow or raven. Although it’s hard for us to imagine a crow among the sparse scattering of stars in Corvus, we can see in classical star atlases how the ancients imagined the raucous bird: bending over to peck at Hydra the Water Snake, the constellation that winds below Corvus.

Corvus from Johann Bode's 1801 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Let’s go outside and stretch our wings.

1) About an hour after your local sunset time, face south. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

Star maps created with Your Sky

2) You’ll find the constellation of Corvus low in the southern sky, just southwest of the brilliant blue-white star Spica (SPY-kuh) in the constellation Virgo the Maiden. If you don’t know how to locate Spica, read this post.

The five brightest stars in Corvus form the small but distinctive asterism known as the Sail. It looks a bit like the wedge-shaped sail of a Chinese junk, with a bit of mast extending below.

Chinese Junk
Image by Wibean

3) The brightest of the four stars that make up the sail shape is the blue-white giant Gienah. Gienah (JENN-uh) is from the Arabic for wing. Moving counterclockwise around the sail shape, we next come to Algorab. Algorab (ALL-gorr-abb) is from the Arabic for raven. Algorab is a binary system, two stars in orbit around each other, although with the naked eye we see their combined light as one star. Algorab’s component stars are a bright white star and a dim orange one; you should be able to see both with even a modest-sized telescope.

The stars of the Sail asterism in Corvus the Crow

Next in line is the yellow-white giant star Kraz. The meaning of its name is unknown. Finally, where the mast meets the sail is the orange giant Minkar (MINN-kahr), from the Arabic for beak.

Dimmer than the four sail stars, the yellow-white dwarf Alchiba (ull-kibb-AH) marks the bottom of the mast. Alchiba is from the Arabic for tent, a harkening to an earlier Arab tradition that saw the star pattern of Corvus not as a bird but as a tent, important shelter for a desert-dwelling people.

Astronomy Essential: The distance from the Earth to the Sun is about 93 million miles.

Because the Earth’s orbit is elliptical, not circular, its distance from the Sun varies a bit over the course of Earth’s year-long orbit. The average distance, however, is about 93 million miles or 150 million kilometers.

Astronomers use a mathematically calculated constant based on the distance between Earth and Sun and call it one astronomical unit or 1.0 AU. They use this as a unit of measure between objects in our solar system. One astronomical unit works out to be slightly less than the average distance between Earth and Sun; it is equivalent to 92,955,807 miles or 149,597,870 kilometers.

Due to the long distances involved even within our own solar system, expressing distances of objects in miles or kilometers quickly becomes cumbersome, making the AU a compact and useful unit of measurement. We can say, for example, that today Neptune is 29.68 AU’s from Earth. Otherwise, we’d have to say it’s 2,758,928,346.9 miles or 4,440,064,781.6 kilometers from Earth. Either alternative is quite a mouthful.