In the fall, the constellation Pegasus, winged horse of Greek mythology, is easy to locate. If you find it hard to imagine connecting the stars to trace out the horse, look for the highly recognizable asterism associated with Pegasus called the Great Square. At southern latitudes, by about 10 P.M. in early October, it should be directly above you. The four stars marking out its four corners aren’t terribly bright, but the other stars in that area of the sky are fairly dim, so the figure should stand out clearly. The eastern side of Great Square also coincides with the 0 marking from which the hours of right ascension start, increasing to the east. Some 20 degrees west and 5 degrees south of Markab, the star that marks the Great Square’s southwest corner, is Enif, the brightest star in Pegasus. Its name means “the horse’s mouth,” and between Markab and Enif is the horse’s neck. Look to the Great Square’s northeast corner for the star Alpheratz, which is not part of Pegasus, but part of Andromeda, the Maiden in Chains. If you trace a line from Alpheratz through Markab, continuing about 40 degrees southwest of Markab, you’ll find the zodiacal constellation Capricornus, Capricorn, or the Sea Goat. Capricorn is distinguished by its brightest star, the brilliant Deneb Algiedi. Return to the Great Square. About 20 degrees east of it, you’ll find another zodiacal constellation, Aries, the Ram. This grouping is easy to identify, since it is marked by two fairly bright stars a mere 5 degrees apart. Last to rise in the sky of fall is Perseus, slayer of snake-haired Medusa and other monsters of Greek mythology. About 45 degrees up in the northeast, it lies across the Milky Way and is marked by its brightest star, Mirfak.

Winter Skies

Winter nights, with the bright arc of the Milky Way overhead, offer more bright stars than are visible at any other season: Sirius, Capella, Rigel, Procyon, Aldebaran, Betelgeuse, Pollux, and Castor. Brightest and most readily recognizable of the winter constellations is Orion, the Hunter, which spans the celestial equator and sports the heavens’ second most familiar asterism (after the Big Dipper): Orion’s Belt, three closely spaced bright stars in a line 3 degrees long. The star Rigel, brightest in the Orion constellation, marks the hunter’s foot, 10 degrees below and to the west of Orion’s Belt. About the same distance and direction above the Belt is Betelgeuse, a reddish star, whose name is Arabic for “armpit of the giant.” And that is precisely what Betelgeuse marks: Orion’s armpit. If you look at the winter star chart on the tear-out card, you’ll also see Bellatrix, which marks the shoulder of Orion’s arm holding his shield, which is an arc of closely spaced, albeit dim stars. Suspended from Orion’s Belt is a short sword, the middle “star” of which is actually a region where stars are being born. (We will discuss the Orion nebula and other regions like it in Article 12, “Solar System Family Snapshot.”) Saiph is Orion’s eastern leg. About 15 degrees to the southeast of this star is Sirius, called the Dog Star, because it is in the constellation Canis Major, the Great Dog. Sirius is the brightest star in the heavens. To the northeast of Orion you will readily see a pair of bright stars close together. These are Castor and Pollux, the Twins, which represent the two heads of the constellation Gemini. Moving in an arc to the northwest of Castor and Pollux, you should see another bright star, this one with a distinctly yellow-gold color. It is called Capella, which means “little she-goat,” and the ancients thought the star was the color of a goat’s eye. Capella is in the constellation Auriga, the Goatherd. Return to Orion. Just to the northwest of his shield, you will find Taurus, the bull, which is marked by Aldebaran, a bright orange star that forms the constellation’s bull’s eye. Early sky watchers imagined Taurus eternally charging the shield of Orion, who stood eternally poised to strike the animal with his upraised club. It is admittedly difficult to imagine the bull in Taurus, though you may at least be able to discern a V-shaped asterism called the Hyades, which is the bull’s mouth. To the northwest of this feature are the Pleiades, or Seven Sisters, a strikingly beautiful cluster of seven stars that are part of an open cluster.

Who Cares?

Enjoy the constellations. The pleasures of getting to know them can occupy a lifetime, and it’s a lot of fun pointing them out to your friends, as well as to sons and daughters. You will also find familiarity with them useful for quickly navigating the heavens. But you won’t be hearing a lot more about the constellations in this article. Recognizing them as the products of human fantasy and not the design of the universe, modern astronomy has little use for them. The Least You Need to Know

For the ancients, even without telescopes, the night sky was a source of great fascination, which we can share.

To view the sky meaningfully, you need a system of orienting yourself and identifying certain key features. Celestial coordinates and altazimuth coordinates offer two such systems. While the celestial coordinate system is how professional astronomers designate location, altazimuth coordinates are used on telescopes that have an altazimuth mount.

Astronomers use angular size and angular distance to describe the apparent sizes and separations of objects in the sky.

Constellations are imaginative groupings of stars perceived as images, many of them influenced by Greek mythology; however, these groupings are arbitrary, reflecting human imagination rather than any actual relationships between those stars.

Constellations are useful as celestial landmarks to help orient your observations.