July 2007

by Deane Morrison

"Planets at play" could be the theme of July's sky show, except that stars also play some pretty boffo supporting roles. But there will be no warm-up act, so hurry up and take your seat for the main event right at nightfall on the 1st.

If you look to the west about an hour and a half after sunset that evening, you'll see bright Venus above the horizon. That other evening "star" close to Venus is Saturn, passing by on its journey into the sun's afterglow. To the upper left of Venus, the bright star Regulus, in Leo, closes in. Pretty as this grouping will be, however, the view on the 16th may beat it. On that evening, Regulus will be closest to Venus, and to the right you'll see a young crescent moon with Saturn just behind its curve.

By the end of the month, Venus, Saturn, and Regulus will all have sunk quite low in the west, making way for the next act, in the east. That one stars Jupiter, which Earth, the speedier planet, passed in June in the race around the sun.

Jupiter comes out as a beacon above the sinuous form of Scorpius in the south-southeast. Compare Jupiter to its temporary neighbor, the bright red star Antares, marking the heart of the scorpion. Antares, whose name means "rival of Mars," has only about 15 times the mass of our sun. But it's an old star entering its death throes, a process that has caused it to swell up to 700 times the diameter of the sun. Antares could, in fact, explode in a supernova any time.

Further east, the Teapot of Sagittarius eternally chases the tail of Scorpius, tipping its spout as if to pour its contents on the scorpion's stinger. Above the Teapot, the tiny Teaspoon of stars hangs unobtrusively.

High in the south, the bright star Arcturus carries its kite-shaped constellation, Bootes, across the sky. Arcturus orbits the center of the Milky Way galaxy in a peculiar path that crosses the disk where most stars, including the sun, are found. It's possible that Arcturus does this because it originated in another galaxy that was cannibalized by the Milky Way. Such mergers are common; our galaxy is currently in various stages of swallowing its close companions, among them the dwarf galaxies known as the Large and Small Magellanic Clouds. Visible from the Southern Hemisphere, these galaxies were discovered by 16th-century Portuguese explorer Ferdinand Magellan and his crew. In 1987 the Large Magellanic Cloud played host to the closest observed supernova explosion since 1604.

East of Bootes, the Summer Triangle of Vega, Deneb and Altair wheels into prominence. The Milky Way runs through the Triangle, extending from Cassiopeia in the northeast down to Sagittarius and Scorpius.

Mars is high in the east at dawn. Still brightening, the Red Planet moves closer to the Pleiades star cluster all month.

The full moon of July was known to Algonquin Indians as the thunder moon, since thunderstorms are now most frequent. It was also called the buck moon, because at this time of year the first velvety points of antlers begin to emerge from the heads of buck deer. The full moon arrives at 7:48 p.m. CDT on the 30th. The moon won't rise until a little more than an hour later, but when it does, it will be the kind of round, softly lit orb that sends poets scurrying for their pens--or, nowadays, their iPods.