Jupiter and Saturn are the jewels of the sky this month. Not only are both approaching optimum brightness, they're also picturesquely located near the Hyades and Pleiades (Seven Sisters) star clusters and Aldebaran, the bright orange-red star in the constellation Taurus. A gibbous, or hunchbacked, Moon joins them this week, making for an irresistible photo opportunity. Saturn rises around 10 p.m., and Jupiter follows a half-hour later.
Saturn is the second largest of the planets, a largely gaseous body almost ten times the size of Earth. It has a very low density — so low, in fact, that it would float in a body of water if one big enough existed. A person standing on Saturn's cloud tops would weigh only 92 percent of what he or she weighs on Earth.
Saturn lies at a 24-degree tilt relative to Earth right now, and the rings appear at their broadest. Anyone with access to a telescope with a magnification power of at least 50 should not miss this chance to view the rings of rock, ice, and dust particles that encircle the planet. The two main rings are divided by a dark area named Cassini's division after its discoverer, Gian Domenico Cassini. Although they're hundreds of thousands of miles wide, the rings are probably less than 100 yards thick. Their color variations reflect their different chemical compositions.
Train the lens on Jupiter, the largest of the planets, to see the colored bands of its atmosphere and even the Great Red Spot, a storm covering an area larger than Earth that's been raging for more than 300 years. The bright colors in Jupiter's atmosphere, composed mainly of hydrogen and helium, come from tiny amounts of such trace gases as methane and ammonia, which show different colors at different temperatures. Red, brown, yellow, and even slightly bluish tones can be seen. Since Jupiter spins completely in just under ten hours, a full night's viewing will reveal nearly all of the planet.
Planets of the Week
Venus rises after sunrise and sets after sunset this week, so it's in the sky mainly during the day. Watch the southwestern sky after the sun goes down to see if you can spot the Evening Star. It sets when the sky is still fairly bright and thus visibility is poor, but it will put on a blazing evening show later in the fall. Mercury is appearing in the evening sky as well, but given its position just above the horizon and the brightness of the surrounding sky, it's difficult to discern. Mars can be seen peeking above the horizon in the very early morning, rising at about 4:30 a.m. in the constellation Leo, the Lion.
Neptune, Uranus, and Pluto reached optimal viewing positions this summer. Pluto, difficult to see even with a mid-sized telescope, lies in the constellation Ophiuchus. Neptune is quite a bit brighter, and Uranus is brightest of the three, but both must be viewed with telescopes. Neptune and Uranus can be found in the constellation Capricornus.
At 9:00 p.m. on September evenings, the Big Dipper, an asterism, or star shape, within the constellation Ursa Major (the Great Bear), sits quite low in the north. The bowl of the Little Dipper, in Ursa Minor (the Little Bear), can be found upside down above it. Trace the Little Dipper's handle to its tip to locate Polaris, the North Star. It marks the northern pole of the celestial sphere and is the extension of Earth's North Pole into space.
The Big Dipper's handle arcs toward Arcturus, the big orange alpha (or brightest) star in the constellation Bootes, the Herdsman, just setting on the northwestern horizon. Look for the pretty half-circle of Corona Borealis, the Northern Crown, almost due west, above Arcturus and just to the left of Bootes. Above Corona Borealis is Hercules and, higher still, Vega, the constellation Lyra's bright blue-white alpha star. Vega marks one corner of the Summer Triangle, made up of three bright stars from three different constellations. Another corner, Deneb, the alpha star in Cygnus (the Swan), is almost directly overhead.
In the northeastern quadrant of the sky, Andromeda, the Princess, stretches her way toward the horizon from the northeastern star of the Great Square of Pegasus, the Winged Horse. Perseus, the Hero, who saved Andromeda from Cetus, the Sea Monster, lies below her and to the left. On a very dark, clear night you might be able to make out a fuzzy patch of light above the middle of the two lines of stars that form Andromeda; this is the Great Andromeda Galaxy, the most distant object the eye can see unaided by binoculars or a telescope. Andromeda's mother, Queen Cassiopeia, above and to her left, now looks like a number 3, while her father, King Cepheus, is high in the northern sky. Just rising in the northeast is Capella, the alpha star in the constellation Auriga, the Charioteer. Capella is the sixth brightest star in the sky and has long been important in navigation.
Look due south to see Capricornus and Delphinus at their most visible. Capricornus, a rather faint assemblage of stars, represents an odd creature called the Sea Goat, while Delphinus appears as a charming little dolphin arcing over the waves. The southern portion of the sky is occupied by many constellations associated with water. In addition to the two aforementioned, Piscis Austrinus, the Southern Fish, and Aquarius, the Water Bearer, can be found here. Farther around to the east are Cetus, the Sea Monster, and Pisces, the Fish. The only bright star in the southeast is Fomalhaut, the alpha star in Piscis Austrinus, close to the horizon.
Halfway up the southwestern part of the sky is Altair, the alpha star in the constellation Aquila, the Eagle. It marks the third corner of the Summer Triangle. The brightest corner, Vega, is in the west. Full darkness brings the full glory of the Milky Way, arching across the sky from the south to the northeast, passing right behind the Summer Triangle. Scan its glowing star clouds with binoculars or a telescope to see star clusters and nebulas by the dozens. Sagittarius, lying toward our galaxy's center, is low in the southwestern sky; it will soon set, not to be seen again until late next spring.