Jupiter and Saturn continue to shine in the late-night sky, appearing as very big and bright stars in the constellation Taurus, the Bull. The Moon is reduced to a thin crescent this week, reaching new phase on the 28th, so the skies will be good and dark for planetary viewing. Saturn rises at about 9:30 p.m., and Jupiter follows within half an hour.
Jupiter and Saturn are the two largest planets in the solar system and have extensive arrays of satellites orbiting them. Some of their moons are large enough to be seen with amateur equipment. Our solar system's largest satellites are (in order from largest to smallest): Saturn's Titan, Jupiter's Ganymede, Callisto, and Io, Earth's Moon, Jupiter's Europa, and Neptune's Triton. All are larger than the planet Pluto, and the two largest exceed even Mercury in size.
Jupiter has at least seventeen moons. Its four largest are called the Galilean satellites, for they were among the first objects discovered by Galileo with his newly invented astronomical telescope in 1610. Io is closest to Jupiter, at 262,000 miles away, followed by Europa, Ganymede, and Callisto, which lies some 1.17 million miles from its parent planet — more than the distance from Earth to the Sun! All of the Galilean satellites can be viewed with binoculars or a small telescope. If you check them periodically over a span of several hours, you can see their positions changing as they orbit the giant planet.
With at least eighteen satellites, Saturn resembles, like Jupiter, a miniature solar system. Many of Saturn's moons are very small, and a few of them, known as shepherd satellites, help keep the rings in shape. The largest, aptly named Titan, after the race of giants in Greek mythology, has a solid rock and ice core surrounded by a thick smoggy atmosphere. Titan and another moon, Rhea, can be seen with amateur telescopes. Because of Saturn's greater distance, though, they can't be seen as easily and readily as Jupiter's large moons.
Planets of the Week
Venus continues to rise after sunrise and set just after sunset this week. Watch the southwestern sky after the sun goes down to see if you can spot the Evening Star. If you have trouble finding it, have patience: it will put on a dazzling evening show later in the fall. Mercury makes a brief appearance in the evening sky as well, but given its position just above the horizon and the brightness of the surrounding sky, it too is difficult to make out. 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 with binoculars and a pair of sharp eyes — or a small telescope.
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.