I got an e-mail the other day from an elderly woman named Leona. From her apartment’s west window she watched a “satellite” travel to the north night after night before she went to bed around 9:30-10 o’clock. Sometimes it was bright, other times fainter, and once in a while it jiggled about. Not many satellites are out that late in the winter, and they rarely take the same path every night. Hmm … was it a plane?
“No, I’d know it was a plane, with those blinking lights,” she said. Its northward path could make it a spy or reconnaissance satellite, but it wouldn’t be out the same time every night. Then it hit me. Jupiter! It’s in the west around 9 and creeps northward as it sinks toward the horizon. We were both tickled to figure out the answer.
You’ve probably noticed that Jupiter has been drifting to the west over the past month. While still high in the southwest shortly after sunset, it’s already halfway to setting by dinnertime. Earth’s incessant orbital motion around the sun means that we’re leaving Jupiter behind; each night it drifts one degree farther to the west and sets four minutes earlier. As long as the planet’s still easy to find, let’s use it for last looks at three of fall’s best-known constellations: the Great Square of Pegasus, Andromeda the Chained Princess and Aries the Ram.
Start your observing session around 7 o’clock by facing southwest into the eye of Jupiter. One fist above and to the right of the planet is the most obvious part of Pegasus the Winged Horse – the Great Square. It’s a big, open box bounded on four corners by stars of similar brightness to those in the Big Dipper. The uppermost star actually belongs to Andromeda but is informally considered part of the Square.
Andromeda begins at the top of the Square and looks like a “two-fist” tall carrot with a narrow bottom and wider top. Depending on how dark your sky is, you might be able to spot the Andromeda Galaxy “two fingers” or some 5 degrees to the right of Beta with your naked eye. Look for a softly-glowing fuzzy spot. A pair of binoculars will show the spiral galaxy, located some 2.5 million light years away, with ease. If you look closely, you can distinguish its brighter core called the nuclear bulge, where most of the galaxy’s older stars are concentrated, from the fainter disk, home to its spiral arms and younger stars.
Returning to Beta, look a fist to the left of the star to find the small, 3-star figure of Aries. Small telescope owners should definitely check out Gamma Arietis, the faintest of the three. To the naked eye it’s a solitary star, but a small telescope will show two side-by-side, pure white stars like a pair of beaming headlights. Gamma is a double star 204 light years away whose companions orbit about one another with a period of at least 5000 years.
Gamma’s also known as Mesarthim (mess-AHR-tim), and while inconspicuous to the eye, it’s famous as “the first star of Aries”. Back in ancient times, Mesarthim was the star closest to the vernal equinox, the spot the sun occupies on the first day of spring. The Babylonians started their calendar year with the sun at the vernal equinox, making Aries the first “sign” of the zodiac. Because of the slow wobble of the Earth’s axis called precession, that location has since slid westward into Pisces.
Mesarthim was a rock star in its prime. These days it’s successfully made the transition to a second career as double star. This weekend we’ll look at another cluster of constellations centered on Orion.