UMN News: Starwatch: September 2008

UMN NEWS

Like a bridge over starry water, the September Milky Way arches above autumn’s aquatic constellations as they flow into the sky.

That isn’t to say that Capricornus, Aquarius, Delphinus, Pisces and Piscis Austrinus are spectacular; in fact, they are all rather on the watery side. But for the devoted fan of the night sky, that just means all the more fun in finding them.

Try starting with the bright star Fomalhaut, the mouth of Piscis Austrinus (the Southern Fish), glimmering low in the southeast. Moving from west to east above the fish, we find chevron-shaped Capricornus (the Sea Goat), then the Y-shaped Water Jar of Aquarius.

In Greek mythology, Aquarius was Ganymede, a handsome Trojan boy whom Zeus plucked from his family and made cupbearer to the gods. Today, Ganymede is the name of the largest moon in our solar system. One of four satellites of Jupiter discovered by Galileo, it has a diameter of 3,200 miles and is larger—albeit much less massive—than the planet Mercury.

East of the Water Jar, the round Circlet of Pisces sits below the Great Square of Pegasus. West of the Great Square, near the edge of the Milky Way, the dolphin Delphinus leaps into a dark sea. Next to Delphinus, the bright Summer Triangle of Vega, Deneb and Altair rides high.

Planets are still on the scarce side, but below the Triangle, mighty Jupiter beams above the Teapot of Sagittarius as it dips its spout toward the horizon. Jupiter is the only bright planet visible for much of the month, but Venus is starting to gain altitude as an “evening star.”

Not to be outdone, Saturn, after passing behind the sun on the 4th, begins an ascent into the morning sky. As it climbs out of the sun’s foreglow, Saturn brightens, but not as much as it might. The planet owes a good deal of its shine to the luminous, grooved loveliness of its rings, but that source is fading as the angle of their dip straightens out. In less than a year the rings will appear edge-on and at their thinnest.

It will be fairly easy to find Saturn in the eastern sky during the hour before sunrise late in the month. On the 26th, a waning crescent moon will be paired with the bright star Regulus like a celestial wink. Look for Saturn about 15 degrees to the lower left of Regulus. The next morning an even wispier old moon will repeat its performance close to the planet.

On the 19th, a waning moon rises close to the Pleiades star cluster and we may see one or more of its stars eclipsed by the moon. Admittedly, it will be hard to see a star, already competing with the moonlight, disappear behind the moon’s bright leading edge. But with binoculars or a small telescope, it may be worthwhile to watch those stars pop into view again in the wake of the moon’s dark trailing edge.

The harvest moon shines the night of the 14th-15th, reaching fullness at 4:13 a.m. The closest full moon to the fall equinox, the harvest moon gets its name by rising only about 20 minutes later from night to night, allowing nearly continuous light for farmers working late to bring in their crops. This is especially helpful for a night or two after full moon, when moonrise occurs in twilight. In comparison, near the spring equinox, the moon rises about 70 minutes later each night around the time of fullness.

The fall equinox ends our summer at 10:44 a.m. on the 22nd. At that moment the sun crosses the equator into the southern sky and the Earth’s sunward face is lit from pole to pole.

The time near the fall equinox is when the sun drops most rapidly through the sky. In the Twin Cities area, we lose about three minutes of daylight per day. But if this seems fast, consider the residents of northern lands like Iceland. Every day the encroaching darkness steals six minutes of their daylight. In the 10 days surrounding the equinox, a whopping hour and four minutes disappears.

No comments yet

Leave a Reply