Observing Highlights for September: The Harvest Moon and Opposition of Uranus

In early August, Mars, Saturn and Spica formed a triangle. Credit: John Chumack

The Harvest Moon, which is defined as the Full Moon closest to the autumnal equinox, can occur in September or October. Most years the Harvest Moon is in September, as it is in 2012. The equinox, and therefore the first day of fall, lands on September 22 at 7:49 a.m. Pacific Time. With days and nights of equal length, the sun rises directly in the east and sets directly in the west. The closest Full Moon to this date is on September 29. The Harvest Moon rises in the east just before it hits 100 percent illumination at 8:19 p.m. pacific.

Moonrise in September occurs as little as a half hour apart from night to night as opposed to other times of the year when moonrise from one night to the next can fall more than an hour apart. The full and nearly full moons after sunset in September provided the extra light for farmers in the fields and earned this moon the name of Harvest Moon.

The planet Uranus reaches opposition, or opposite the sun in the sky when it rises at sunset and sets at sunrise, on the same night as the Full Moon, September 29. Normally opposition is considered the best time to observe a planet, but when it occurs simultaneously with a Full Moon in the same region of sky, this is not the case. The moon will be just four degrees above Uranus, washing out the sky around the dim planet.

Saturn and Mars are still keeping close quarters in the west after sunset, but Saturn sinks toward the horizon as Mars speeds away. By September 4, Mars is already leaving the constellation Virgo as it crosses the border into Libra.

The constellation Libra is home to two stars with the fun-to-pronounce names of Zubenelgenubi and Zubeneschamali, or the southern and northern claw. Libra was once part of the constellation Scorpius the Scorpion, thus the claw reference. On September 14, Mars will be just below Zubenelgenubi, the lower of the two stars that make up the triangular roof-shape of Libra (which looks a bit like a ramshackle house). Through binoculars, you’ll see that Zubenelgenubi is a double star system, with the brighter component at magnitude 2.7 and the dimmer at 5.1.

On September 17, a crescent moon passes below Spica and Saturn and then skims about two degrees left of Mars on September 19. Mars will continue heading east across the constellations, entering the Milky Way in October.

With the onset of autumn, the season’s constellations will be rising in the east in the evening. The hallmarks of fall include Pegasus with its Great Square, Andromeda with its famous galaxy, Perseus with the Double Cluster, and Auriga with the sparkling star Capella.

For those up late on September evenings, look for the Pleiades cluster rising with the planet Jupiter just behind. Late at night on September 7, a half moon lies less than three degrees from Jupiter. The orangish star Aldebaran is to the right of the moon along with the Hyades cluster.

Kelly Kizer Whitt loves clean, clear, and dark skies. Kelly studied English and Astronomy at the University of Wisconsin-Madison and worked for Astronomy magazine. She writes the SkyGuide for AstronomyToday.com. You can follow her on Twitter at twitter.com/Astronomommy.