September's Observing Highlights: Elusive Mercury and the Harvest Moon
Fall's cooler evenings arrive with a cluster of planets in the west at twilight. Venus, Mercury, and Saturn are joined by the moon early in the month. Mercury was a morning object in August but has edged into the evening sky for September.
Venus is a brilliant magnitude -4.0 and looks like a plane with its landing lights on except that it doesn’t move. By September 5, Venus is a degree and a half above the star Spica. To Venus's upper left is the Ringed Planet Saturn, and to Venus's lower right lies Mercury. Because Mercury is so low and in the fading light of sunset, it may take a while before you can spot it. On September 7, a line consisting of Saturn, Venus, Spica, and Mercury spans 40 degrees from the southwest to the west. In addition, a thin crescent moon can be found below this line. The recently set sun will make it difficult to catch all the objects, however.
On September 8, catch a pretty photo-op when the crescent moon lies two degrees to the left of Venus, with Spica almost four degrees to Venus’s lower right. The next night the moon is wide left of Saturn. Saturn and Venus have been closing in on each other and will get to within three and a half degrees on September 17. Saturn is in the constellation Libra above Venus, while Venus lies in the constellation Virgo. On the next night, September 18, the planets keep their distance but Venus crosses the border to join Saturn in Libra. Following this date the planets start to drift apart.
Another chance to catch Mercury is on September 24 when Mercury will be less than a degree above Spica in the west-southwest. Mercury shines brighter at magnitude -0.1 and Spica shines at magnitude 1.0. Binoculars may show them both in the twilight.
Early risers can catch Mars in the Beehive Cluster in the east on both September 8 and 9. The Beehive Cluster in Cancer appears dimly as a patch of fuzzy stars without optical aid, but a small telescope can show about 40 of the more than 1000 stars. Use binoculars to get a good view of the reddish planet floating in front of the grouping of stars. Also in predawn skies on September 29, the moon hovers just below Jupiter.
The annual favorite, the Harvest Moon, occurs on September 19, with the moon reaching peak fullness at 4:13 a.m. PDT. At this time of year, the moon rises only about 30 minutes later than it did the night before, which is in contrast to nearly an hour difference on other nights of the year. This is because the ecliptic (and path of the moon) is very shallow to the horizon in autumn for the Northern Hemisphere. Therefore the Harvest Moon is stunning during full but also for the days surrounding full.
The Harvest Moon is not always in September. Occasionally the Harvest Moon occurs in October, because it’s the Full Moon that occurs closest to the autumnal equinox. In 2013 the Harvest Moon occurs on September 19 because the equinox is just days later, on September 22 at 1:44 p.m. PDT.
(Photo: John Chumack.)
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.