September 2013 Starwatch
image showing planet alignment in the sky

by Deane Morrison

Saturn swoops down on Venus in the middle of September, a month chock full of celestial events.

Wasting no time, September opens with a waning moon high in the east the morning of the 1st. The moon forms a diamond-shaped figure with bright Jupiter highest, fainter Mars lowest, and the brighter Gemini twin, Pollux, opposite the moon. Look for this gathering about an hour before sunrise.

The evening of the 5th, the bright star Spica, in Virgo, makes its closest pass to Venus as it heads down into the sunset. Use binoculars to find the star about three full moon widths below and left of Venus.

On the 8th, a waxing crescent moon appears next to Venus. With its cusps pointing away from the planet, it’ll look as if the moon is turning its back on Venus. Waiting in the wings is Saturn, above and left of the pair.

The moon moves on, but Saturn closes in on Venus, passing 3.5 degrees (7 full moon widths) above our sister planet on the 18th. Saturn ends the month well to the right of Venus in the west-southwest.

As Saturn takes its slow fall in the evening, Mars makes a quick journey in the morning. Between the 7th and the 9th, the Red Planet glides across the subtly lovely Beehive star cluster. Mars will still be below the Gemini twins and Jupiter, but plenty high enough to see an hour before sunrise. While Mars and the Beehive is a combination not to be missed, to fully appreciate it you really must see it through binoculars. So grab yours before heading outside.

Some 10 days later, September presents us with an annual treat: the harvest moon.

Traditionally, the full moon closest to the autumnal equinox is given the title "harvest moon," and some years it falls in early October. But this year's arrives just three days before the equinox, so there's no contest.

The name “harvest moon” arose because for a few days around full moon, moonrise occurs only about 25 to 30 minutes later each night, ensuring an early-evening light source for farmers working late to harvest their crops. This year it reaches fullness on the 19th at 6:13 a.m., so you may want to enjoy it the night before.

The harvest moon phenomenon mirrors what the sun does in March. Around the spring equinox, the sun moves most rapidly north, and the farther north it gets, the earlier it rises. Full moons, being opposite the sun in the sky, move most rapidly north as the sun is heading most rapidly south--which is around the time of the autumnal equinox. The moon’s orbit around Earth still makes it rise later each night, but the interval is briefest this time of year.

The equinox arrives at 3:44 p.m. CDT on the 22nd, when the Earth will be lighted from pole to pole. From then until the spring equinox, the farther north you go, the shorter the days.

The University of Minnesota offers public viewings of the night sky at its Duluth and Twin Cities campuses. For more information and viewing schedules, see:

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