Minnesota Starwatch: September 2014
From Deane Morrison
University of Minnesota
A terrific harvest moon and a meeting of celestial rivals are among the delights in store for us this September.
On the other hand, Venus gets lost in the sun's foreglow as it heads behind the sun. Our sister planet falls away, leaving Jupiter as the sole bright morning planet. Climbing higher every day, Jupiter is well up in the east by dawn and ends the month rising four hours ahead of the sun.
On moonless mornings, look above Jupiter for the faint, lovely Beehive star cluster; binoculars will help. The two objects move further apart each morning, but you can find the Beehive late in the month by imagining a line connecting the bright star Regulus, below Jupiter, with Pollux, the brighter Gemini twin, above Jupiter. The Beehive will be about 60 percent of the way up from Regulus, about the same distance west of the line as Jupiter is.
In the evening sky, look low in the southwest and watch Mars speeding eastward, away from Saturn and into the stars of Scorpius. On the Sept. 28, it glides above the gigantic red star Antares, the heart of the scorpion. This is a rare chance to see Antares, whose name means "rival of Mars," close to its planetary namesake. However, a waxing crescent moon may wash out some of the duo's brightness that night, so try comparing the red rivals on the near-moonless Sept 26.
September's harvest moon shines the night of Sept. 8-9. This will be another perigee full moon ("supermoon"), when the moon reaches the closest point to Earth in its orbit at a near-full phase. It rises about 21 hours past perigee and, in most locations, less than 90 minutes shy of full. So if you have a chance to watch this lovely moon lift above the horizon, don't miss it.
Traditionally, the full moon closest to the autumnal equinox is given the title "harvest moon," and some years it falls in early October. This year it's almost a tossup: The intervals between each month's full moon and the equinox differ by just 31.5 hours.
The name "harvest moon" comes from the moon's habit of rising only about 25 to 30 minutes later from night to night when it's near full, and this only happens near harvest time. This gives farmers an early-evening light source as they work late to bring in their crops. It happens because the sun is moving rapidly south and rising later each day, and so the near-full moon is moving rapidly north and rising relatively earlier each night.
The equinox arrives at 9:29 p.m. on the Sept. 22. From then until the spring equinox, the days will get longer as you move south and shorter as you move north.
You may catch the seldom seen zodiacal light in the east an hour or two before dawn in the last 10 days of September. This faint glow, emanating from sunlight reflected off dust in the plane of the solar system, was dubbed the "false dawn" by the Persian poet Omar Khayyám in his celebrated collection, the "Rubaiyat."