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The Sky This Week, 2011 September 6 - 13

Shine on Harvest Moon...
Supernova 2011fe in galaxy M101, 2011 August 28
Imaged by Richard Schmidt at Burleith Observatory,
Wasnington, DC.
57-min. integration made with a 32-cm PlaneWave astrograph, Kron-Cousins I band filter.
The SN was at 12.4 magnitude as of 28 August.

The Moon brightens the evening sky this week as she waxes toward the full phase and heads toward northern hemisphere skies. Full Moon occurs on the 12th at 5:27 am Eastern daylight Time. Since this Full Moon falls closest to the autumnal equinox it is almost universally known as the Harvest Moon. This tradition comes down to us from farmers in Medieval Europe, who took advantage of the unusual geometry of the Moon’s orbit and their high northerly latitude to help them bring in their crops for the winter. At this time of the year the Moon’s orbit intersects the eastern horizon at a shallow angle for Northern Hemisphere observers. The farther north you go, the shallower the angle appears. The net effect of this is that over the course of several nights around the time of the Full Moon Luna appears to rise just a few minutes later on consecutive nights. Most of the U.S. the difference is between 25 to 35 minutes later on successive nights, but in northern Europe the difference is only 10 minutes per night. In the Nordic countries above 62 degrees north latitude the Moon actually rises earlier on successive nights around the time of Full Moon. The "extra" light provided by the rising Moon thus allowed farmers to work well past sunset to harvest their fields, thus giving this particular Full Moon its very appropriate name in sky lore.

The waxing Moon will brighten the sky for those who wish to track down Comet 2009 P1 Garradd and the brightening supernova in the galaxy M101 that we featured in last week’s edition. Fortunately both objects should be visible for the next several weeks. The comet remains an 8th magnitude fuzzball that will slowly drift westward from the western edge of the Summer Triangle toward the constellation of Hercules over the next few weeks. It should be relatively easy to find in binoculars or a small telescope near the stars 109, 110, and 113 Herculis except for the few nights before and after Full Moon.

As for the supernova, it is still gradually brightening in the galaxy M101. It is expected to peak in brightness at around magnitude 10 in the next week, then gradually decline over the course of the next several months. At this brightness it will easily show up in a three-inch telescope, but the host galaxy will probably be washed out by moonlight. This is the brightest supernova to grace northern skies in several decades, and it belongs to a class of these objects that are extremely important to astrophysicists. Type-Ia supernovae are a specific class that all flare to the same intrinsic brightness, so they are very effective "standard candles" for measuring distances to their host galaxies. M101 is thought to be some 21 million light-years away, and observations of SN 2011fe should help refine this estimate.

The only bright planet to currently grace the night sky is mighty Jupiter, who rises due east at around 9:30 pm EDT by the end of the week. You can catch a good glimpse of him just before midnight if you’re willing to stay up late, but for most of us the best time to view him is before sunrise when he’s high in the southern sky. Morning twilight begins at around 5:30 am, and the giant planet is bright enough to be seen during most of the brightening dawn. Binoculars will show Jupiter’s four bright Galilean moons, and even a small 60-millimeter aperture telescope will reveal his more prominent cloud belts. Larger telescopes will show greater detail, so I’m really looking forward to getting him in the sights of our nearly 120 year-old 12-inch telescope!

If you are up before the Sun, look just above the eastern horizon half an hour before sunrise for the fleet planet Mercury. This week the swift world may be found close to the bright star Regulus, and on the morning of the 9th the two objects are only 3/4ths of a degree apart. See if you can spot them in the gathering twilight with your binoculars. Mercury should be the brighter of the pair.

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