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The Sky This Week, 2015 September 22 - 29

Equinox, Harvest Moon, and a total lunar eclipse!
Total Lunar Eclipse, 2014 October 10, 10:38 UT
Imaged with a Canon EOS Rebel T2i DSLR, 250mm f/8, 1/4s @ ISO 1600

The Moon brightens the evening sky this week, passing through the faint autumnal constellations as she waxes to Full Moon on the 27th at 10:51 pm Eastern Daylight Time. At this time Luna will undergo a total eclipse by the shadow of the Earth; more on this below. September’s Full Moon is known almost universally in the northern hemisphere as the "Harvest Moon" due to the peculiar geometry of its orbit with respect to the eastern horizon that occurs each year at this time. At northern temperate latitudes the plane of Luna’s orbit intersects the eastern horizon at a shallow angle that becomes even shallower at higher latitudes. This causes the Moon to rise at around the same time on the nights immediately before and after Full Moon. Before the invention of artificial lighting the extra light provided by the rising Full Moon gave farmers a little extra light to bring in their crops. Typically the interval between successive moonrises averages about an hour later on successive nights. Here in Washington the difference is about 40 minutes at this time of the year. In Scotland the difference is about 25 minutes, while the northernmost regions of Norway see successive moonrises that are just three minutes later each night. Venture above 75 degrees north latitude and the Moon will actually rise earlier each night around the full phase!

This year’s Harvest Moon will also provide us with a very nice total eclipse that will be a "prime time" event for the eastern United States. Luna enters Earth’s penumbral shadow at 8:10 pm EDT, and over the course of the next 45 minutes you may begin to notice a slight darkening of the Moon’s upper left limb. At 9:07 pm the Moon enters the umbral shadow and is completely eclipsed beginning at 10:11 pm. Mid-eclipse occurs at 10:47 pm, and totality ends at 11:23 pm. Luna exits the umbra at 12:27 am and the penumbra at 1:24 am. Much ado has been trumpeted about this eclipse occurring in conjunction with the closest lunar perigee of the year, but rest assured that there’s very little substance in this phenomenon. Luna’s disc will be about 7% bigger than average, but most of us would have a hard time detecting this with the unaided eye. There is also a fair amount of nonsense circulating on social media calling the eclipse a "blood moon" with the implication of dire effects of earth-shattering import. Rest assured that there have been thousands of total lunar eclipses across recorded human history, and whatever calamities that have been associated with them have usually been the fault of poor human judgment. Enjoy this eclipse for what it is: one of Nature’s wonderful treats for the eye.

The autumnal equinox falls on the 23rd at 4:21 am EDT. At that instant the center of the Sun’s disc stands directly over the Equator at a point in the Indian Ocean about 1000 kilometers east of southern Somalia. If Old Sol were a point source of light and Earth didn’t have an atmosphere the length of day and night would be exactly 12 hours. However, the Sun subtends a disc that’s half a degree in apparent diameter and we have a significant atmosphere that sustains us, so our day of "equal night" falls on the 26th.

Our only bright evening planet remains Saturn, who may be found in the southwestern sky as evening twilight deepens. You have a limited window to observe him, however, as he sets well before 10:00 pm. His rings can be spotted in just about any telescope, but your chance of seeing a nice crisp image will be severely limited by the dense layers of our atmosphere that you’ll have to peer through.

The best planet viewing is before dawn in the eastern sky. Here you’ll find the dazzling glimmer of Venus well up as twilight gathers. You should also see bright Jupiter rising out of the horizon haze by 6:00 am. Halfway between them you’ll find the much fainter glow of Mars close to the bright star Regulus in Leo. These planets will gradually move toward the evening sky over the next several months, promising to give us an interesting springtime sky to look forward to.