Skywatch Line for Friday, September 16, through Sunday, September 18, 2016

This is Dudley Observatory’s Skywatch Line for Friday, September 16, through Sunday, September 18, written by Sam Salem.

The Full Moon occurs at 3:05pm on Friday. The Harvest Full Moon rises shortly after sunset, at 7:11pm, and sets shortly before sunrise, at 6:14am. The Harvest Moon is the full Moon closest to the Fall Equinox. Every Full Moon rises around the time of sunset, and on average each successive moonrise comes about 50 minutes later daily. But when a full Moon happens close to the Fall Equinox, the Moon (at our latitude) rises only about 30 to 35 minutes later daily for several days before and after the full Harvest Moon. It happens because the Moon’s orbital path makes a narrow angle with the evening horizon near the Fall Equinox. In the nights after the full Harvest Moon, the Moon rises in the east relatively soon after sunset making it seem as if there are several full Moons for a few nights in a row. That fact was important to farmers in earlier times. It meant there was no long period of darkness between sunset and moonrise for several days after full Moon. Farmers could work on in the fields, bringing in the crops by moonlight, hence the name “Harvest Moon”.

Lunar sightseeing can be done during this full Moon weekend. The most striking detail seen along the lunar terminator would be absent when the Moon is full. However, you can use your telescope to scan the entire circumference of the limb and notice a trace of shadow detail, especially on Saturday and Sunday nights when the phase is a little past full. The Moon is very bright when it’s full. For comfortable observing, use a Moon filter to cut the brilliance without eliminating detail. Because there are no shadows at full Moon, the dark and light areas seen are variations in the reflectivity of different parts of the Moon. The biggest difference is between the dark maria (the lunar “seas”) and the light highlands. This is due to the compositions of the two surfaces. When the Moon is full, the great crater Tycho is one of the most noticeable lunar features. It features impressively long bright rays that span two-thirds of the lunar disc. Inspect Tycho with a telescope at moderate magnification and you’ll easily see the dark halo that encircles the crater. Inspect the lunar disk for more ray craters, including Copernicus, Proclus, Kepler and Aristarchus.

Spica, the brightest star in constellation Virgo, is about 3 degrees lower right of Venus on Saturday and Sunday, soon after sunset. Spica sets at 7:53pm and Venus sets at 8:03pm on Saturday.

Shortly before sunrise, follow Capella and Sirius into daylight. Look up north for Capella, the brightest star in constellation Aurigae and look up south for Sirius, the brightest star in the night sky.

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