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The Sky This Week, 2017 September 26 - October 3

Follow the Moon through the autumn sky.
Gibbous Moon, imaged 2017 March 9
from Alexandria, Virginia by Geoff Chester
with an Explore Scientific AR102 4-inch f/6.5 refractor,
Antares 1.6X Barlow lens, and Canon EOS Rebel T2i DSLR.

The Moon waxes in the evening sky this week, climbing northward along the ecliptic as she drifts through the dim stars of the autumnal constellations. By the end of the week she appears nearly full, lighting up the twilight and early evening with her pale glimmer. The next Full Moon is the Harvest Moon, which occurs on the 5th at 2:40 pm Eastern Daylight Time. This particular Full Moon gets its name from the interesting geometry it has with respect to the northern hemisphere horizon. For several nights before and after the Full Moon the time differences between successive moonrises amounts to about half an hour at the latitude of Washington. Typically this difference is about an hour. The farther north you go, the shorter the interval between successive moonrises. In Scotland the difference is about 20 minutes, and locations north of the Arctic Circle see Luna rise earlier on the nights around Full Moon! In popular folklore, this extra bit of natural light augmented the fading glow of twilight, allowing farmers to have a little more time to bring in their crops before the first frost, thus leading to the aptly named "Harvest Moon".

You’ve probably noticed that the length of day is now changing rather quickly from day to day. This effect seems particularly noticeable to me at this time of the year, especially in the mornings. I now find myself waking up in the morning in darkness. There’s still a bit of time to enjoy daylight in the evening, but we now dine in darkness. This is all a result of the equinox, where we find the Sun moving at its most rapid rate in declination. The length of day now changes by just over 2.5 minutes per day, or about 20 minutes per week! Fortunately for stargazers this means that we can begin our observing at a decent hour and enjoy the last of summer’s stars and the deep-sky clusters and nebulae hidden among the star clouds of the Milky Way.

The main attraction this week, though, is the Moon. As Luna moves through the dim stars of autumn we can follow the progress of the terminator line from night to night as it reveals new landscapes on each successive night. Even though the features of the Moon’s surface have remained essentially unchanged for countless eons, there are subtle changes in the angle of sunlight which can make well-known craters, mountains, and plains take on a new and unique look every time you view them. The Moon is far and away the best target for the small telescope, and I often find myself staring at Luna’s battered surface for long stretches through my 4-inch refractor.

Jupiter can still be glimpsed in the glow of evening twilight, but you’ll need to be on your toes to do so. He appears in the west shortly after sunset, and half an hour later he’s just five degrees above the horizon. You’ll be hard-pressed to see him in another week.

Saturn lingers in the southwest thanks to the rapidly changing time of sunset. You should be able to spot him about half an hour after sunset, and as the sky darkens he’ll become the brightest the brightest object in this part of the sky. You have about an hour or two to get a decent view of him in the telescope before he becomes too low for a steady view.

Venus and Mars spend the week moving closer together during the morning twilight hours. You’ll have no trouble spotting Venus, but Mars may require binoculars to tease out of the gathering glow. Venus will rapidly catch up to the red planet during the course of the week, and on the morning of the 5th the pair will be just a quarter of a degree apart. If you check out the scene in binoculars you may notice another faint object just north of Venus; this is the 4th-magnitude star Sigma Leonis. All three objects should fit nicely in a low power telescope field.

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