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The Sky This Week, 2013 September 17 - 24

It's Harvest Moon time.
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Jupiter and Io, 2013 SEP 17, 10:22 UT
The first Jupiter image of the new observing season!


The Moon climbs through the dim autumnal constellations this week, shedding her abundant light on an otherwise rather empty part of the sky. She ends the week among the rising stars of the winter sky, rising just before midnight. The Full Moon falls on the 19th at 7:13 am Eastern daylight Time. Since this is the Full Moon that occurs closest to the Autumnal Equinox it is almost universally known in the Northern Hemisphere as the Harvest Moon. It gets this name because it happens to occur at the time of year when Boreal farmers are bringing in their crops for the winter. It also happens to undergo a rather interesting phenomenon that’s associated with its apparent path around the sky. You’ve probably noticed that over the past week the Moon hasn’t risen very far above the southern horizon. If you trace its apparent path around the sky you’ll also notice that this path will intersect the eastern horizon at a shallow angle. Even though the Moon moves about 13 degrees along the Ecliptic each day, if you measure its elevation with respect to the eastern horizon for several days around the date of Full Moon you’ll find that its apparent elevation only changes by some five to six degrees. This means that at the latitude of Washington moonrise on successive nights around Full Moon takes place just over a half hour later each night instead of the more typical hour. This effect becomes more pronounced at higher latitudes. In Scotland the difference is just 20 minutes, and in Iceland it’s less than 10 minutes! The net effect of all of this is that, traditionally, farmers in northern Europe could rely on the nearly-full Moon to extend the time they had to work in their fields to bring in the harvest. Today’s farmers probably don’t need the extra bit of light afforded by this phenomenon, but many are still keenly aware of the folklore. This is actually one of two Harvest Moons that occur each year; the other helps our friends "down under" bring in their crops in March.

Full Moon isn’t really the best time to explore Luna’s varied landscapes through the telescope, though. With sunlight falling almost directly down on lunar features all sense of topography is lost, and only differences in reflectivity mark her most prominent features. Her surface also appears very bright, almost blindingly so, if you look at her with optical aid. However, as a whole the Moon is a very poor reflector of light. Her average surface albedo is about seven percent, roughly the same value as a dark business suit!

The Autumnal Equinox occurs on the 22nd at 4:44 pm EDT. This is the moment when the center of the Sun’s disc reaches an ecliptic longitude of 180 degrees, which also happens to correspond with the time it crosses the celestial equator and enters the southern hemisphere sky. You’ve probably noticed the rapid change in the length of daylight in the past few weeks, especially in the time of sunset. This is the time of year when length-of-day changes most rapidly, losing about two minutes at sunrise and sunset each passing day. By the end of the week daylight will be shorter than night for the next six months.

Venus and Saturn continue their race against the advancing Sun. You can see the two objects close together during the first evenings of the week, but Saturn is going to lose this race pretty quickly. The two planets are within four degrees of each other on the 17th and 18th, but by week’s end Venus will have left Saturn some 10 degrees in her wake. The ringed planet will soon be lost in the glow of twilight, but Venus will muster enough momentum to stay prominent in the sky until early next year.

Jupiter is now very well-placed for viewing as morning twilight begins to tint the eastern horizon. Old Jove is holding court in the middle of the constellation of Gemini, surrounded by the bright stars of the Great Winter Circle. I’ve managed to get up before the Sun on some of the crisp cool mornings of late and have enjoyed seeing the giant planet along with his glowing attendants. Winter nights won’t seem so long with these bright companions in the sky!

Mars hasn’t quite cleared my roofline before dawn, but he is still easy to spot as morning twilight gathers. The red planet is now setting his sights on Regulus, the heart of Leo, the Lion. He’ll pass very close to the star in mid-October. Right now he doesn’t offer much for the telescope, but this spring we’ll have a chance to watch him brighten and grow in the eyepiece as he moves toward opposition.

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