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The Sky This Week, 2011 September 20 - 27

The equinox means longer nights...

Jupiter and its moon Io, 2011 August 12, 08:37 UT 
Imaged from Fishers Island, New York

The Moon wanes in the morning sky this week, shrinking through her crescent phases as she plunges toward the eastern horizon. New Moon occurs on the 27th at 7:09 am Eastern Daylight Time. Luna begins the week among the stars of Gemini. Before dawn on the 23rd you’ll find her about five degrees southeast of ruddy Mars in the faint constellation of Cancer. On the 25th her slender crescent will be about seven degrees south of the bright star Regulus in the constellation Leo in the gathering morning twilight.

The autumnal equinox spells the official end of summer in the Northern hemisphere on the 23rd at 5:05 am EDT. At that moment the center of the Sun’s disc will stand directly over the earth’s equator at a point near the border of Sudan and Kenya in East Africa. The equinoxes are the times of the year when we notice the most rapid changes in the length of daylight. In this case we’re losing between three and four minutes of light with each passing day. The word "equinox" implies that the length of day and night are equal on the 23rd, but since the Sun isn’t a point source of light and we view it at sunrise and sunset through a refractive atmosphere the actual day of exactly 12 hours of daylight won’t occur until the 26th. From then until March 16 next year the nights will be longer than the days. This is not necessarily bad news for skywatchers. We’ll have more time to enjoy the delights of the night under crisp, haze-free skies!

The skies of autumn present some great opportunities to explore sights both near and far away. In the early evening the sky is still dominated by the bright stars of the Summer Triangle, Vega, Deneb, and Altair. At 9:00 pm these stars and their respective constellations straddle the meridian from nearly overhead to the south. Passing through the triangle, and neatly dividing the sky from the southwest to the northeast, is the subtle band of the Milky Way, our home galaxy. Take the time to explore this ghostly glow with a pair of binoculars or a small telescope at low power. From a dark location you’ll see knots of stars and glowing wisps of gas arrayed against a seemingly fathomless backdrop of uncountable stars. As the night presses on toward midnight the triangle is replaced by the "Great Square" of second-magnitude stars that make up the body of Pegasus. As we focus our gaze in this direction we’re looking out of the plane of the Milky Way toward intergalactic space. If you focus your binoculars about 15 degrees northeast of the upper-left star in the square you’ll encounter the fuzzy swatch of light that betrays our largest galactic neighbor, the Great Andromeda Galaxy. Located some 2.5 million light years from us, it is the most distant object that most of us can see with the naked eye!

By 9:00 pm you’ll notice a bright object climbing up from the haze of the eastern horizon. Two weeks ago, when I was watching the stars rise over Bar Harbor in Maine, I thought that I was looking at the landing light on a distant airplane. It was several minutes before I realized that I was looking at Jupiter, tinged a reddish hue by atmospheric scattering. The giant planet will beckon you for a telescopic glimpse by 11:00 pm, and he’s well worth obliging if you’re still up at that hour. Jupiter presents the largest apparent disc of any planet (except Venus when she’s near inferior conjunction), and that disc is a jumble of subtle pastel shades created by Old Jove’s turbulent cloud patterns. Small telescopes will show his two main equatorial belts, but even a six-inch instrument will show much finer detail which can change from hour to hour. Throw in the antics of his four large moons and you have entertainment that will last all night long!

If you’re up before the Sun you can now easily spot ruddy Mars in the faint constellation of Cancer. Located between the Gemini twins and the rising head of Leo, Mars gets a visit from the Moon on the morning of the 25th. Telescopically he’s little more than a small pink dot; his apparent disc is only 1/10th the size of Jupiter’s!