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Crescent Moon

Photo of the Week. A five-day old waxing crescent descends the blue sky.

Astronomy news for the week starting Friday, September 16, 2011.

The Moon begins our week in its waning gibbous phase in which we see mostly daylight on the lunar disk with the nighttime side there, but unseen, as a dark crescent. Gibbous finally gives way to third quarter on Tuesday, September 20, in the morning before Moonset in North America, allowing you to see a fine third quarter in daylight hours. Passing from the stars of eastern Taurus, we then get to catch a few days of the waning crescent. The lunar week features planetary bookends. The night of Friday the 16th, the gibbous Moon will be set to the northeast of Jupiter, while on the morning of Friday the 23rd, the crescent will glide five degrees south of Mars, with great Leo seen rising to the east of the pair as dawn approaches.

Obviously, our current planetary jewels are Jupiter and Mars. Jupiter, slowly retrograding westward in southern Aries, is now rising brilliantly to the north of east around 9 PM Daylight Time, less than half an hour past the end of twilight. Follow it across the sky to 3:30 AM, when it crosses the meridian to the south and enters the western celestial hemisphere. By then, Mars, rising in the northeast about 2 AM, is well up, the two planets beautifully punctuating the heavens. Leaving bright Gemini to the west, Mars, moving to the east against the stars, passes into dim Cancer as our week begins.

The big event, however, involves ourselves, as on the morning Friday the 23rd, at 4:05 AM CDT (5:05 EDT, 3:05 MDT, 2:05 PDT), the Sun passes the autumnal equinox in Virgo, which marks the beginning of fall in the northern hemisphere (spring in the southern). By the time we are graced with daylight, autumn will already be in progress, summer behind us. On that day, the Sun will rise very close to due east, will set due west, be up for 12 hours (actually a bit more because of the solar diameter and refraction in the Earth's atmosphere), and be down for 12. The day also marks the more or less formal setting of the Sun at the north pole and its rising at the south pole (though for reasons given above, the Sun is already up at the south polar station).

With the Moon gone from the evening sky, if you are in a dark location, you might once again admire the Milky Way, made of the combined light of the billions of stars in the disk of our Galaxy. Highly irregular, the Milky Stream is given great character by the Galaxy's dark, star-birthing dust clouds. From Cygnus, overhead in mid-evening, the path is split in two by the Great Rift, the brighter eastern side passing to the south through Aquila, Scutum (where it gathers into a fine star cloud), and into Sagittarius.
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