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Tropical dawn

Photo of the Week. Tropical dawn.

Astronomy news for the week starting Friday, September 19, 2008.

The Moon begins our week in the latter stages of its waning gibbous phase as it heads toward third quarter the night of Sunday the 21st around the time of Moonrise in North America. It then enters the waning crescent phase. The morning of Thursday the 25th, the Moon can be seen heading toward the Sickle of Leo, while the following morning, that of Friday the 26th, our ever-present companion will be seen just to the southwest of the star Regulus. Two days before the quarter, on Friday the 19th, the Moon passes perigee, when it will be at its closest point to the Earth.

Three planets shine in our skies, two difficult to see, one readily visible. Try to find Venus low above the horizon in bright southwestern twilight. The planet will gradually become increasingly visible as autumn progresses, eventually growing quite obvious. In the morning, Saturn rises just after the commencement of dawn, the ringed planet having moved eastward into south-central Leo. In between is the one you will not have to "look for," Jupiter , which shines brightly in the south just to the west of the celestial meridian in northern Sagittarius as darkness falls. The giant planet is now strictly an evening object, as it sets before true local midnight (around 12:30 AM Daylight Time).

It's the Earth that grabs our attention, however, as the Sun passes the autumnal equinox in Virgo on Monday the 22nd (a bit early as a result of the addition of February 29th in our current leap year), bringing autumn to the northern hemisphere (spring to the southern). The event itself occurs in the morning, at 10:44 AM CDT (11:44 EDT, 9:44 MDT, 8:44 PDT), giving us a nice chance to celebrate with another cup of morning coffee. On that date the Sun will rise (almost) due east, set due west, be up for 12 hours and down for 12, hence the term "equinox." (Upward lofting of the Sun as a result of refraction by the Earth's atmosphere and the extended solar diameter actually give us a bit more than 12 hours of daylight.) On this special day the Sun technically rises at the south pole (for six months of daylight) and sets at the north pole, ever-deepening twilight leading to nearly three months of complete darkness.

In the evening, look for bright Arcturus, the brightest star of the northern hemisphere (number four overall), sparkling over in the northwest. Higher up is Vega, which comes in at northern's number 2. To the south of Vega is bright Altair. Flanked by two outlier stars that make it look like an airplane slowly flying across the sky, it's the luminary of Aquila, the celestial Eagle.
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