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Astronomy Picture of the Day

Cloud Shadows

Photo of the Week.. Cloud shadows highlight sunlit air.

Astronomy news for the week starting Friday, September 20, 2002.

Fall is upon us, the Moon is in her glory, Venus bursts at the seams, and autumn colors will soon follow.

As the tilted Earth orbits the Sun, the Sun appears not only to pass against the constellations of the ancient astronomical Zodiac, but also moves back and forth across the celestial equator. The night of Sunday, September 22, the Sun will move across the equator into the southern hemisphere at the autumnal equinox in Virgo, and astronomical autumn will begin in the northern hemisphere, spring in the southern. The event takes place at 11:55 PM Central Daylight Time on the 22nd, but at 12:55 AM Eastern Time on the 23rd (9:55 PM Pacific Time on the 22nd). Given that the time is near midnight, the Sun will be slightly north of the equator on the 22nd, and about the same small angle to the south of it on the 23rd. On both days it will rise very nearly at the east point of the horizon, set very nearly west, with days and nights very nearly equal in length. (In fact, the day is slightly longer because sunrise and sunset are reckoned from appearance and disappearance of the upper solar edge, or limb, not the center, and refraction by the Earth's atmosphere, which raises the horizon Sun by half a degree.) Autumnal equinox time also means that the Sun technically sets at the north pole, rises at the south pole for a six month stay, and also passes overhead at the Earth's equator.

Almost exactly a day before the equinox, the Moon passes its apogee, where it is farthest from the Earth, and on Saturday the 21st passes its full phase, making it an almost perfect Harvest Moon that lies close to the vernal equinox in Pisces. The term refers to the large amount of nearly full moonlight seen in the evening at this time of year, when the ecliptic to the east lies at a low angle against the horizon, and the delay in moonrise from one night to the next is at a minimum -- just what is needed to bring in the crops in the old days before artificial lighting. Just a few days after full Moon, on Thursday the 26th, Venus reaches its greatest brilliancy for this western evening appearance. But because the ecliptic lies so flat against the evening western horizon as well, this nearest of planets will not be very high in the sky. Nevertheless, it will be a glorious sight in deepening twilight. Venus will now quickly disappear from the evening sky, to appear in November in the morning. A day after Venus's greatest brilliancy, Mercury passes inferior conjunction with the Sun, and flips to the morning side as well. Between morning and evening, keep your eye out for Saturn, which rises around 11:30 daylight time, and then for Jupiter, which lofts itself over the horizon around 2:30 AM.

As fall begins, the summer stars slip away. With the Sun at the autumnal equinox, the winter solstice and Sagittarius are directly south at sunset. Watch for the Sagittarius's upside-down 5-star Dipper to the south-southwest as twilight nears its end.

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