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.
