Photo of the Week. A complex lightning display rips
the sky from a thunderhead 60 miles away. The curve of stars up
and to the right is Corona
Australis, the Southern Crown. See full resolution.
Astronomy news for the two weeks starting Friday, September 9,
The next skylights will appear September 23.
The Moon begins our session in its first
quarter on Friday the 9th, to the northeast of Saturn and above Mars
,leaving the planets
behind as it grows in the waxing
gibbous phase that terminates at full Moon on Friday the
16th, when it will undergo another minimal penumbral
eclipse that in any case is nowhere visible from the Americas.
Of more significance, this full Moon is the famed Harvest Moon. At
this time of year, the evening ecliptic in the east bears its
most shallow angle to the horizon with the result that the
intervals between successive Moonrises from one night to the next
are minimized, giving us lots of early evening moonlight for
outdoor activities, such as harvesting. Two days after full, the
Moon passes perigee, where
it is closest to Earth, making it even brighter, though not really
noticeably so. The Moon then gibbously
wanes, terminating our fortnight at third
quarter on the morning of Friday the 23rd with the moon high
in the sky. Look the night of Tuesday the 20th to see the Moon to
the west of Aldebaran. After
occulting the star during the day, the Moon will be just east of
the star the following night.
The big event is the passage of the Sun across the autumnal equinox in Virgo at 9:21 AM Central Daylight
Time (10:21 EDT, 8:21 MDT, 7:21 PDT) on Thursday the 22nd, which
begins autumn in the northern hemisphere, spring in the southern.
Discounting the effects of atmospheric refraction and the finite
angular diameter of the Sun, at that time the Sun rises due east,
sets due west, days and nights are of equal length, and the Sun
rises at the north pole and sets at the south pole. Moving south
against the constellations of the Zodiac, it will bottom out 24.4
degrees south of the celestial equator on December 21, the first
day of winter in the northern hemisphere.
Mars and Saturn are quickly separating as Mars moves to the east.
Both still in the southwest in early evening, they set just after
11 PM at the beginning of our fortnight, while by the time of the
equinox Saturn (still north of Antares in Scorpius) sets about 10
PM, while Mars lingers for about an hour. With a good horizon,
Venus is visible in western twilight, while
Mercury disappears, as it goes through inferior conjunction
with the Sun (on this side of the Sun) on Monday the 12th.
Ever so slowly, the stars of summer are being replaced by those of
autumn. As the Big Dipper
glides off into the northwest, the "W" of Cassiopeia climbs the northeastern sky. In mid-evening
look about halfway up the sky to find bright Altair, the luminary of Aquila the Eagle, which is nicely
set within the Milky Way. While
rarely so represented, Altair with its two flanking stars (Tarazed to the north, Alshain) look much like a bird in
flight, and have been taken for an airplane with wing lights. Up
and to the left of Altair, find the compact and delightful Delphinus, the Dolphin, which looks
rather lie a hand with its finger pointing south.