Astronomy news for the week starting Friday, September 17,
2010.
As usual, we start our week (and a busy one it is) with the Moon,
which begins in the waxing gibbous
phase, then rolls through its full phase
the morning of Thursday, September 23, while high in the southwest,
allowing a fine sight of the near-perfect phase. We then catch a
bit of the waning gibbous the following
morning. On Monday the 20th, the brightening Moon passes a few
degrees north of
Neptune (the planet near-invisible), then north of Jupiter and Uranus the same morning it is full, which
just enhances the beauty of the full phase. Then two days before
full, on Tuesday the 21st, the Moon passes apogee, where it
is farthest from Earth.
All that remains of our fine evening planetary sky is
Venus, which is still very much visible in evening twilight,
the bright planet setting just before the end of twilight. Though
Mars is still there, it is very difficult to see to the
northwest of the bright planet. Venus, which has been steadily
brightening, finally starts to finish the show with maximum
brilliance on (again) Thursday the 23rd, after which it quickly
leaves the evening sky as it approaches conjunction with the Sun
about a month from now.
Venus is nicely replaced, though, by Jupiter, which passes
opposition with the Sun on Tuesday the 21st,
when it rises at sunset, sets at sunrise, and crosses the meridian to the south at local midnight.
That same day, Uranus does the same thing. Obviously close to each
other, on the following day Jupiter passes conjunction with, just
under a degree south of, more distant Uranus. These planets lead
us into the later morning hours, when as twilight brightens the
eastern sky you might spot Mercury, which
passes greatest western elongation (18 degrees west of the Sun) on
Sunday the 19th, making for one of the better apparitions of the
year.
All this activity is set against the background of the passage of
the Sun across the autumnal
equinox in Virgo on the
evening of Wednesday the 22nd (at 10:09 PM CDT, 11:09 EDT, 8:09
PDT). As the Sun thus crosses the celestial equator to the south, fall
begins in the northern hemisphere, spring in the southern. On that
day (ignoring refraction by the Earth's atmosphere and the Sun's
half-degree angular diameter), the Sun will rise due east, set due
west, be up then down for 12 hours apiece, will rise at the south
pole and set at the
north pole. With Jupiter, Uranus, and the full Moon opposite
the Sun, they on the other hand are tightly clumped around the VERNAL equinox in Pisces, the whole affair as
fascinating as the now-gone evening
planetary lineup.
The bright Moon dims out the constellations, though the first
magnitude (and brighter) stars remain. Look for Antares low in the southwest.
Directly north of it lies dim Ophiuchus and Serpens, the latter the only
constellation to come in two parts. The northern hemisphere does
better, with Arcturus in the
northeast in early evening, Vega
nearly overhead, Deneb to the east
of it, and Altair to the south of
both of them, the trio making the bright Summer Triangle. As dawn breaks, you can then admire
the Winter Triangle of brilliant
Sirius, the "Dog Star" in Canis Major, reddish Betelgeuse in Orion, and to the east of the star,
Procyon in Canis Minor.
