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Shafts of sunlight

Photo of the Week. Sunbeams.

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.
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