This is the Skywatch Line for Monday and Tuesday, October 1st and 2nd. The Sun sets at 6:36 PM; night falls at 8:10. Dawn breaks at 5:19 AM and ends with the Sun rising at 6:53.
The planetary parade is ending. While we see four bright planets during civil dusk, two are gone shortly after sunset.
Venus, in Libra, the brightest at minus 4th magnitude, lies very low on the western horizon. As it approaches Earth, it grows in size to 47 arc-seconds, but its disk shrinks to 16% illuminated. At civil dusk, it is only one degree high, which means that the observer should seek an unobstructed horizon. Venus sets at 7:14 PM.
Jupiter, also in Libra, is next brightest at minus 1st magnitude, and moderately low in the southwest at 12º altitude and 32 arc-seconds in size. Telescopic observers can witness the Jovian moon IO begin to cross the planet’s face at 7:41 Monday night; Tuesday can see it reappear at 8:01 PM. However, these events occur when the planet is quite low. Jupiter sets at 8:28 PM.
Looking eastward, Saturn, in Sagittarius, glows with 5th magnitude. This month, it dims slightly and shrinks to 16 arc-seconds in size. At about 24º high in the south, it is high enough for enjoyable views of its ring system. The Cassini space probe orbiting Saturn recently made a startling discovery: a dust storm on the moon Titan. Up until now, Mars was the only planet, besides Earth, that exhibited dust storms. Titan’s storm originated in the equatorial dunes and went planet-wide. However, unlike Earth and Mars, Titan’s atmosphere is a mix of methane and ethane, not oxygen and water. The Ringed Planet sets at 10:57 PM.
Mars, in Capricornus, is the night’s last bright planet. It shines at minus 1st magnitude, but fades to .6 at month’s end. In our telescopes, Mars appears about 88% lit and 15 arc-seconds in size. Civil dusk finds it 20º high in the East. Mars is best observed at 8:51 PM and sets at 1:25 AM.
Neptune rises in Aquarius at 5:40, glows with 7th magnitude and appears a tiny 2 arc-seconds in size. It is highest at 11:16 PM, when it is best observed; it sets at 4:57 AM. Uranus, rises in Aries at 7:24 PM, shines with 5th magnitude and is 3 arc-seconds in size. By Midnight, it is 47º high in the south and is best observed at 2:09 PM. Both planets lie in crowded star fields and require detailed charts to aid the observer.
Tuesday, the Moon rises in Gemini at 11:13 PM. At 22 days old, it appears about 52% illuminated and blazes with minus 10th magnitude. Wednesday’s Moon, still in Gemini, is only 38% lit, slightly dimmer at minus 9th magnitude and rises at 12:12 AM.
The Moon may hinder observation of Comet 21P/Giacobini-Zinner. The 7th magnitude comet is located in the dim constellation Monoceros, which lies beneath Gemini. Visible after midnight, the comet is reportedly binocular-visible; astronomical websites provide finder charts.
At nightfall, the Milky Way streams overhead from North to South. Many of the most famous constellations are found along this river of stars: Cassiopeia, Cygnus, Aquila, Sagittarius, also lesser known Delphinus and Scutum. Scutum is located halfway between Aquila and Sagittarius. A bright condensation can be spotted in dark, rural skies. This is the “Wild Duck” cluster. Binoculars show it as a bright knot of stars; telescopes reveal a myriad of stars. Reference books say it contains 2900 stars and is fifteen light years in diameter. This cluster, the eleventh on Messier’s list, is quite close, about 5500 light years away. It shines with the brilliance of 10,000 suns.