This is the Dudley Observatory Skywatch Line for Wednesday, October 11th, and Thursday October 12th, written by Louis Suarato.
You’ll have about 4 hours to observe in a moonless sky before the 54% illuminated, waning gibbous Moon rises at 11:11 Wednesday night. During that time, Saturn’s initial appearance is 17 degrees above the southwestern horizon in the constellation Ophiuchus. To Saturn’s left, or south, the Milky Way stretches through Sagittarius, Scutum, and Aquila. The Milky Way is a barred spiral galaxy estimated to have a diameter between 100,000 and 180,000 light-years, containing 100 to 400 billion stars, and at least 100 billion planets. Our solar system is located about 26,000 light-years from the Milky Way’s center, about two-thirds out from the center, on the Orion arm, or spur. The arms of the galaxy are where most of its stars are born. NASA’s Wide-field Infrared Survey Explorer, or WISE, has discovered more than 400 stellar nurseries which trace the shape of the galaxy’s arms. When we look at the Milky Way through Sagittarius during the summer months, we are looking toward the center of the galaxy. As you follow the Milky Way northward, you are seeing the Sagittarius arm and Orion spur. Beyond Cassiopeia, you are looking at the Orion spur, and Perseus arm. During the winter months, we are looking away from the center, toward the thinner outer bands.
The Last Quarter Moon occurs at 8:25 a.m. Thursday. Mars rises at 4:55 a.m., followed by Venus 17 minutes later. Our two neighboring planets will be 4 degrees apart in the pre-dawn sky. Comet C/2017 O1 (ASASSN) will be 7 degrees above Auriga’s brightest star, Capella, Thursday night. Look over the northeastern horizon before moonrise for this 8th to 10th magnitude comet. This comet is expected to pass through its perihelion, its closest approach to the Sun on October 14th, at a distance of 139.3 million miles. After passing the Sun, Comet ASASSN’s distance from Earth will decrease, reaching a minimum of 67 million miles on October 18th.