This is the Dudley Observatory Skywatch Line for Wednesday, September 26th, and Thursday, September 27th, written by Louis Suarato. The 96% illuminated, waning gibbous Moon rises in the constellation Pisces at 7:56 p.m. Wednesday. There are two separate motions that affect our perception of the Moon’s path across the sky. The first is Earth’s rotation toward the east. As the Earth rotates away from the Moon, it appears to move across the sky a the rate of 15 degrees per hour, the result of dividing a 360 degree circle by 24 hours. The second is the Moon’s own eastward motion at the rate of .55 degrees per hour, or 13.2 degrees per day. This eastward motion by the Moon, changes its position in front of fixed background stars from night to night. As the Moon is rising, Jupiter is setting in the west-southwest. With Venus and Jupiter gone from the night sky, Mars and Saturn remain the only naked-eye visible planets. Saturn sets at 11:14 p.m., and Mars will set at 1:31 a.m. Thursday. As for a not-so-easily visible planet, use binoculars, or a small telescope, to look for Uranus 5 degrees above the Moon at 3 a.m., Thursday..
Comet 21P/Giacobini-Zinner has moved beyond Gemini and into the constellation Monoceros. Look about 20 degrees to the upper left of Sirius, which is the very bright star above the southeastern horizon at 4 a.m., for this green comet. Comet 21P will be 2 degrees to the east of the Rosette Nebula. The Rosette Nebula, also known as Caldwell 49, is a large cluster of stars surrounded by a nebulous cloud. It is comprised of nebulous regions NGC 2237, NGC 2238, NGC 2239, and NGC 2246, and an open star cluster NGC 2244, within the nebula. The Rosette Nebula, located about 5,000 light-years from Earth, was discovered in 1690 by John Flamseed.
September 27 is the birth date of astronomer Benjamin Gould, the first director of the Dudley Observatory. Born in Boston, Massachusetts in 1824, Gould served as director from 1855 to 1859. Gould was also in charge of the longitude department of the United States Coast Survey, where he was one of the first to determine longitude by telegraphic means. Gould’s greatest work was compiling a catalog of stars in the southern hemisphere while he was in Argentina.