This is Dudley Observatory’s Skywatch Line for Friday, September 21 through Sunday, September 23, written by Sam Salem. On Friday, the Sun rises at 6:42am and sets at 6:55pm; the waxing gibbous Moon sets at 3:01am and rises at 5:35pm. Full Moon occurs on Monday at 10:52pm. On Saturday after dark, watch the Great Square of Pegasus balances on one corner about three fists at arm’s length to the left of the gibbous Moon. The Square’s upper right side points down toward the Moon.
On Saturday, the Sun crosses the celestial equator at 9:54pm, marking the equinox and the beginning of fall season in the northern hemisphere. On the day of the equinox, the Sun rises due east and sets almost exactly due west. Day and night are almost equal. Coincidentally, when summer turns to autumn, Deneb takes over from brighter Vega as the zenith star after nightfall.
Venus reaches its peak brightness of magnitude –4.8 on Friday evening. Unfortunately, the planet is very low in the west at sunset and difficult to sight. In telescopes Venus displays a tiny, snow-white crescent. This distinctive shape is best observed in the afternoon, around 3:30pm when the planet is highest and its brightness isn’t overwhelming.
Mars, in southern Capricornus, fades from magnitude –1.7 to –1.5. It is still a trace brighter than Sirius. Mars shines highest in the south about an hour after dark and sets around 2am. In a telescope Mars shrinks from 18½ to 17 arc-seconds wide. Jupiter, at magnitude –1.9, shines ever lower in constellation Libra in the southwest in twilight. Jupiter is still very close to the double star Zubenelgenubi, or the southern claw in Arabic, in constellation Libra, but it is slowly drifting farther away from it. Saturn, at magnitude +0.4, sits above the spout-tip of the Sagittarius Teapot in the south at dusk. It’s located well to the right of brighter Mars. Saturn sets by midnight.
This weekend represents the last best chance to enjoy Comet 21/P Giacobini-Zinner. After this weekend, moonlight will start lighting up the predawn sky, where the comet is found. 21/P is beginning to fade as it recedes both from the Sun and the Earth. Find Giacobini-Zinner this weekend moving its way through southern Gemini and into constellation, Monoceros. On Monday morning, the comet drifts alongside the 4th-magnitude open cluster, NGC2264, the Christmas Tree Cluster. This same NGC number also refers to the much fainter Cone Nebula. Try to capture the cluster, nebula and comet together this weekend. The Moon sets as twilight begins. For this reason, Sunday might be a better choice to capture the scene with your camera.
Sunday marks the 172nd. Anniversary of the discovery of planet Neptune. The German astronomer Johann G. Galle discovered Neptune on September 23 1846, after only an hour of searching, within one degree of the position that had been computed by Urbain-Jean-Joseph Le Verrier. Irregularities in the orbit of Uranus led to the belief that another celestial body might be responsible for the gravitational pull. The size and position of Neptune were calculated independently by the English astronomer John Adams and the French astronomer Urabain-Jean Le Verrier based on Neptune’s influence on Uranus’ orbit. Neptune goes around the Sun once roughly every 165 Earth years. In the year 2011, Neptune completed its first orbit since being discovered.