This is Dudley Observatory’s Skywatch Line for Friday, September 18, through Sunday, September 20, written by Sam Salem.
On Friday, Sun rises at 6:39am and sets at 6:59pm; Moon rises at 7:51am and sets at 8:05pm. The Moon reaches perigee, the Moon’s closest point to Earth in its monthly orbit, on Friday. At that time, Moon travels away from the Sun at a maximum orbital speed. After sunset, spot the very slim crescent Moon sitting just above the western horizon, and a palm’s width to the upper right of Mercury. Find an unobstructed horizon in the direction of sunset. Look westward, near the sunset point on the horizon, for a pale crescent, some 45 minutes after sundown. The Moon and Mercury sit low in the west, sinking with the not-yet-visible stars of Virgo. The Moon and Mercury will both fit into the field of view of binoculars. Ensure that the Sun has completely disappeared from view before using them. This young Moon takes place just a few days before the Northern Hemisphere’s autumn equinox. At sunset on the autumn equinox, the ecliptic hits the horizon at its shallowest angle for the year.
Venus, at magnitude –4.2 in dim constellation Cancer, rises in deep darkness, two hours before dawn begins, in the east-northeast. Venus sits about 20 degrees below Pollux and Castor, in constellation Gemini. By the time dawn gets under way, Venus shines fairly high in the east. To Venus’ upper right is Procyon, in constellation Canis Minor. Venus and Procyon form a nearly equilateral triangle with Pollux above. Right, or lower right, of Procyon shines brighter Sirius, in constellation Canis Major. In a telescope, Venus continues to shrink slowly into the distance. It’s now about 18 arcseconds in diameter. As it rounds toward pass behind the Sun, it’s becoming more gibbous. It’s now 65% sunlit.
Mars shines big, bright, and close as it approaches its October 13th opposition. It rises in the east around the end of twilight, shining bright orange, at magnitude –2.2. Mars climbs higher through the evening and stands at its highest, and telescopic best, around 3am, beaming high in the south. It’s near the dim, 4th-magnitude, Knot of Pisces. Mars is becoming less gibbous, about 95% sunlit, as it approaches “full Mars” at opposition. In a telescope, look for its white South Polar Cap, now much shrunken as summer advances in Mars’s southern hemisphere.
Jupiter, at magnitudes –2.5, and Saturn, at magnitude +0.4, shine in the south in early evening and move to the southwest later in the night. Saturn remains 8 degrees to the left of Jupiter. Very high above the two planets shines Altair, in constellation Aquila. Altair is a white-hot star, 11 times as luminous as the Sun, and just 17 light-years away. Much closer below Jupiter after dark is the handle of the Sagittarius Teapot. The brightest top star of the Teapot handle is Sigma Sagittarii, or Nunki. At magnitude 2.0, Nunki is an even hotter blue-white star, 4.5 times the Sun’s diameter, 3300 times as luminous, and 230 light-years away.
This weekend, the delicate crescent will sink lower in the deepening twilight, leaving a dark sky overnight for deep sky observing. Use your scope to explore Andromeda Galaxy (M31), the triangle-shaped Triangulum Galaxy (M33), and the planetary Ring Nebula (M57) in the northern constellation of Lyra. Early in the evening, you may even catch 9th-magnitude Comet 88P/Howell, about 3.5 degrees west of globular cluster M80 in constellation Scorpius.