Skywatch Line for Friday, September 22, through Sunday, September 24, 2023

This is Dudley Observatory’s Skywatch Line for Friday, September 22, through Sunday, September 24, written by Sam Salem.

On Friday, Sun rises at 6:42am and sets at 6:53pm; Moon rises at 2:46pm and sets at 11:05pm. First-quarter Moon occurs on Friday, at 3:32pm. After dark it’s just over the spout, or handle, of the Sagittarius Teapot.

Saturday is the equinox night. On Saturday, the fall season begins in the Northern Hemisphere at 2:50 a.m. EDT. That’s when the Sun crosses the equator heading south for the season, when day and night are of approximately equal length. There’s about eight more minutes of daylight, at mid-temperate latitudes, on the day of an equinox. The reasons are the Sun’s geometry as a disk and the atmospheric refraction. Most almanacs regard sunrise as when the leading edge of the Sun first touches the eastern horizon. They define sunset as when the Sun’s trailing edge finally touches the western horizon. This alone provides an extra 2 1/2 to 3 minutes of daylight at mid-temperate latitudes. Atmospheric refraction raises the Sun about 1/2 degree upward in our sky at both sunrise and sunset. This advances the time of actual sunrise, while delaying the time of actual sunset. The result is several minutes of extra daylight, not just at an equinox, but every day.

Venus, at brilliant magnitude –4.8 in dim constellation of Cancer, is getting ever higher in the east before and during dawn. It’s at its peak brightness as the “Morning Star.” In a telescope it’s a thickening crescent. Venus now rises a good two hours before dawn’s first light, on the horizon far under Castor and Pollux.

Mercury is very low in the east at dawn. Look for it about 45 minutes before sunrise very far to the lower left of bright Venus. Mercury brightens fast. It’s at magnitude -0.5 on Saturday morning. That’s a doubling of its light in just five days.

Jupiter, at magnitude –2.7, rises in the east-northeast not long after dark. Watch for it under the brightest stars of the constellation of Aries. Jupiter dominates the eastern sky by late evening and shines highest in the south during the early-morning hours.

Saturn, at magnitude +0.5 in dim constellation of Aquarius, is the “star” rather low in the southeast in twilight. It’s two weeks past opposition. Saturn shines at a good height for telescopic observing by 10 p.m., by which time Fomalhaut is twinkling two fists at arm’s length below it. Saturn stands highest in the south around 11.

Saturn can guide you to the lonely, but bright star Fomalhaut. If you are under dark skies, they are near the faint arrowhead-shaped constellation Capricornus the Sea-Goat. They’ll rise in the east around sunset, and they are visible all night. Fomalhaut, aka Alpha Piscis Austrinus, is also called the Loneliest Star because it is the only bright star in a wide stretch of sky. Fomalhaut arcs in solitary splendor across the southern sky in autumn. Some call it the Autumn Star. In 2023, Fomalhaut isn’t so solitary. A bright planet, Saturn, appears near it in the sky. Fomalhaut will be the one that’s twinkling since Saturn will shine with a steady light.