This is the Dudley Observatory Skywatch Line for Monday and Tuesday, September 21st and 22nd written by Joe Slomka.
The Sun sets at 6:55 PM; night falls at 8:30. Dawn breaks at 5:07 AM and ends with the Sun rising at 6:42.
The Moon officially turned First Quarter early Monday morning, but rose in our skies in the afternoon. The eight-day-old Moon is already quite bright, at magnitude minus 10.1 and inhabits Sagittarius both nights. It appears a bit more than half illuminated and sets after midnight.
Saturn is the only other brilliant object in our sky. Saturn still resides in Libra and appears midway between Libra and the head of Scorpius. By nightfall, it is only ten-and-a-half degrees high and sets about 9:30 PM.
Twilight’s end reveals the distant planets Neptune and Uranus. Neptune is nearly eighth magnitude in Aquarius, and Uranus is a brighter 5.7 in Pisces. Both require charts, which are found in astronomy magazines, websites and apps. Neptune is best observed at 11:30 PM, and Uranus at 2:04 AM. Neptune sets at 5 AM, while Uranus remains up past sunrise.
By 10 PM, Perseus is moderately up in the eastern sky. At 11:30 PM on Tuesday, the star Algol (also known as Beta Persei) dims. Algol, the “Demon Star,” varies its light every 2 days, 20 hours and 49 minutes. It fades from second magnitude to third – easily seen by the naked eye. The entire cycle takes about nine hours. John Goodricke theorized that a dimmer star was partially eclipsing a brighter star. In 1889, the new technique of spectroscopy verified this theory. The main star is one hundred times the Sun’s luminosity. The eclipsing star is actually slightly brighter than our Sun. There is a third star that orbits the system once every 1.8 years, but plays little part in the occultation. The system is about 96 light years away and the most easily studied “eclipsing binary.” Astronomy magazines and websites provide timetables of its eclipses. Begin observing about 10 PM.
Astronomical Dawn sees the arrival of three bright planets. Venus rises first, at 3:27 AM. It blazes in the dim constellation Cancer. Under magnification, it appears about a third illuminated. Mars emerges a half hour later and is about eleven degrees to Venus’ lower left. Mars lies about one-and-a-half degrees from Leo’s brightest star, Regulus. Jupiter brings up the rear, rising before 5 AM. All three are great observing targets and all will remain in our skies for the rest of the year. They also experience several close conjunctions later this month.
The Vernal Equinox takes place at 4:21 AM Wednesday. This happens when the Sun dives below the projection of Earth’s equator on to the sky. The Sun also rises exactly East and sets exactly West. The word “equinox” means, “equal night.” Indeed, day lasts 12 hours and night also lasts 12 hours. This event signals the beginning of Fall for the Northern Hemisphere and Spring for those in the Southern Hemisphere.