Wednesday, September 22
The autumnal equinox occurs at 3:21 P.M. EDT as the Sun crosses the celestial equator, heading south. The event also marks the first day of fall in the Northern Hemisphere and the first day of spring in the Southern Hemisphere. Those above the equator have likely already noticed the Sun setting earlier each day, as the long daylight hours of summer begin to give way to the shorter days and long, cold nights of winter.
Let’s take the opportunity today to start saying goodbye to a summertime friend: the Summer Triangle. This famous asterism, or unofficial star pattern, comprises three stars from three different constellations: Vega in Lyra, Deneb in Cygnus, and Altair in Aquila. (Remember them from last night? They’re bright and easy to find, appearing shortly after sunset.)
The Summer Triangle is so named because it rides high in the summer nighttime skies, rising as the Sun sets and typically appearing overhead around midnight. Tonight after sunset, you’ll find this stellar triad still high in the southwest, but now heading toward the horizon as the night marches onward. The Summer Triangle will set a little earlier each night as fall stretches on, until its stars eventually disappear altogether from the night sky during winter.
Sunrise: 6:48 A.M.
Sunset: 6:57 P.M.
Moonrise: 8:05 P.M.
Moonset: 8:17 A.M.
Moon Phase: Waning gibbous (97%)
Thursday, September 23
Mercury passes 1.7° south of Spica at 8 A.M. EDT; however, neither is visible at that time. Instead, try looking west as soon as the Sun sinks below the horizon, where Mercury and Spica, still roughly the same distance apart, are setting quickly. See how soon you can spot the pair in binoculars or a telescope, with slightly brighter Mercury sitting to Spica’s lower left. You’ll need a clear horizon to find them and if you can get to higher ground, there’s an even better chance you can catch the pair before first Mercury, then Spica, sets.
Spica, also known as Alpha (α) Virginis, is some 1,900 times brighter than the Sun, which is why it’s so bright in our sky, even at a distance of 250 light-years. It is also a tight binary system, with its two components separated by some one-tenth the distance between Earth and the Sun. Both are blue-white class B stars, and at least one of the two is massive enough to end its life as a supernova when its fuel has been exhausted.
Sunrise: 6:49 A.M.
Sunset: 6:55 P.M.
Moonrise: 8:29 P.M.
Moonset: 9:19 A.M.
Moon Phase: Waning gibbous (93%)
Friday, September 24
The Moon passes 1.3° south of Uranus at noon EDT, although the pair won’t rise until about two hours after sunset. Uranus climbs above the horizon first, followed by the Moon about 25 minutes later. That 25-minute window is a great time to see if you can spot the distant ice giant on its own, although you will likely be somewhat hindered by the turbulent air that clings to the horizon. To find Uranus, first locate bright (magnitude 2) Hamal in northwestern Aries. Drop about 12° down from that star toward the horizon (southeast) to reach Uranus. It’s currently magnitude 5.7 and should be visible with binoculars.
Once the Moon rises, it’s nearly 6° from Uranus. Now several days past Full, the Moon has become a clearly waning gibbous less than 90 percent lit. Turn binoculars or a small scope to the terminator — the line where light and dark meet — to see features on the Moon’s eastern limb (west in our sky) start to fall into darkness. Peeking out from shadow is the western portion of the Sea of Tranquillity, bordered to the northwest by the Sea of Serenity.
Sunrise: 6:50 A.M.
Sunset: 6:53 P.M.
Moonrise: 8:55 P.M.
Moonset: 10:20 A.M.
Moon Phase: Waning gibbous (87%)
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