Sky This Week

What to look for in the Night Sky this week with a quick Survey at the bottom.


■ The 1st-magnitude star very high in the south after dark is Altair. To check that you're looking at the right one, look for its little marker Tarazed, 3rd magnitude, about a finger-width at arm's length to its upper right.

About a fist to Altair's upper left is the little constellation Delphinus, the Dolphin, featured in the September Sky & Telescope, page 45.

Not quite as far straight above Altair is smaller, fainter Sagitta, the Arrow.

Venus, Regulus, and Mercury at dawn, mid-September 2023

When looking for Mercury far below Venus in early dawn this week, don't confuse it with twinklier Regulus along the way.


■ These closing days of summer always find the Sagittarius Teapot moving westward from south during evening, and tipping increasingly far over to the right as if pouring out summer's end.


■ Look very low in the west-southwest in early twilight for the waxing crescent Moon. Can you see Spica twinkling 3° or 4° lower right of it? Use binoculars.

Then look due west, about 25° to the right of the Moon, for Comet Nishimura at perihelion. Use those binoculars or a wide-field telescope at low power. You have only a narrow time window between when twilight is still too bright and the comet gets too low and sets.


■ By about 11 p.m. bright Jupiter is nicely high and shining precisely due east. (The exact time depends on your location.)

Look lower left of Jupiter, by about 1½ fists at arm's length, for the Pleiades.

A similar distance below the Pleiades, Aldebaran is rising.

Nearly three fists left of the Pleiades shines Capella.


■ Algol, in the bottom of Perseus now making its way up the northeastern sky, should be at its minimum brightness, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 10:01 p.m. EDT. Compare it to Gamma Andromedae (Andromeda's bright foot), mag 2.1, and Delta Persei, mag 3.0 (Delta is currently about the same height as Algol).


■ This is the time of year when, by late evening, the dim Little Dipper in the north "dumps water" into the bowl of the Big Dipper way down below. The Big Dipper will dump it back in the evenings of spring.


■ Arcturus shines in the west these evenings as twilight fades out. Capella, equally bright, is barely rising in the north-northeast (depending on your latitude; the farther north you are the higher it will be.) They're both magnitude 0.

Later in the evening, around 9 or 10 p.m., Arcturus and Capella shine at the same height. When will this happen? That depends on both your latitude and longitude.

When it does, turn around and look low in the south-southeast. There will be 1st-magnitude Fomalhaut at about the same height too — exactly so if you're at latitude 43° north (from Boston to Buffalo, Milwaukee, Boise, Eugene). Seen from south of that latitude, Fomalhaut will appear higher than Capella and Arcturus are. Seen from north of there, it will be lower.


■ First-quarter Moon (exactly so at 3:32 p.m. EDT). After dark it's just over the spout of the Sagittarius Teapot, as shown below (for Northern Hemisphere observers).

Moon passing the Sagittarius Teapot, Sept 22-23, 2023

Every 24 hours the Moon moves about 13° east along its orbit with respect to the stars. That's the distance from the spout to the handle of the Sagittarius Teapot.


■ Now the Moon shines by the Teapot's handle.

■ It's equinox night; fall begins in the Northern Hemisphere tonight at 2:50 a.m. EDT (11:50 p.m. PDT). That's when the Sun crosses the equator heading south for the season. The days are getting shorter.

■ Coincidentally, when summer turns to autumn is about when Deneb takes over from brighter Vega as the zenith star after nightfall (for skywatchers at mid-northern latitudes).


■ The starry W of Cassiopeia stands high in the northeast after dark. The right-hand side of the W (the brightest side) is tilted up.

Look along the second segment of the W counting down from the top. It's not quite horizontal. Notice the dim naked-eye stars along that segment (not counting its two ends). The brightest of these, on the right, is Eta Cassiopeiae, magnitude 3.4. It's a remarkably Sun-like star just 19 light-years away, and it has an orange-dwarf companion, magnitude 7.3, making it a lovely binary in a telescope (separation 13 arcseconds).

Farther left along the segment is a fainter, wide naked-eye pair if you have a dark sky: Upsilon1 and Upsilon2 Cassiopeiae, 0.3° apart. They're yellow-orange giants unrelated to each other, 200 and 400 light-years distant from us. Upsilon1 is slightly fainter; that's the farther one.



Mercury is very low in the east at dawn, as shown near the top of this page. Starting around September 18th, look for it about 45 minutes before sunrise very far to the lower left of bright Venus. Mercury brightens fast: from magnitude +0.2 that morning to –0.5 on the 23rd. That's a doubling of its light in just five days.

Venus (a brilliant magnitude –4.8, in dim Cancer) is getting ever higher in the east before and during dawn. This week 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  a weird UFO of a thing on the horizon far under Castor and Pollux.

Mars is out of sight behind the glare of the Sun and will remain so for the rest of the year.

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

Hi-res image of Jupiter, Ganymede, and Ganymede's shadow, Sept 9, 2023

Jupiter, Ganymede, and Ganymede's shadow on September 9th, imaged by S&T's Sean Walker from New Hampshire. South is up. Note the detail visible even on Ganymede's disk. "I recently completed my observatory and upgraded the primary scope to a C14," he writes. "Yesterday morning brought excellent seeing conditions and permitted imaging at the resolution limit of the optics." It also required sophisticated video-frame stacking and image-processing techniques; don't expect to do nearly this well without lots of study and practice!

Saturn (magnitude +0.5, in dim 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 9 or 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 (and Dione) Aug 30, 2023

Saturn on August 30th, imaged by Christopher Go. South is up. Saturn was barely past opposition, so the globe's shadow on the rings behind it was barely becoming visible on the east side (at the lower right here). The tiny dot just south of (above) Saturn is Dione.

Uranus, magnitude 5.7 in Aries, is nice and high in the early-morning hours, 7° or 8° east of Jupiter.

Neptune, magnitude 7.8 at the Aquarius-Pisces border, is fairly high in the southeast by 10 p.m., 24° east of Saturn. Neptune comes to opposition on September 19th.

Courtesy of Sky & Telescope senior editor Alan MacRobert
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