Sky Tour Podcast for September 2024
This month’s episode is sponsored by Celestron, manufacturer of high-quality telescopes and an industry leader in developing exciting optical products with revolutionary technologies.
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I’m Kelly Beatty of Sky & Telescope magazine, and tonight we’re going on a tour of the stars and planets that you’ll see overhead during September. First, we’ll check the calendar for this month’s equinox and Harvest Moon; then we’ll track down the bright planets, explore some lesser-known constellations, and bounce around the Summer Triangle. It’s a great introduction to the late-summer sky, all in about 10 minutes. So grab your curiosity, and come along on this month’s Sky Tour.
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Ask a stargazer about this month’s astronomical significance, and the answer you’ll most likely get is: “the autumnal equinox.” It marks the celestial end of summer and the beginning of autumn in the Northern Hemisphere. This year, September’s equinox takes place on the 22nd at 8:44 a.m. Eastern Daylight Time. At that moment the Sun shines directly overhead as seen from the equator. Days and nights are both 12 hours long — that’s where the word equinox comes from — and no matter where you live the Sun rises due east and sets due west.
There’s another celestial event associated with this equinox, and that’s the Harvest Moon. Traditionally, according to records going back more than 300 years, it’s the name assigned to the Full Moon that falls closest to the autumnal equinox. The Harvest Moon gets its name from a geometric oddity. Because the lunar orbit makes a shallow angle with respect to the eastern evening horizon at this time of year, the Moon rises only about 30 minutes later on successive evenings, not the usual 50 minutes or so. So it never really gets dark between sunset and moonrise for several successive evenings, a boon to farmers working late to try to harvest their crops at day’s end. 
September’s full Moon occurs on the 17th. So that means the Moon will be in the evening sky for the first half of the month and relegated to late-night and predawn appearances for the second half. New Moon falls on the evening of September 2nd in the Americas, first quarter on the 11th, and last quarter on the 24th. 
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Let’s return to that Harvest Moon for a minute. This one will be extra special, because that big bright orb will undergo a partial lunar eclipse that will be visible throughout the western hemisphere. But like the barely-there lunar eclipse last March, this one will be challenging to get excited about. 
You might recall that Earth projects two shadows out into space: a deep complete umbra, where Earth covers all the Sun, and a penumbra, where we only block part of it. On 17th the Moon only barely grazes the umbra. So here’s what to look for: 
We can’t detect the first vestiges of the penumbra. But by about 9:30 p.m. Eastern Daylight Time, look for a smudgy darkening on the upper-left edge of the lunar disk. That signals that the eclipse is under way. Then comes the partial phase, which lasts for just over an hour. Watch for a dark divot to appear at the top of the Moon. It’ll slide left to right, getting deepest at 10:44 p.m. Eastern Time. For those of you on the West Coast, the Moon will rise just before this maximum, which for you occurs at 7:44 p.m. Pacific Time. 
Then the Moon gradually exits the umbra until 11:17 p.m. Eastern time, and you’ll probably lose sight of the dusky penumbra around midnight (or 9 p.m. on the West Coast).
Now, this is the kind of eclipse that only eclipse-lovers can really appreciate. But take heart! The next three lunar eclipses after this one will all be complete cover-ups. The first of these is early on March 14th, and it’ll visible from everywhere in North America. The bad news is that mid-eclipse comes at 3 a.m. on the East Coast and midnight on the West Coast. But we’re stargazers, and we can handle these late-night assignments, right? Right??
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The bright planets are scattered all over the morning and evening sky during this month. Mercury is worth getting up early for as September begins. You won’t have another chance to see it this clearly until mid-December. So get out there early in the month for your best chance of spotting it. The optimum time is about 30 minutes before sunrise. The best days to look for Mercury will be around September 4th, but on the 9th this fleet-footed planet will be very close to the bright star Regulus down in the twilight glow.
If you can’t spot Mercury, don’t fret — your predawn searching won’t be in vain. Jupiter is very obvious well up in the eastern sky. Mars is to its lower left by about twice the size of your clenched fist held at arm’s length, tangled in the legs of Gemini’s twins. And to Jupiter’s lower right is the magnificent hourglass shape of Orion. He won’t make it to the evening sky for some months, so congratulate yourself for getting this sneak peak of a coming attraction.
Two planets await you in the evening sky. Venus is lurking low in the evening twilight after sunset. But with an unobstructed view toward west, you should pick it up pretty easily beginning about 30 minutes after sunset.
Now swing your gaze clear across the sky, to the eastern horizon, and in an hour or so you’ll be able to spot Saturn. This ringed planet reaches opposition, that is, opposite the Sun in the sky, on September 7th. Now, the Moon is also at opposition, sort of, on the 17th. That’s when the lunar eclipse occurs. So it stands to reason that the Moon and Saturn should be close together in the sky on the 17th — and, boy, are they ever!
On that day, before dawn, those of you in western North America can watch the nearly full Moon cover up Saturn. For example, from San Francisco, this disappearance starts at 4:06, and it’ll take about a minute for the Moon’s orbital motion to push completely across the planet. You won’t see this by eye, because the Moon will be very bright, but it’ll be a real treat to witness through binoculars or a telescope. So that’ll be the payback you West Coasters get for missing a chunk of the lunar eclipse the following evening.
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Soon after the Sun goes down, as evening twilight is fading away, look almost directly above the sunset point, and you’ll see a bright star well up in the sky. That’s Arcturus, an ancient Greek name meaning “Guardian of the Bear” — in this case the constellation Ursa Major, the Big Bear, which is off to its right by about four fists. Arcturus is a red-giant star about 37 light-years away. When the light you’re seeing now left Arcturus, President Ronald Reagan visited the Berlin Wall, and the most popular movie was Beverly Hills Cop III. My, how time flies. Arcturus is the 4th brightest star in the nighttime sky, both because it’s relatively close by and because it outshines the Sun by 170 times.
Now swing your view way to the left and a little farther down, by about five fists. That warm-hued star is the red supergiant Antares, marking the heart of the constellation Scorpius. If you have a clear view toward south, look about three fists to the left of Antares for the Teapot-shaped stars of Sagittarius.
Shift your gaze even farther to the left, and you encounter the constellation Capricornus, which is supposed to represent a quirky creature with the head of a goat and the tail of a fish. Despite being so well known as a constellation of the zodiac, Capricornus consists of fairly dims stars. Look for a stacked pair of stars that are on the dim side of medium-bright. Traditionally these mark the Sea-Goat’s head. But I find it easier to make out Capricornus if I imagine those two stars as the right-hand bow of a round-bottomed ship, kind of like Noah’s ark. Do you see it that way too?
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Let’s go after some stars that are slam-dunk bright, even if you have a lot of light pollution. Lift your gaze way up to find a bright star that’s almost directly overhead. That’s Vega [VAY-guh]. It’s relatively close as stars go, just 25 light-years away. And, by the way, Vega plays a big role in the classic science-fiction movie Contact.
Since Vega is right overhead at nightfall, let’s use it to find three nearby compact, four-sided patterns. The first of these is to Vega’s immediate left, toward east. You’re looking for a parallelogram about as long as three outstretched fingers. These are the main stars of the constellation Lyra, the Lyre, and Vega is its alpha star.
Now look to Vega’s north by about 1½ fists. This time look for a compact trapezoid, with two modestly bright stars and two faint ones, also about three fingers across. This is the head of Draco, the Dragon, and its long line of stars snakes from the head to form a curving line between the Big and Little Dippers. 
The third quartet is to the west of Vega. Its stars form a trapezoid that’s bigger than Draco’s, roughly the size of your clenched fist, but a little dimmer. This is known as the Keystone of the constellation Hercules, a great hero in Roman mythology, whose muscular torso and legs are represented by stars splaying out from the corners of the trapezoid. Look beyond the Keystone toward west, a little more than one fist, to spot a lovely semicircle of stars called Corona Borealis, the Northern Crown. This is the area that astronomers are hoping to see a nova erupt dramatically into view before the end of the year. For more details about that, check out SkyandTelecsope.org.
These four star patterns might not be obvious if you have a lot of light pollution, but here’s one that’ll be unmistakable. Start at Vega and slide your gaze by about 2½ fists eastward, in the direction opposite Arcturus, to a prominent star. That’s Deneb. It’s roughly 100 times farther away than Vega — but it’s still quite the sparkler, isn’t it? That’s because Deneb is a real powerhouse of a star, pumping out nearly 200,000 times more light than our Sun does.
Now look for a third bright star, Altair, about three fists to the south of Vega and four from Deneb. Week by week this trio of easy-to-spot stars, known collectively as the Summer Triangle, is gradually migrating westward in the sky. 
If you’re lucky enough to live in a really dark location, far from city lights, you’ll have no trouble seeing a delicately glowing river of light that we call the Milky Way vaulting high across the sky. That’s the combined light of countless stars in our galaxy’s midsection. As it turns out, some of its richest concentrations of stars are in the vicinity of Deneb. Can you see a faint glow up there? If you’re not sure, try again closer to midnight, when many lights have been shut off and Deneb itself is almost overhead. If you still can’t see the star cloud, you can blame light pollution, which sadly has robbed most of us of the chance to see the Milky Way at all.
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In the next few weeks you’ll surely hear about a comet that is passing through the inner solar system. It’s called Comet Tsuchinshan-ATLAS named jointly for the telescopes in China and South Africa that discovered it. As of September 1st, no one is quite sure how bright this comet might get. But if would want to try to spot it, go out 45 minutes before dawn on September 29th or 30th, and locate the super-thin crescent Moon over in the east. The comet is to the Moon’s lower right by two fists on the 29th and 1½ fists on the 30th. Use binoculars for this. And if you don’t spot it, don’t worry. Astronomers expect it to reappear in the evening sky early next month, maybe bright enough to see just by eye. Fingers crossed!
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Thanks for letting me expand your celestial horizons for another month. If you want more tips for viewing the night sky, including a free, interactive star chart for any time or date, check out our website, SkyandTelescope.org.
If you haven’t already subscribed, you can find Sky Tour on Apple Podcasts, Spotify, Stitcher, or wherever you listen. And if you’ve enjoyed listening, please leave a rating or review! It will help spread the word about Sky Tour, and I really welcome your feedback.
And if you want to explore the solar system and universe more deeply, check out the full line of binoculars and telescopes available at Celestron.com.
Sky Tour is a production of Sky & Telescope, a division of the American Astronomical Society, and is produced by me, Kelly Beatty.
Next month we’ll spend some time with Cassiopeia and find out why she’s so vain. Until then, I wish you Clear Skies.