On clear September evenings, I often find myself looking up just after dusk, expecting the sky to feel a little less like summer. Yet there, still commanding the overhead darkness, are Vega, Altair, and Deneb — the three brilliant stars of the famous Summer Triangle.

Their familiar pattern can make the season feel as though it is lingering above us, even as cooler nights and earlier sunsets suggest otherwise. In fact, even as late as mid-November, the Triangle is still nearly overhead soon after sunset at dusk. That contrast raises a simple but intriguing question: if this bright celestial landmark remains so prominent well into autumn, why do we still call it the Summer Triangle?

Why not simply call it the Autumn Triangle?

Why it belongs to summer

The answer lies not in the calendar, but in how the Triangle moves across the sky over the course of a summer night. The chief reason is that, at this time of year, the Triangle does not remain visible all night. By dawn, it has begun to drop beyond the west-northwest horizon. During summer, by contrast, it remains with us from dusk until dawn: already evident in the east-northeast at nightfall, near overhead around the middle of the night, and descending in the west-northwest as daybreak approaches.

A matter of distance and brightness

The stars that compose the Triangle — Vega, Altair, and Deneb — lie at very unequal distances from us, so their apparent brightness can give a misleading impression of their true luminosity.

The closest of the three is Altair, 16.7 light-years away, followed by Vega at 25 light-years. Deneb, however, is vastly farther off, at roughly 1,600 light-years, though various sources place it between 1,400 and 2,600 light-years. Even though we see Deneb by light that may have begun its journey during the last days of the Roman Empire, it still shines at first magnitude (+1.3, to be exact) because it is a brilliant blue supergiant, at least 55,000 times brighter than the sun.

But their apparent superiority is largely a consequence of distance: Vega and Altair are only a few dozen light-years away, while Deneb lies well over a thousand light-years from Earth. Compared with the stellar lighthouse of Deneb, Vega and Altair are, comparatively speaking, penlight flashlights.

How to say "Vega"

Before continuing with this discussion about the Summer Triangle, it is worth pausing briefly on the pronunciation of Vega, a point that became especially noticeable after the 1997 science fiction drama Contact. Based on Carl Sagan's 1985 novel, the film follows astronomer Dr. Ellie Arroway, played by actress Jodie Foster, as she discovers a radio signal from Vega containing a sequence of prime numbers.

Throughout the film, the star is pronounced "Vey-gah," perhaps echoing Chevrolet's Vega, the subcompact automobile sold in the 1970s. Astronomers, however, generally use a different pronunciation.

The traditional astronomical pronunciation is Vē-ga, with the stress on the first syllable. In professional astronomical usage, Dr. Arroway and her colleagues would almost certainly have said "Vee-ga," not "Vey-gah." George Lovi (1939—1993), who was widely respected for his knowledge of star nomenclature, often reminded audiences that "Vee-ga is a star and Vey-gah is a car."

Who named the Summer Triangle?

There has long been debate over who first christened the bright stellar trio of Vega, Deneb, and Altair as the Summer Triangle. Some credit the legendary British astronomer Sir Patrick Moore (1923—2012), who wrote, "During a 1958 television broadcast on the BBC, I introduced the nickname of the Summer Triangle, and everyone now seems to use the term, even though it is completely unofficial." Yet several years earlier, in his 1952 sky guide, The Stars — A New Way to See Them, H.A. Rey (1898—1977) referred to the three stars as a "right triangle," and in his 1954 children's book, Find the Constellations, Rey used the Summer Triangle moniker.

Still others believe the term was popularized through U.S. navigator training manuals during World War II. Interestingly, astronomer Oswald Thomas (1882—1963) described Vega, Altair, and Deneb as "Grosses Dreieck" ("Great Triangle") in the late 1920s and as "Sommerliches Dreieck" ("Summerly Triangle") in 1934.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope, The Old Farmer's Almanac and other publications.

Today (Sept. 8), skywatchers can first catch a striking predawn pairing of the crescent moon and Jupiter, then — where conditions and location allow — attempt the much harder daytime occultation, when the moon hides the planet from view.

Before sunrise: Find the moon and Jupiter

Set your alarm for 5 a.m. local time on Tuesday and look low toward the east-northeast sky. Check in advance for tall obstructions that could block your view of this delicate predawn tableau. A slender waning crescent moon — just under 10% illuminated and resembling a "celestial smile" — will be immediately apparent. About 5 degrees below it, roughly half the width of your clenched fist held at arm's length, will be brilliant Jupiter.

Jupiter rises in the east-northeast shortly before 4 a.m. local daylight time, but it does not climb fairly high until just before dawn's first light. The planet is in the dim zodiacal constellation Cancer; one of my astronomy mentors at New York's Hayden Planetarium, Dr. Ken Franklin, used to call Cancer "the empty space between the Sickle of Leo and the Gemini Twins." If you set up a telescope before dawn, you can be among the first to see what became of Jupiter's tumultuous cloud belts while the planet was hidden in the sun's glare from early July through mid-August.

Later in the day: The daytime occultation

Another moon-Jupiter show awaits later in the day: a daytime occultation, when the thin waning crescent moon will hide Jupiter from much of North America. Unfortunately, a swath of the western United States — anywhere west and south of a line running roughly from Bellingham, Washington, to Rockport, Texas — falls outside the occultation's visibility zone.

The occultation occurs during the afternoon, when the moon and Jupiter will appear somewhat lower than the sun and well off to its right. Jupiter will disappear behind the moon's sunlit limb, with the moon taking roughly a minute or more to inch across the giant planet's face. Approximately 50 minutes later (give or take ten minutes), Jupiter will gradually reappear from behind the moon's dark edge.

Jupiter will vanish (going from north to south) between 2:55 and 3:30 p.m. for those in the Eastern Time Zone, 1:45 to 2:20 p.m. for the Central Time Zone, and 12:30 to 1:00 p.m. in the Mountain Time Zone.

To find out when these events occur at your location, use the accompanying map. Interpolate between the ten selected cities to find the local daylight time of Jupiter's disappearance (top) and reappearance (bottom) for your site.

How to find them in daylight

The chief challenge will be finding the moon and Jupiter at all, so give yourself plenty of time. The easiest method is to locate the pair before sunrise and track them into daylight. Much will depend on the transparency of your local sky — how deep its blue color is. If the day is hazy, the search will be harder.

1. Start before sunrise, when the crescent moon and Jupiter are easiest to identify.
2. As daylight strengthens, keep tracking the pair or return to the same region of sky about 30 degrees — roughly four fist-widths at arm's length—from the sun, mostly to the right and, for most locations, somewhat lower.
3. Sweep the area with a finderscope or binoculars, remembering that the crescent moon's low surface brightness may make it hard to distinguish from the blue sky.

Once you have the moon, the next challenge is Jupiter. In a telescope, it will appear far smaller than the moon — about one-sixtieth of the moon's diameter — and it will have an even lower surface brightness. Watch for Jupiter to return along the moon's dark limb, a captivating sight as the planet seems to emerge literally from out of the blue.

If you miss it

Sighting the moon and Jupiter in daytime will be challenging, so do not despair if you fail to find them. In less than a month, on the morning of Tuesday, Oct. 6, the moon and Jupiter will have another interaction, but this time about eight hours earlier, before sunrise in a dark or twilight sky. That resultant event should be a spectacular sight, easily visible with the naked eye, binoculars, or a telescope. Space.com will provide details as the date approaches, so stay tuned!

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope, The Old Farmer's Almanac and other publications.

Soon, the sky's most glamorous evening performer will take its leave.

Since late March, Venus has been the unmistakable "Evening Star," gleaming shortly after sunset and stealing attention from nearly everything else in the twilight. In June, it helped stage a striking three-planet lineup after sundown, outshining both Jupiter and Mercury.

Now the curtain is beginning to fall: Venus is sinking night by night and will trade its evening role for a morning one before October is over.

A familiar dance around the sun

Venus moves to a remarkably regular beat. Our nearest planetary neighbor circles the sun in an almost perfect path, completing 13 orbits in 8 Earth years. From Earth, that rhythm makes Venus appear to trace five great loops across the sky in the same span. It is a long cycle noticed by skywatchers — and important to the ancient Aztec and Mayan civilizations — and it gives each Venus year its own recognizable personality. In 2026, the pattern closely echoes 2018, differing by only about two days.

Key dates this month

Venus is about to blaze at peak brilliance, but this farewell performance comes with a challenge. All month long, the planet will slide lower in the southwest after sunset, shortening the window for a good view. If the horizon is clear and the weather cooperates, these are the evenings to circle on your skywatching calendar.

• Sept. 7: On Labor Day, Venus sets about 1¼ hours after sunset.
• Sept. 14: Look low in the west-southwest 30 to 45 minutes after sunset, when a waxing crescent moon appears around 5½ degrees to the left of Venus.
• Sept. 18: Venus reaches greatest brilliancy, shining at a dazzling magnitude of
  -4.8 near the horizon.
• Late September: The planet slips quickly from view, setting just 35 minutes after sundown by month's end.

Why this farewell is so low-key

Ordinarily, the weeks between greatest elongation and greatest brilliancy are prime time for Venus watchers. Not this year. For evening observers, this apparition has been more frustrating than triumphant, with Venus lingering low in the glow of twilight instead of climbing into a commanding position.

The best views came back in June, when Venus lingered until around 11 p.m. Since then, it has been ducking out earlier each night. By Aug. 15, the evening of greatest elongation — when Venus reached its greatest apparent distance east of the sun — it was already setting just as evening twilight faded.

For observers at mid-northern latitudes, the finale is especially low to the horizon. Around 40 degrees north latitude, Venus will stand only about 5½ degrees above the southwest horizon at sunset by the end of September. That is barely more than half the width of your clenched fist held at arm's length — a beautiful but fleeting target in the fading light.

A crescent world in the twilight

Through a telescope, Venus now shows one of its most striking faces: a narrowing crescent. As its sunlit side turns farther away from Earth, that crescent grows thinner and longer, becoming a delicate arc of brilliant light. With very steady binoculars, you may even glimpse the shape during the next couple of weeks, though the planet's low altitude will make the view shimmer and blur.

The morning star returns

By the end of the first week of October, Venus will have slipped from the evening sky altogether. It reaches inferior conjunction on Oct. 24, passing about 6 degrees south of the sun. Around that time, spotting it on nearby evenings or mornings will be extremely difficult.

But Venus never stays away for long. In November, it reappears before dawn, opening a brilliant new morning apparition. By the winter holidays, it will shine high in the eastern sky as a fitting "Christmas Star," rising more than two hours before daybreak — and nearly four hours before the sun.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope, The Old Farmer's Almanac and other publications.