Summer Triangle Stays in the Evening Sky as Autumn Deepens

The Summer Triangle, a famous pattern of three bright stars, has not left the night sky even though summer is long over. According to EarthSky, stargazers in the Northern Hemisphere can find it in November and December, high in the west in the early evening.

The group is not a constellation. EarthSky describes it as an asterism, meaning an obvious pattern or group of stars that carries a popular name. Each of its three stars belongs to a different constellation, and together they mark out the signature star formation of the Northern Hemisphere's summer sky. After the September equinox, the trio keeps showing up as the weeks of autumn pass.

The Summer Triangle by the numbers

• 5th-brightest: Vega, in Lyra the Harp, is the brightest of the three and ranks fifth among all stars in our night sky. EarthSky says it has a blue-white color.
• 25 light-years: This is the distance to Vega, according to EarthSky.
• 16.7 light-years: Altair, in Aquila the Eagle, is the second-brightest of the trio and sits this far away. EarthSky says it is easy to recognize because two fainter stars flank it closely.
• 1,400 to 2,600 light-years: Deneb, in Cygnus the Swan, is the faintest of the three to the eye. EarthSky describes it as a luminous supergiant and one of the most distant stars visible without a telescope or binoculars, at roughly this range of distances.
• 10 to 11 p.m.: In November, at mid-northern latitudes, Altair, the southernmost star of the group, sets around this time.
• Four minutes: The triangle returns to the same spot in the sky about this much earlier each day.

Why Deneb looks dim but is not

The three stars do not look equally bright, and distance explains much of the difference. Deneb is the faintest of the trio as seen from Earth, but EarthSky notes that this is for a good reason: it lies far beyond its two companions. The report gives a wide range for its distance, from about 1,400 to about 2,600 light-years, which is many times farther than Vega or Altair.

Altair is the closest of the three at 16.7 light-years. Vega, at 25 light-years, is also relatively near by comparison.

Why it matters for anyone who likes to look up

For casual observers, the practical value is that a familiar sight remains available after the summer season ends. The triangle needs no equipment to spot, since EarthSky points to its stars as bright and easy to pick out with the unaided eye. Because the shape is so recognizable, it also works as a signpost for finding the surrounding constellations, each of which contains one of its stars.

The timing has changed since summer, though. EarthSky says that around the June solstice the triangle rises in the east as darkness falls and shines all night. In November, it instead appears high in the west after sunset and then slides westward as the evening goes on. All three stars stay above the horizon until mid-to-late evening, so the early part of the night is the best time to look.

How to find it and track it

EarthSky suggests looking west in the early evening and noting where the triangle sits at a given time. Because it comes back to that same place about four minutes earlier with each passing day, or two hours earlier with each passing month, observers can predict where it will be on later nights and watch it move across the sky through the season.

Altair, the southernmost of the three, is the first to disappear, setting around 10 to 11 p.m. in November at mid-northern latitudes. That means the window for viewing the full triangle narrows as the night goes on, and anyone who wants all three stars at once should head outside earlier rather than later.

The details here come from EarthSky's guide, which describes the asterism as a feature of both the autumn and early winter evening sky in the Northern Hemisphere. The report does not describe any new discovery or unusual event, only the pattern's regular return to view for those who want to look for it.

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Around Prague, based on reporting by EarthSky