Atlas · August 19, 2026
The Summer Triangle: What Its Three Sides Actually Measure
Fondateur de SkyChart
Look up around midnight at the moment: the three brightest stars above you form a triangle that takes up half the sky. Everyone points at it, nobody ever gives its dimensions. We measured them in the catalogue that runs the app: 23.85° from Vega to Deneb, 34.20° from Vega to Altair, 38.01° from Deneb to Altair. One side is therefore half as long again as another: the Summer Triangle is nowhere near equilateral, and that is the first thing to know if you want to recognise it without hesitating.
The Triangle drawn on the real sky, with stars down to magnitude 6.2 taken from the same catalogues as the app. Under each vertex, its magnitude. On each side, its computed angular length. In green, the four Messier objects that fall inside. Constellations carry their three-letter IAU abbreviation, which is the same in every language.
SkyChart illustration
Three stars, three constellations, and no official standing
The Summer Triangle is not a constellation. The International Astronomical Union recognises 88 of them, with boundaries fixed in 1930, and this is not one. It is an asterism: a convenient figure, drawn by stars that belong to three different constellations. Vega sits in Lyra, Deneb in Cygnus, Altair in Aquila. None of the three was ever meant to go with the other two; we are the ones joining them up.
| Star | HIP | Right ascension | Declination | Magnitude | B−V index | Constellation |
|---|---|---|---|---|---|---|
| Vega | 91262 | 18 h 36.9 | +38.784° | 0.03 | 0.00 | Lyra |
| Altair | 97649 | 19 h 50.8 | +8.868° | 0.76 | 0.22 | Aquila |
| Deneb | 102098 | 20 h 41.4 | +45.280° | 1.25 | 0.09 | Cygnus |
The three vertices, as our catalogue records them
The magnitude column holds a surprise for anyone who has looked at the sky without ever reading the figures: the three stars are not remotely equal in brightness. Vega at 0.03 is three times brighter than Deneb at 1.25. Out in the field the difference is obvious once you think about it, and invisible until you do: that is what a logarithmic scale does to you.
A triangle that is nothing like equilateral
The three sides measure 23.85°, 34.20° and 38.01°, a perimeter of 96.1 degrees. The shortest, Vega to Deneb, is only 63 % of the longest. The area enclosed is 418 square degrees, that is 1.01 % of the whole celestial sphere, which holds 41,253. One hundredth of the sky in a single figure: that is what makes it recognisable at a glance, and also what makes it impossible to photograph on a phone without a very wide lens.
These lengths are angular distances, not real ones. They say nothing about how far away the three stars are, and the three have no relationship in space at all: they merely happen to lie in the same direction from here. The Triangle is an effect of perspective, valid from the Earth and from nowhere else.
Vega, the zero point of every colour
The last column of the table repays a pause. The B−V index measures a star's colour: negative for blue ones, positive for red. Our catalogue gives Vega as exactly 0.00. That is neither a coincidence nor a rounding: Vega is the star that was used to define the zero of the scale. Every stellar colour published for the past century is measured against it. When you look at Vega, you are looking at the standard.
The other two vertices sit at 0.09 and 0.22, so barely yellower. All three stars of the Triangle are white, which is why the figure looks so homogeneous even though its three brightnesses differ by a factor of three. Next door, a star like Tarazed, just above Altair, reads 1.51: that one is frankly orange, and the eye can tell.
What is inside it
We ran the app's deep-sky catalogue through a spherical point-in-triangle test. The result: 17 objects fall inside, of which twelve open clusters, two globular clusters, two nebulae and one planetary nebula. Four of them are in Messier's catalogue.
| Object | Type | Magnitude | Constellation |
|---|---|---|---|
| M27, the Dumbbell | planetary nebula | 7.4 | Vulpecula |
| M29 | open cluster | 6.6 | Cygnus |
| M56 | globular cluster | 8.4 | Lyra |
| M71 | globular cluster | 6.1 | Sagitta |
The four Messier objects enclosed by the Triangle
But the prettiest result is not in that table. Testing the drawn figures of all 88 constellations, we find that two entire figures fit inside the Triangle: Sagitta, whose five pattern stars are all in, and Vulpecula, whose two are as well. They are among the smallest constellations in the sky, and they go entirely unnoticed next to the three beacons framing them. Lyra and Cygnus, by contrast, spill well outside.
How to find it tonight
In late August the Triangle is at its highest in the middle of the night. From Paris, Deneb culminates at 86.4° around 00:38, that is four degrees from the exact zenith: you have to lie down to look at it comfortably. Vega culminates at 79.9° around 22:34, Altair at 50.0° around 23:48. Finding it is easy: it is the only bright figure directly above your head, and Vega is its brightest star. From London, Dublin or Edinburgh, subtract an hour from those times.
One detail of latitude, to finish. From Paris, at 48.86° north, a star never sets if its declination exceeds 41.14°. Deneb, at 45.28°, is therefore circumpolar: it is above the Paris horizon every night of the year, January included. Vega, at 38.78°, misses that status by 2.4 degrees and does set, but only just. You would need to be at 51.2° north for it to stop setting: Lille, at 50.63°, is not enough; London, at 51.51°, manages it.
What these figures do not tell you
The positions come from the catalogue built into the app, in J2000 coordinates. Stars have proper motion: over a century, Vega moves about half an arcminute. On the scale of a 24-degree side that changes nothing measurable, but the Triangle is not eternal: in a few tens of thousands of years it will no longer look like itself.
The side lengths and the area are computed on the sphere, not on a flat map. That is why the three angles of the triangle add up to more than 180 degrees, and it is that excess which gives the area directly. On any given projection, the three sides will look unequal in some other way.
The membership test uses the catalogued positions of the objects and the constellation patterns we draw, not the official IAU boundaries. A constellation whose pattern fits inside the Triangle may have a boundary that does not; that is the case for Sagitta and Vulpecula, whose official rectangles spill slightly outside.
Finally, our catalogue does not carry stellar distances, so you will find no light years here. That is deliberate: we publish only what the engine computes. To see the Triangle at your own hour and from your own position, the SkyChart sky map draws it live. And if you are after the Milky Way that runs through it, we computed what it really amounts to at our latitudes.
This article was translated from the French edition by AI, then reviewed before publication. The figures are not translated: coordinates, magnitudes, angular lengths and star counts are computed by SkyChart and are identical in both editions.