Observing · August 16, 2026

The Milky Way From Western Europe: 12 Degrees Up, and One Hour to See It

Mid-August is the window every guide recommends for photographing the Milky Way, and they are right: the core of our galaxy, in Sagittarius, transits early in the night, and this week the Moon is a crescent that sets early. What those guides almost always leave out is the rest. From Paris, the galactic centre culminates at 12.2 degrees of altitude. And it culminates at 21:55, in full twilight, more than an hour before the sky is dark.

That is not bad news, it is useful news: it tells you where to look, when to go out, and why the photograph you saw on social media was not taken here.

An altitude that depends on nothing but your latitude

The galactic centre, marked by the radio source Sagittarius A*, sits at 17 h 45 min 40 s of right ascension and −29° 00′ 28″ of declination. Its maximum altitude fits on one line: 90° minus your latitude, minus 29°. For Paris that gives 12.1°, which refraction lifts to 12.2°. No date, no hour and no instrument will change that number: latitude alone decides it.

Nine degrees separate Lille from Ajaccio, and twenty-two separate Lille from the Canaries. That is why the Milky Way images doing the rounds are so often shot in Chile, on Tenerife or in the American southwest: it is not only a matter of dark skies, it is first a matter of latitude. All times in this article are Paris time.

Why twelve degrees cost almost a magnitude

At twelve degrees of altitude, your line of sight crosses 4.6 times the thickness of atmosphere it would cross overhead. That is the air mass, and we compute it here with the Kasten and Young formula, which stays valid near the horizon where a plain 1/sin(h) blows up.

At a good site, each air mass costs about 0.2 magnitudes in the visible. From Paris, the atmosphere therefore takes close to a full magnitude off the galactic core, a factor of two and a half in received flux, before a single street lamp enters the picture. From La Palma it takes 0.37. Same object, same sky, and it arrives twice as bright.

And that thickness of air does not merely dim: it scatters. The artificial light of an entire region ends up precisely in the slice of sky where you are hunting for the dark lanes of Sagittarius. It is the combination of the two that explains how you can have a fine sky overhead and nothing at all in the south.

The real window, on the night of 16 to 17 August

Here is the exact timetable for the night in Paris. The Sun sets at 21:04, the Moon, a 20 % crescent, sets at 22:16, and astronomical night runs from 23:11 to 04:38. So the Moon is entirely out of the way by the time the sky is dark: that is this week’s good news.

The conclusion fits on one line: you have from 23:11 to 00:30, no more, and you must aim south-south-west, between 196 and 213° of azimuth, at an altitude falling from 11 to 5 degrees. Which means a clear southern horizon is not a comfort, it is the condition. A line of hills ten degrees high, and there is nothing to see. Transit moves about four minutes earlier each day: in a week the same scene will sit half a degree lower at the same hour of full dark, and by late September the season is over.

The consolation prize is directly overhead

The galactic core is not the whole Milky Way. At 23:15 from Paris, Vega stands at 78.7°, Deneb at 73.4° and Altair at 48.9°: the Summer Triangle is close to the zenith, and the band of the Milky Way runs right through it. That stretch, the Cygnus stretch, you look at through one air mass instead of five.

It is also the finest in binoculars: the Great Rift in Cygnus, the dark tear that splits the band in two, and the Deneb star field, one of the richest in the northern sky. Less colour, many more stars. For an observer at these latitudes it is objectively the best use of an August night.

And the darkness?

Latitude is not something you can fix in one evening. Sky background is. The Milky Way needs, at minimum, a sky whose naked-eye limiting magnitude clears 5.5, which rules out the whole periphery of any large city straight away: we computed a limiting magnitude of 3.6 at Châtelet in central Paris in our study of the Bortle scale, against 6.8 on the Millevaches plateau. And once you are there, allow twenty to thirty minutes without looking at a single screen: dark adaptation is free, and it is the most profitable equipment there is.

To find exactly where Sagittarius will fall above your own horizon, the SkyChart sky map recomputes the scene for your latitude and your hour.

The limits of these figures

Altitudes and azimuths are geometric, corrected for standard refraction, and computed against a mathematical horizon: yours is higher. Air masses follow the Kasten and Young formula of 1989. The coefficient of 0.2 magnitudes per air mass is a typical good-site value in the visible band: it climbs easily to 0.3 or 0.4 in humid or aerosol-loaded air, which widens the north-south gap further. Finally, the Milky Way is an extended object of very low contrast: none of these values predicts what your eyes will see, they only say where and when it is worth trying.

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