Atlas · September 8, 2026

The Andromeda Galaxy Is a Naked-Eye Object, if You Know Where to Rest Your Gaze

The light entering your eye when you look at the Andromeda Galaxy left it roughly 2.5 million years ago. It is, by an enormous margin, the oldest and most distant thing a human eye can perceive without an instrument: the next photon on the list, from the Triangulum Galaxy, already demands an exceptional sky and a trained observer. And September is precisely the right month: the Moon deserts the evening sky from the 5th to the 17th, and M31 already stands 45 degrees high at 23:00 local time early in the month, 49 degrees mid-month, before culminating at 82 degrees above the horizon of Paris later in the night.

A huge object, not a faint one

The classic misunderstanding gets the problem backwards. M31 has an integrated magnitude of 3.4, which would make it an easy star, but that light is smeared over 177 arcminutes, nearly three degrees: six full Moons in a row. Its surface brightness is therefore low, and it is the sky background, not the galaxy, that decides everything.

Concretely, on the Bortle scale:

• Bortle 7 to 9, city centres: nothing with the naked eye. In 10×50 binoculars the core shows as a small fuzzy ball, which is already a respectable thrill.
• Bortle 5 and 6, the suburbs: the core can be glimpsed naked-eye with averted vision, if you know exactly where to look.
• Bortle 4 and darker: the galaxy becomes obvious and elongated, and grows in length as your eyes adapt. Under a truly dark sky it stretches beyond two degrees.

Averted vision is not astronomers' folklore: the centre of the retina is blind to faint light. Look at a point beside the galaxy and it will appear in the corner of your eye, then vanish the moment you stare at it.

The path, in three steps

1. Find Cassiopeia, the W standing high in the north-eastern evening sky. The deeper of its two notches, on the Schedar side, works as an arrow: it points at the galaxy, some fifteen degrees away.
2. Or start from the Great Square of Pegasus, due east. Its north-eastern corner is Alpheratz, the first stone of the arc of Andromeda: two stars along the arc and you are at Mirach.
3. From Mirach, climb perpendicular to the arc: μ Andromedae, then ν Andromedae, two steps of about four degrees each, a fist at arm's length for the pair. Just past ν: the smudge.

In binoculars the same walk takes ten seconds, and M31 shares the field with ν Andromedae. If your instrument is steady, look for its two companions, M32 pressed against the disc and M110 on the far side, shier.

Why September rather than December

M31 is visible six months of the year, but September stacks three computed advantages: the moonless window falls exactly in the early evening from the 5th to the 17th, the galaxy rides high, 82 degrees on the meridian of Paris, which lifts it out of the horizon haze, and the nights are still mild. In December it culminates at 21:00, but the Moon, the weather and the frost interfere every other year.

What these figures do not tell you

The 2.5 million light-year distance is a rounded average of recent measurements: the best estimates still scatter by a few percent. The 82-degree culmination is for Paris; it reaches 84 from Lille and drops to 76 from Ajaccio, and from anywhere in Western Europe the galaxy passes high enough to ignore the horizon. The real limiting factor is your sky background, and that cannot be computed from an armchair: the SkyChart sky map displays M31's real light at its exact position above you, along with the Bortle class of where you stand, and will tell you honestly whether tonight is worth the trip.

This article was translated from the French edition by AI, then reviewed before publication. The figures are not translated: magnitudes, sizes and altitudes are computed by SkyChart and are identical in both editions.

• Beginners
• September 2026 sky
• Bortle scale