How to observe the northern lights

A few classic mistakes to avoid:

• Looking at your phone: every glance at the screen destroys 15 minutes of dark adaptation. Night mode mandatory (red filter), brightness at minimum.
• Using binoculars or a telescope: useless. Auroras are diffuse and extended, observed with the naked eye. Binoculars can help spot fine structures (rays, folds) but are never required.
• Looking towards the city: the aurora is north. If your city is north, change site.
• Giving up after 5 minutes: a faint aurora takes several minutes to emerge in scotopic vision. Give yourself at least 30 minutes.

The human eye has two photoreceptor types: cones (daytime, colour) and rods (night, black-and-white, very sensitive). Switching from day to night vision takes about 20 to 30 minutes, during which the pupil dilates and the retina regenerates its rhodopsin stock.

To protect this adaptation:

• Use a headlamp with a red LED only. Red does not destroy adaptation.
• Silence all phone notifications. If you must consult the Pulsar app, use its built-in red mode.
• Turn your back to any even-faint parasite lights (passing car headlights, guesthouse lighting).

Do not expect the saturated Instagram image. Here is what the eye actually sees from France, by increasing intensity:

• Kp 6, dark site: grey glow barely brighter than the background sky, skimming the northern horizon. Invisible to untrained eyes, photographable.
• Kp 7, dark site: greenish or reddish glow, visible once adapted. Faint column definition.
• Kp 8, dark site: clear show, visible columns and bands, distinct green or red. Comparable to a faint Scandinavian aurora.
• Kp 9, dark site: major show, tall columns, intense colours, dynamic folds. Equivalent to a 'moderate' night under the auroral oval.

Long-exposure cameras reveal colours the eye does not see (rods are poorly sensitive to long wavelengths). Do not be disappointed by the gap between eye and photo: both are real.

For naked-eye observation in autumn or winter, at 3am, in 2°C:

• Warm layered clothing: inertia is the real enemy. An old down jacket, a hat, gloves, two pairs of trousers. Cold saps patience.
• Folding chair or mat: you observe for a long time. Seated, you last for hours.
• Thermos of hot tea: psychology matters.
• Red-LED headlamp: counts for safety on the drive.
• Charged phone with the Pulsar app, red mode on.
• Compass or phone compass app to precisely orient due north.

No binoculars, no telescope, no camera mandatory. If you want to photograph, see the next section.

Once on site:

1. Orient yourself due north (compass or Polaris). Polaris is found via the Big Dipper: extend the line of the two stars at the back of the bowl by 5x.
2. Scan the northern horizon 60° left and right. An aurora can appear off-centre if the oval is asymmetric.
3. Look for a glow different from the rest of the sky: greener, redder, purpler, or just lighter. Light pollution halos are yellowish and stable; an aurora is cooler and more dynamic.
4. When in doubt: take a phone photo in night mode, 6-10 seconds, high ISO. If an aurora is there, it will show up.