Northern lights, month by month

• A place is under the auroral oval on an ordinary night where its geomagnetic latitude is 60° or more. NOAA's Space Weather Prediction Center puts the oval's edge near 66° on the quietest nights (Kp 0), 2° nearer the equator for each step of Kp, the index of magnetic activity from 0 to 9: 60° at Kp 3.
• A dark night has 6 hours or more with the sun 12° or more below the horizon, past nautical twilight.
• A month is good where 2 nights in 3 are dark, and fair where 1 in 2 are.
• Cloud is not counted, and a cloudy night hides the lights: the months say when the sky is dark enough over a place under the oval, not whether the lights will show. That is a forecast's, on the night.
• Geomagnetic latitude along the real field, AACGM-v2 (Shepherd 2014) on the International Geomagnetic Reference Field (IGRF-14, 2025.0; IAGA), at the aurora's height; darkness from the sun's position by NOAA's solar equations, after Meeus; the oval's reach from NOAA's Space Weather Prediction Center.

A test on figures computed from where each place is, the same at every place and from every city: the same month always gives the same answer. It is "good" or "fair", and a month that passes neither is simply not listed. It is separate from the month's own verdict, which also weighs holidays and crowds. The sun keeps the same dates to within a night from year to year.