Aurora season: how many hours are dark enough, month by month

The rule behind the table

Aurora is faint compared with daylight and twilight. Aurorix counts an hour as dark when the sun is more than 12 degrees below the horizon, which is the end of nautical twilight. The table counts the hours of the 15th of each month that meet that limit, using the NOAA Global Monitoring Laboratory solar equations.

The numbers are an upper bound. Clouds, the moon and your Kp decide how many of those hours are useful.

Reading the table

From May to July none of the six places has a single dark hour on the 15th. April is the last month with any: Rovaniemi, Reykjavik, Fairbanks and Yellowknife still get 3 to 5 hours, Abisko only 20 minutes and Tromsø none. In August only Yellowknife has a dark spell, about 2 hours. By the 15th of September every place has between 5 and 8 hours, and by the 15th of December between 14 and 16.

The table is for the 15th only. Darkness grows fast in autumn: at Tromsø it rises from 5.5 hours on 15 September to 10.3 hours on 15 October, about 10 minutes more each day, so a 15 September figure is not a good guide to 30 September.

Midnight sun and polar night

Tromsø has midnight sun, with the sun above the horizon all day, from 19 May to 27 July, and polar night, with the sun never rising, from 28 November to 14 January. Polar night does not mean total darkness at midday: the sun is a few degrees below the horizon, which is why December has 16 hours below −12 degrees and not 24.

You can check this with the solar altitude formula. At the winter solstice the sun at noon in Tromsø is 90 − 69.65 − 23.44 = −3.1 degrees. On the summer solstice the sun at midnight is 69.65 + 23.44 − 90 = +3.1 degrees, above the horizon.

Equinoxes and geomagnetic activity

Geomagnetic activity tends to be higher around the March and September equinoxes. Russell and McPherron explained this in 1973 as an effect of the way Earth's magnetic axis is tilted to the solar wind field at those times of year. The effect is statistical: it raises the odds of an active night, it does not guarantee one.

It combines with darkness in a useful way: September already has several dark hours and falls in an equinox peak, whereas midsummer has neither. Planners often favour September to March.

Hours with the sun below −12 degrees

Calculated with the NOAA solar position equations for the 15th of each month in 2026, UTC calendar day, sampled every five minutes.

Quellen

• NOAA GML: general solar position calculations
• Russell and McPherron (1973), Semiannual variation of geomagnetic activity, J. Geophys. Res. 78
• Wikipedia: Tromsø (midnight sun and polar night dates)