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Dark sky hours: when it gets truly dark tonight
Sunset is not darkness. Full darkness begins when the Sun is 18 degrees below the horizon, sometimes two hours later, and in the far north some nights never get there.
Real darkness at Minneapolis, MN, Tue, Oct 13
9.7 h
Sun below 18°: 8:08 PM to 5:50 AM CDT
| Dark sky (Sun 18° down) | 8:08 PM to 5:50 AM CDT |
|---|---|
| Moon | 13% lit |
| Magnetic latitude (AACGM) | 54.2° |
The next seven nights (CDT)
| Night of | Dark sky | Moon at mid-darkness |
|---|---|---|
| Tue, Oct 13 | 8:08 PM to 5:50 AM | down |
| Wed, Oct 14 | 8:07 PM to 5:51 AM | down |
| Thu, Oct 15 | 8:05 PM to 5:52 AM | down |
| Fri, Oct 16 | 8:03 PM to 5:53 AM | down |
| Sat, Oct 17 | 8:02 PM to 5:55 AM | down |
| Sun, Oct 18 | 8:00 PM to 5:56 AM | down |
| Mon, Oct 19 | 7:58 PM to 5:57 AM | up |
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Astronomers call the sky fully dark once the centre of the Sun is more than 18 degrees below the horizon: the last glow of twilight is gone and the faintest objects, a weak aurora among them, can show. This tool computes that moment and the moment the sky starts to brighten again, for the place and the night you choose, from the same solar equations as the NOAA solar calculator, tested on this site against the JPL DE421 ephemeris to within two minutes. In Minneapolis on December 15 the dark sky lasts 11.7 hours and on June 15 only 3.3. Fairbanks gets 13.0 hours in mid-December and none at all from April to August. The table under the result gives the next seven nights and whether the Moon is up in the middle of each.
How the dark window is computed
For each night the tool starts at local mean solar noon, the moment the Sun crosses your meridian on average, and steps through the next 24 hours five minutes at a time, computing the Sun's altitude from its declination and the equation of time, the method published by the NOAA Global Monitoring Laboratory. When the altitude crosses -18 degrees, the crossing is refined to the second. The same is done at -12 degrees for nautical twilight. If the Sun never goes below the threshold the night is reported as having no darkness; if it never comes back above it, as polar night.
Refraction, the bending of light that lifts the Sun near the horizon, is left out: at 18 degrees below it does not apply. The result is accurate to a minute or two, which is far below the hour-by-hour resolution of the aurora forecast.
Why darkness decides more nights than Kp
Over a year, the places with the most aurora are also the ones with the shortest nights in summer. A town in the northern lower 48 gets roughly twice as many dark hours in December as in June, and the far north gets none in June. Add the cloud climate, and the clear dark hours of a month, the figure on every state and city page, vary more than the aurora itself. In Minneapolis the clearest dark month by that measure is January; in Fairbanks it is January. The best time guide goes through the seasons and the equinox effect.
Latitude and season, in numbers
Two effects combine. Farther north, summer nights shorten and winter nights lengthen; and at any latitude, twilight lasts longer the farther you are from the equator, because the Sun sinks at a shallower angle. That second effect is why a city in the northern states loses so much darkness in June: the Sun goes down, but it never gets far enough below the horizon. On June 15, full darkness lasts 3.3 hours in Minneapolis, 5.0 in Denver and none in Edinburgh. On December 15 the order reverses: 11.7 hours in Minneapolis, 13.0 in Fairbanks. The tool shows the next 30 nights for any place, and the state and city pages give the 15th of every month.
The Moon in the table
The last column says whether the Moon is above the horizon at the middle of the dark window, from the low-precision lunar formulas of the Astronomical Almanac. The Moon does not stop the aurora, but SWPC notes that a full Moon lowers its apparent brightness, and on the edge of your range it is often the difference between a glow you notice and one you miss. Moon-free nights around the new Moon are the ones to aim for when you have a choice.