ToolComputedCited
Is it dark enough, and for how long?
Answer
Anchorage gets 11 h 5 min of civil night on the evening of October 1, 2026, 7 h 37 min of it astronomical, with 4 h dark inside 10 PM to 2 AM.
| Civil nightSun at or below -6° | 11 h 5 min |
|---|---|
| Nautical nightSun at or below -12° | 9 h 24 min |
| Astronomical nightSun at or below -18° | 7 h 37 min |
| Dark inside 22:00 to 02:00Civil night in the SWPC window | 4 h |
| Civil dusk | 20:16 |
|---|---|
| Nautical dusk | 21:07 |
| Astronomical dusk | 22:01 |
| Solar midnight | 01:48 |
| Astronomical dawn | 05:37 |
| Nautical dawn | 06:31 |
| Civil dawn | 07:22 |
Worked example
Tromsø on the evening of October 1, 2026: Tromsø gets 10 h 45 min of civil night on the evening of October 1, 2026, 5 h 31 min of it astronomical, with 4 h dark inside 10 PM to 2 AM. Civil dusk falls at 19:11 and civil dawn at 05:57 Europe/Oslo time.
The table below runs the same calculation for all 10 Glow places on tonight's date in each place's own time zone.
| Place | Civil | 22:00 to 02:00 | ||
|---|---|---|---|---|
| Fairbanks | 11 h | 6 h 58 min | 4 h | 20:10 |
| Anchorage | 11 h 5 min | 7 h 37 min | 4 h | 20:16 |
| Seattle | 11 h 15 min | 8 h 50 min | 4 h | 19:21 |
| Minneapolis | 11 h 15 min | 8 h 58 min | 4 h | 19:25 |
| Duluth | 11 h 15 min | 8 h 53 min | 4 h | 19:20 |
| Portland | 11 h 15 min | 9 h 1 min | 4 h | 18:53 |
| Buffalo | 11 h 16 min | 9 h 3 min | 4 h | 19:27 |
| Reykjavik | 10 h 59 min | 7 h 4 min | 4 h | 19:48 |
| Tromsø | 10 h 45 min | 5 h 31 min | 4 h | 19:11 |
| Yellowknife | 11 h 3 min | 7 h 24 min | 4 h | 19:56 |
Civil, nautical and astronomical night
Twilight is defined by how far the sun sits below the horizon. Civil night starts when the sun's center is 6 degrees down, nautical night at 12 degrees and astronomical night at 18 degrees. Each step removes more scattered sunlight from the sky.
Glow starts the aurora window at civil dusk. A bright display can show against a deep blue civil sky, and at high latitudes in late spring and late summer civil night may be the only night there is. The nautical and astronomical rows tell you how much darker the sky gets after that.
The calculator samples the sun's altitude every 5 minutes from local noon to the next noon and interpolates each crossing, so durations are good to about a minute. It uses the time zone you pick for every clock time.
The 10 PM to 2 AM row
SWPC's viewing tips say the best aurora is usually within an hour or two of midnight, between 10 PM and 2 AM local time. The last duration row counts the civil-dark minutes inside that window, up to 4 hours.
A place can have a long night and a short window, or the reverse. In high-latitude summer, dusk can come after 22:00 or not at all. In winter the window is always dark at every Glow place, and the question moves to the moon and the weather.
White nights and polar night
North of about 60 degrees, the summer sun can stay above -6 degrees all night. The calculator then reports no civil night instead of inventing a dusk time. Near the winter solstice the reverse happens farther north: places above the Arctic Circle see no sunrise, but the sky at noon can still be civil twilight. Glow treats that noon as twilight, not night, because the sun sits above -6 degrees.
The calculation is geometric. It ignores terrain, buildings, cloud, air pollution and refraction beyond SunCalc's standard model.
Questions
- How dark does it need to be to see the northern lights?
- Glow starts at civil dusk, when the sun is 6 degrees below the horizon. Faint aurora needs a darker sky; the nautical and astronomical rows show when that comes.
- Why does the night run from noon to noon?
- A night crosses midnight. Starting at local noon keeps dusk and dawn on one row and handles places where it is still dark at noon.
- Why is the 22:00 to 02:00 figure capped at 4 hours?
- That window is exactly 4 hours long. The row shows how much of it is civil-dark on your date.
- Does daylight saving time change the result?
- It changes the clock times, not the darkness. Pick the correct IANA time zone and the table uses that zone's offset for the date.
- Why do my times differ from another website by a minute?
- Glow interpolates between 5 minute samples of the sun's altitude. Other sites may solve for the crossing directly or use a different refraction model.