How dark does it need to be to see the aurora?

Twilight is defined by how far the center of the sun sits below the horizon. The US Naval Observatory uses three thresholds. Civil twilight ends at 6 degrees below, when outdoor work normally needs artificial light. Nautical twilight ends at 12 degrees, when the horizon fades from view at sea. Astronomical twilight ends at 18 degrees.

Sunrise and sunset use a different threshold. USNO sets them at a zenith distance of 90.8333 degrees, a sun center 0.8333 degrees below the horizon. A day with no sunrise is not a dark day. At Tromsø near the December solstice the sun never rises, but at noon it sits above -6 degrees, so the sky is in civil twilight.

No official source gives a single sun angle at which aurora becomes visible. It depends on how bright the aurora is. So Glow picks a stated, conservative threshold and applies it everywhere: civil night, the sun at or below -6 degrees. The live logbook, the dark hours tool and every month table use the same line.

Nautical and astronomical night are shown beside it. If you are chasing faint aurora or photographing it with long exposures, the -12 and -18 degree columns are the ones to plan on. At high latitude in spring and autumn the gap between them can be large, because the sun sinks at a shallow angle and lingers in each band.

Moonlight is a separate layer. A bright moon lights the sky even in astronomical night, so the darkest sky needs both the sun below -18 degrees and the moon below the horizon. The moon guide counts those minutes for each town.

The table compares civil, nautical and astronomical night on the coming December 15 and on March 15, near the equinox, for each Glow place.

In December the far-north towns have long civil nights. Tromsø reaches 19.2 hours on December 15 of 2026. Its astronomical night is shorter because the sun spends part of the long night between -6 and -18 degrees. In the lower 48, Minneapolis has 14.0 hours of civil night that date.

In March the order flips. Lower-latitude towns keep more astronomical darkness than the far north, where twilight lasts longer. That matters for photographers planning a spring trip.

• The dark hours calculator gives civil, nautical and astronomical night for any place and date, with the minutes inside 22:00 to 02:00.
• The aurora season finder draws the full year as a ribbon, so you can see when each band opens and closes.
• Each place page, such as Yellowknife, shows tonight's darkness timeline beside the live OVATION cell.

Darkness is computed, not observed. Glow's figures ignore terrain, buildings, weather and the atmosphere beyond standard refraction. A mountain to your southwest can end twilight early; city light can keep the sky bright long after astronomical night begins. SWPC's first viewing tip is simple: get away from city lights.