Astronomical, Nautical, Civil Twilight — and When Aurora Becomes Visible

Aurora is happening on most nights of the year in the auroral oval — you just can't see it while the sky is still lit. Twilight defines the viewing window, and at aurora latitudes it behaves in surprising ways. One number runs the whole show: how far the sun sits below the horizon.

The three bands

Civil twilight (sun 0° to −6°): the sky is still bright enough to read by; horizon clearly visible. No aurora visible, full stop.

Nautical twilight (−6° to −12°): the horizon is detectable but the sky is genuinely darkening. Bright displays begin to bleed through near the −12° mark — a faint glow, not a show.

Astronomical twilight (−12° to −18°, and beyond): below −18°, no sunlight at all contributes to sky glow. This is full astronomical darkness — the aurora's stage. Faint structures and true colours only appear here.

Why −18° matters more than sunset time

Sunset is a civil convention; aurora cares about the −18° line. In autumn and spring that line might arrive 90 minutes after sunset — which is why "the sun set an hour ago and there's still glow on the horizon" is normal, not a forecasting failure. Our engine computes the sun's altitude hour by hour for every city and requires real darkness before it will show anything but NO.

The high-latitude exception

Above roughly 67° latitude, summer breaks the system: the sun never reaches −6°, let alone −18°, for weeks around the solstice — the midnight sun. Tromsø cannot host aurora viewing in late June no matter what the Sun does. Our engine returns a hard NO during those weeks and says why, instead of showing a misleading "0%". The flip side: the same latitudes get nearly 24-hour darkness in deep winter, which is partly why aurora-zone success rates are so high.

Using twilight in practice

Your city page's darkness factor already computes the sun's altitude for every hour of the night — the practical takeaway is when to show up. Plan to arrive about 30 minutes after true darkness begins, not at sunset: that's 90–120 minutes after sundown at mid-latitudes, later in spring. In the aurora zone during winter, true darkness spans most of the day, so the twilight factor matters less than cloud and moon. And in polar summer, don't fight the physics — no darkness means no viewing, and the pages say so plainly rather than showing a fake percentage.

Common twilight mistakes

Going out too early: the hour after sunset is civil twilight; you will see streetlights, not stars. The single most common first-timer error. Leaving too early: the darkest, best hours are 21:00–01:00 local — leaving at 22:00 forfeits the peak. Confusing sunset with readiness: nautical and astronomical twilight delay the real window; trust the sun-altitude math on the city page, not your instinct about "how dark it looks".