Night Sky Alert
Methodology

How the score works

No black box. Here's the exact formula, what feeds it, and where it can be wrong.

The formula

event chance  E = 1 − (1 − 1.0·aurora)(1 − 0.9·launch)(1 − 0.7·meteors)(1 − 0.5·station)(1 − 0.15)
sky quality   Q = (1 − cloud) × min(1, dark hours ÷ 4)
score         = round(100 × E × Q)

moonlight dims meteors by up to 60% (scaled by how lit the Moon is and how long it's up),
and faint aurora by up to 18%. Launches and the space station are bright enough to ignore it.

Bands: 75 and up: Go outside · 50–74: Worth a look · 25–49: Maybe · under 25: Stay in.

Each ingredient

Aurora (0–1). We take the highest Kp forecast by NOAA for tonight's dark hours and compare it with what your location needs. A rule of thumb puts the visible edge at a magnetic latitude of about 66° − 2.6 × Kp. Your magnetic latitude comes from the AACGM-v2 model of Earth's real field (a simple tilted dipole is off by about 5° over Europe and Australasia). One Kp below the need scores 0, one above scores 1. During the night, NOAA's 30–90 minute OVATION nowcast can raise it. This is a heuristic we keep calibrating against real sightings.

Rocket launch (0–1). Scheduled launches during tonight's dark window: full credit within 150 km of the pad, fading to zero at 1,000 km, with a bonus for twilight launches whose sunlit plumes can be spectacular.

Meteor shower (0–1). Our grouping of each shower's typical strength (major 1, medium 0.6, minor 0.3) × closeness to peak night (peak 1, one night off 0.6, two nights off 0.25) × how high the shower's radiant climbs at your latitude (the sine of its highest altitude; zero if it never gets 10° up). That's why the Quadrantids count for little in Sydney and the Eta Aquariids count for more in the south.

Space station (0–1). The best visible pass tonight: 40°+ high scores 1, 20–40° scores 0.6, lower scores 0.3.

Clouds. MET Norway's hourly forecast for your city, averaged over the dark hours. Low cloud and fog count fully; thin high cloud counts half. If the forecast hasn't loaded, we assume 50%.

Darkness. Hours with the Sun more than 12° below the horizon. Four or more hours earns full credit. Near the poles in summer the Sun never sinks that far, so those nights score zero and the city page says when proper darkness returns.

Both hemispheres. Kp is a planet-wide index, and the geomagnetic latitude rule works the same north and south, so the aurora ingredient covers the aurora australis too. Look toward the nearer pole.

Known limits

  • Cloud forecasts can be wrong, especially for scattered cloud.
  • The aurora rule of thumb is approximate: storms vary, and the view line shifts with longitude.
  • Launch times slip often, sometimes by days.
  • We don't yet account for light pollution where you stand. Driving away from city lights always helps.

Update log

  • Oct 16, 2026: aurora reach now uses AACGM-v2 magnetic latitudes (fixes optimistic odds in Europe and pessimistic odds in Tasmania and New Zealand).
  • Oct 16, 2026: worldwide coverage, southern lights, meteor radiant altitude, polar summer handling.
  • Oct 16, 2026: first version (United States and Canada).

Sources: NOAA SWPC (Kp forecast, OVATION), MET Norway Locationforecast (CC BY 4.0), The Space Devs Launch Library 2, CelesTrak, American Meteor Society calendar, GeoNames. Full list on Data & credits.