How the score works
60% weight
Cloud cover
Layer-weighted across the best 2-hour window of the night. Low clouds block everything, thin high cirrus still lets bright stars through.
40% weight
Moon darkness
A new moon boosts the score. A full moon near midnight washes out all but the brightest stars. We ease the penalty after midnight when the moon is setting.
How it actually works
Behind the score
The 60/40 split above is the headline. The full version adds a visibility gate (why 100% cloud always means 0), moon-illumination tiers and a late-night discount — with two worked examples: tonight, and a hypothetical 9.5 night.
Read the methodology →