Tonight's skywatch forecast

How the score works

Here's the thing about skywatching. Most nights aren't worth the cold, and the ones that are can be gone in an hour when the clouds roll in. So I wanted one number that tells you whether to bother. The score is out of 10 and it rates the best three hours of the night, not the whole thing, because almost nobody stays out from sunset to sunrise.

Clouds, up to 5 points

Half the score is clouds because if it's overcast nothing else matters. We pull the hourly cloud forecast from Open-Meteo for the spot you picked. A clear sky gets all 5 points and a sky that's completely covered gets none.

The Moon, up to 3 points

A full Moon high in the sky washes out faint stars and most meteors. A thin crescent, or a Moon that hasn't risen yet, costs you almost nothing. The points depend on how much of the Moon is lit and how high it is during your best three hours.

Darkness, up to 2 points

The sky isn't properly dark until the Sun is 18 degrees below the horizon (astronomers call that the end of astronomical twilight). Up north in summer that never happens, so a clear June night in Edmonton or Edinburgh still won't score a 10. Sorry, I don't make the rules.

One catch. The Moon, darkness and bonus points all shrink as the clouds build up, so a dark moonless night under total cloud still scores low. That's on purpose.

Bonus points

A meteor shower near its peak adds up to 1 point if its radiant (the spot the meteors seem to come from) gets up above your horizon. A visible Space Station pass adds half a point. A pass only counts when the station is still in sunlight and you're in the dark, which is the only time you can actually see it. The orbit comes from Celestrak and the math runs on your phone. The total tops out at 10.

What it can't tell you

Light pollution. A 9 in the middle of a city is still a city sky, so get out of town if you can. It also can't tell you about smoke, haze or fog, and a cloud forecast is only a forecast, so check again before you head out. Every time on the forecast is shown in the local time of the place you picked.

Saw something odd while you were out there? Check if it was a planet first. Venus has fooled more people than every other "UFO" put together.