Stargazing Night Planner

What this tool answers, and what it cannot

Most “is tonight good for stargazing?” questions collapse into two facts you can know in advance and one you cannot. You can know exactly how long the sky will be properly dark, and exactly how much of that darkness the Moon will be sitting in. You cannot know from geometry whether it will be cloudy. This page does the first two precisely and refuses to guess at the third.

What the stargazing score actually measures

It measures one thing: how many minutes of the night are both astronomically dark and free of the Moon. Astronomical darkness begins when the Sun is 18 degrees below the horizon; the Moon counts as up whenever its centre is above the horizon, sampled every 5 minutes across the night. The moonlit minutes are subtracted from the dark ones, and what is left is scored against a 360-minute reference — six hours of moonless dark scores 100, three hours scores 50, and a night with no astronomical darkness at all scores 0. Nothing else feeds the number.

That reference is a deliberate plateau, not a cap on how good a night can be. Six hours of genuinely dark, Moon-free sky is already more than a single observing session uses; a nine-hour polar-winter night is not meaningfully “better” for the person standing under it, so the score stops climbing rather than rewarding latitude.

Does this tell me whether tonight will be clear?

No, and it is important that you know that. Every figure on this page comes from orbital geometry — where the Sun and the Moon are, for a date and a point on Earth. Geometry cannot see cloud, haze, humidity, atmospheric seeing or the skyglow from the town you are standing in. A score of 100 means the sky will be dark and moonless if it is clear; it does not promise that it will be clear. Check a weather forecast for cloud and a light-pollution map for skyglow before you drive anywhere.

Why the Moon matters so much to the score

Because a bright Moon erases faint objects even in a perfectly clear sky. A full Moon raises the brightness of the sky background by roughly three magnitudes, which cuts the number of stars a naked eye can pick out from a few thousand to a few hundred and makes the Milky Way effectively invisible. That is why this tool subtracts moonlit time instead of reporting darkness on its own: a nine-hour winter night with a full Moon riding high from dusk to dawn scores 0, and that 0 is the honest answer for someone hoping to see anything faint.

A worked example you can reproduce above. Press the Roskilde chip (55.65°N) and set the date to 20 September 2026: astronomical night runs 7h 23m, but the Moon is above the horizon for 2h 29m of it, so the moonless figure is 4h 54m and the score is 82. Now move the date a week either way. On 13 September the dark window is shorter, 6h 43m — but the Moon is a 9%-lit crescent that has set before darkness begins, so every one of those minutes is moonless and the score is 100. On 27 September the dark window is longer, 8h 01m — and a 98%-lit Moon is up across the whole of it, so the moonless figure is 0m and the score is 0. The night itself lengthens by only about forty minutes a week around the equinox; the usable part of it swings from all of the night to none of it. That gap is the entire reason this tool exists, and it is why a “how long is the night?” answer on its own misleads.

Reading the twilight ladder

The timeline is banded by how far the Sun has sunk. Civil twilight (0 to −6 degrees) is bright enough to read outside and useless for faint objects. Nautical twilight (−6 to −12) is when the horizon and the brighter stars are both visible. Astronomical twilight (−12 to −18) still has a measurable glow in the sky. Only below −18 degrees is the Sun’s contribution gone, and that band — the darkest one on the chart — is the only part the score counts.

Why the score falls to zero every summer

Above roughly 49 degrees of latitude the Sun stops dipping a full 18 degrees below the horizon around midsummer, so astronomical darkness never arrives and the score is 0 by definition rather than by bad luck. In London the gap runs from about 22 May to 20 July in 2026 — sixty consecutive nights with no astronomical darkness at all. In Copenhagen, four degrees further north, the same gap stretches from early May to early August; north of the Arctic Circle it widens into continuous daylight. When it happens this page says so in words instead of printing a small number that looks like a near miss.

How accurate are the times?

The solar events use the NOAA solar-position algorithm and land within about a minute of published tables at mid-latitudes. The Moon is the weaker half, and you should know by how much. This planner uses a leading-term lunar model: one periodic correction each to the Moon’s longitude, latitude and distance. Measured against the fuller 30-term Meeus series that drives our own Moon Phase page, its moonrise and moonset times differ by a median of about 4 minutes across the 365 nights of 2026 at 55 degrees North, and by up to about 20 minutes on the worst of them. Those minutes feed the score, so read the moonless figure as good to about a quarter of an hour rather than to the minute. Near the poles, or on a date when the Moon only grazes the horizon, the error grows. Times are printed on the whole-hour UTC offset in the Clock box, which knows nothing about daylight saving, so check it if a time looks an hour out.

Does the town search send my location anywhere?

No. The first time you type in the town box, the page loads a list of 29,919 places from this same site and then does all the matching in your browser. Nothing you type is uploaded, no coordinate is sent to a geocoding service, and if you use the latitude and longitude box instead, the town list is never requested at all.

If you want the daytime side of the same maths — golden hour, blue hour and the twilight windows for photography — the sunrise and sunset calculator covers it, and the moon phase calculator goes deeper on the Moon itself, including its altitude, compass bearing and distance.