Tonight's sky

Is tonight worth
the drive?

Enter where you are and this works out the rest: when real darkness starts, how much the moon will ruin, how high the galactic centre gets, and which targets clear the horizon while it is dark. Everything runs in your browser.

Working out where the sun, the moon and the galactic centre will be.

This panel runs entirely in your browser. Nothing about your location leaves the device.

How the numbers work

No magic,
just geometry.

Every figure on this page comes from the same low-precision solar and lunar series that planetarium software uses, calculated live for your coordinates. Here is what each one actually means.

Astronomical darkness

The sky is only properly dark once the sun sits more than 18 degrees below the horizon. Between sunset and that point you get civil, nautical and astronomical twilight in sequence, and each one is brighter than beginners expect. Faint nebulosity that appears at 11pm often simply was not there at 9:30.

Above roughly 48 degrees of latitude, that never happens at all in midsummer. If you live in Scotland, southern Canada or northern Europe, deep-sky season has a hard stop in May and reopens in late July.

Moon interference

Phase alone is a poor guide. A 60 percent moon that sets at 11pm is a better night than a 30 percent moon that rises at 1am and sits there until dawn. What matters is how much of your dark window has a lit moon above the horizon.

That is the number this page calculates: the fraction of astronomical darkness with the moon up, multiplied by how lit it is. Narrowband filters change the maths entirely, which is why moonlit weeks are good weeks for hydrogen-alpha.

Target altitude

An object at 20 degrees is looking through roughly three times as much atmosphere as one directly overhead. That means more extinction, more turbulence, more light dome and more atmospheric dispersion. Thirty degrees is a reasonable working floor; above 45 is comfortable.

This is also why latitude quietly decides what you can shoot. The galactic centre reaches about 27 degrees from Joshua Tree and about 60 from the Atacama, and no equipment closes that gap.

The score

Forty points for how many hours of astronomical darkness you get, up to eight. Thirty-five points for the absence of moonlight during those hours. Twenty-five for how many catalog targets climb above 30 degrees. Add them up.

It is deliberately simple and it is deliberately transparent, because a number you cannot interrogate is worth nothing. It also knows nothing about clouds, wind, road closures or whether you have work in the morning.

Milky Way core, northern hemisphere

The season, month
by month.

Rough guidance for mid-northern latitudes, around 30 to 45 degrees north. Southern observers get a longer season and a much higher core. Use the desk above for your actual coordinates.

MonthWindowWhat it means

FebruaryPre-dawn onlyThe core clears the horizon an hour before twilight. Cold, short and worth it if you want an empty location.

MarchVery early morningA usable window opens around 4am. This is when the season really starts for most northern shooters.

AprilAfter midnightThe core is up for a few hours before dawn and sits reasonably high by the end of the night.

MayLate nightRises around midnight. The first month where you can shoot, sleep, and still function the next day.

JuneMost of the nightHighest and longest for northern latitudes, but short nights at higher latitudes cut into the darkness.

JulyPrimeThe best combination of core altitude and true darkness for most of the northern hemisphere.

AugustEvening primeUp as soon as it gets dark and setting before dawn. Perseid season overlaps, which is a bonus.

SeptemberEarly eveningSetting earlier each week. You get one good window right after astronomical dark.

OctoberClosingA short low window immediately after dusk. The season is essentially over for the north.

November to JanuaryBelow the horizonThe galactic centre is behind the sun. Shoot winter targets: Orion, the Pleiades, Andromeda and the California Nebula.

Then check the things geometry cannot see