Home / Science / Astronomy and space / Observing / Dark-sky calendarTool #45

Dark-sky calendar: Moon-free dark hours for every night of the year

See a whole year of nights at once: twilight, true darkness and moonlight for your place, the Moon-free dark hours in each month, and the best nights for deep-sky observing and imaging.

Keep it one tap away for everyday use.

Bookmark Dark-sky calendar?

Press Ctrl + D now. Your browser opens its own bookmark box, where you choose the name and folder.

  1. Your place
  2. Each night
  3. Best dates
Nothing is uploadedWorks offline once open

Each row is one night, January at the top. Time runs left to right in local solar time, so midnight is when the Sun is lowest, whatever the clocks say. Black is true darkness with no Moon: the time for faint galaxies and nebulae.

What it does

Faint galaxies and nebulae need a sky that is truly dark, with the Sun well below the horizon and no Moon. How much of that you get changes through the year and with your latitude: in summer at northern latitudes it can vanish completely, and the Moon takes away a week or two of every month. This calendar draws every night of a year as one row of a picture, so you can see the dark windows at a glance and plan trips, imaging projects and star parties months ahead.

How to use it

  1. Set your place at the top. A nearby city is close enough; your choice is shared with the other astronomy tools.
  2. Pick the year with the arrows.
  3. Choose when the Moon spoils the dark: whenever it is up, or only when it is more than 10 % or 25 % lit. Imagers and visual observers of faint objects usually count any Moon; a thin crescent barely matters for many targets.
  4. Read the picture: each row is a night, January at the top. Time runs left to right in local solar time, so the middle of each row is the darkest moment of the night. Black is true darkness with no Moon. Shades of grey are the three twilights, and the pale gold band is the Moon up during dark hours.
  5. The table gives each month’s dark hours, Moon-free dark hours, the number of nights with at least 4 Moon-free dark hours, and the best night. Every night (CSV) downloads astronomical dusk and dawn, dark hours, Moon-free hours and Moon phase for each night, in your place’s time zone.

How it works

The Sun and Moon positions come from astronomy-engine, accurate to about an arcminute. True darkness is the time with the Sun more than 18° below the horizon (after astronomical dusk and before astronomical dawn). Dusk and dawn are found to within a minute. Within the dark hours the Moon is checked every 10 minutes and counts as up when its centre is above the horizon.

Worked example

For Toronto in 2027, the year has 2,977 hours of true darkness, of which 1,577 are free of the Moon. That gives 177 nights with at least 4 Moon-free dark hours. June has only 54 Moon-free dark hours in the whole month, while December has 195. In London there is no true darkness at all from late May to late July, so June has none, and in Edinburgh the gap is longer still.

Frequently asked questions

Why local solar time? Clock time shifts by an hour at daylight saving changes, which would break the picture. In solar time, midnight is always the middle of the night. The CSV gives clock times.

Does it include clouds or light pollution? No. It shows when the sky can be dark. Use a forecast for clouds, and the sub-exposure calculator or a light pollution map for sky brightness.

Is my location sent anywhere? No. Everything is worked out in your browser, and the place is only kept in your browser if you ask it to remember it.

Last updated 2026-11-08.

Change log for Dark-sky calendar

  1. Add five astronomy tools: cosmology, sub-exposure, backfocus, magnitude-flux, dark-sky calendar Details on GitHub

Every change to fibub

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