Free planning tools · Plan an imaging night

Darkness and Moon planning calendar

Find astronomical darkness and dark intervals with the Moon below the horizon for up to 31 nights at your chosen location.

No account needed. Results update here as you change the inputs. Measurements stay in this tab unless you choose to share a link; shared links contain the inputs, including any location coordinates.

Your inputs

The starting values are examples. Replace them with your own measurements, prices or documented requirements.

What to have ready
  • Latitude and longitude for the observing site.
  • The IANA time zone for that location, such as Europe/London or Europe/Berlin.
  • The first evening and a range of 1–31 nights.
Measurements for Darkness and Moon planning calendar

Try an approximate city location, then replace it with your observing site.

Calculated from your inputs

Your plan

The example will calculate here when JavaScript is ready.

What to do next

    Calculation details

    Inputs used in this result

    Compare a change before committing

    Keep this plan, change the inputs and see the difference. The comparison stays in this tab.

    Which nights offer dark time with the Moon below the horizon?

    Choose an evening date, location and matching time zone to plan a run of observing nights. The calendar separates all astronomical darkness from the overlap when the calculated Moon centre is below the modelled horizon. Open a night to see the actual local intervals.

    For a club observing weekend, a shared link makes the location and dates visible alongside the schedule. The page calculates locally and does not request your GPS position or look up a weather service. Coordinates are approximate until you enter your own.

    How to use this planner

    1. Choose a location example or enter your own coordinates and zone.
    2. Open the nights you are considering and compare the local start and end times.
    3. Take a usable window into the integration and battery planners, then check weather and target altitude separately.

    The calculation

    Astronomical darkness means the geometric Sun is below −18°. Each observing window runs from local noon on the selected date to local noon the next day. Solar altitude is sampled every five minutes and crossings refined; Moon values are sampled at the midpoint of that window.

    A worked example

    London near the June solstice has no astronomical darkness. Winter nights have a longer window. Europe/London automatically changes between GMT and BST for the selected dates.

    Example inputs are demonstrations. Replace them with your own measurements, prices or documented requirements.

    Assumptions and limits

    Approximate planning values from SunCalc 1.9.0, not a forecast or precision ephemeris. Choose a time zone matching the coordinates. Each evening covers local noon to noon the next day, including daylight-saving changes. Darkness uses the geometric Sun below −18°. Moon-below-horizon intervals use calculated apparent Moon-centre altitude below 0°, rather than the conventional upper-limb moonrise/set definition. Five-minute sampling with refined crossings can miss very short intervals. Terrain, weather, target altitude and light pollution are excluded. Midpoint Moon illumination does not describe the whole night or predict sky brightness.

    SunCalc calculation methods (BSD-2-Clause)

    Astronomical twilight uses the US Naval Observatory’s −18° solar definition. SunCalc license notice.

    Questions about this calculation

    Why can a UK summer night show no astronomical darkness?

    Astronomical darkness in this tool requires the Sun below −18 degrees. Some locations and dates never reach that threshold, even though useful observing or brighter-target imaging may still be possible.

    Is Moon-below-horizon time a precision moonrise and moonset forecast?

    No. The tool uses the calculated apparent Moon centre below 0 degrees, with sampled and refined boundaries. Conventional rise/set definitions, terrain and actual refraction differ; use the result as an approximate planning screen.

    Put the result in context