Dark-Sky Calendar Planner
Find nighttime windows with the Sun below your chosen twilight threshold and, optionally, the Moon below the horizon. Rank up to 31 nights by usable duration.
Starts with an editable example. Replace the dates and rules with your own.
Your calculation
Ready when you are
Choose your dates.
Select your counting rules and calculate.
Calculation details
| Step | Value |
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How to use this calculator
Search for a city or enter coordinates and a time zone. Choose evening dates, the Sun-angle threshold, Moon constraint and minimum window length.
Each evening covers local noon to the following noon. Exact altitude and horizon crossings divide the night into intervals. Moon-free means the Moon's upper limb is below a level horizon with standard refraction, not merely a low illumination percentage.
A worked example
For New York on November 6-8, 2026, astronomical darkness with the Moon below the horizon produces three windows of at least 30 minutes. The longest is approximately 644 minutes.
Does a window guarantee good stargazing?
No. The planner checks Sun and Moon geometry. Clouds, light pollution, terrain, elevation and observing conditions are not predicted.
Which calendar and years are supported?
Use Gregorian dates from 1900 through 2100, with results in that range. Daily plans allow up to 366 dates; dark-sky searches allow 31 nights. Durations, capacities and policy rules are supplied by you. Astronomy uses the bundled Astronomy Engine, sea-level geometry and browser time-zone rules. Timed exports use UTC instants; date-only events do not assign a clock time. Copy, CSV and print include calculation assumptions. Nothing is booked or scheduled outside this page.
What happens to my dates?
Calculations run on your device. Saving a calculation is optional and keeps it in this browser. A setup link contains the dates and settings you entered; anyone with the link can read them. Use one tab per saved calculator. Clear saved data from Privacy & settings.