Dark Sky & Moon Window Calculator
Find astronomical darkness and the longest moon-below-horizon observing window for a location. Inputs stay in your browser.
The primary window is the longest overlap between astronomical darkness (Sun below −18°) and the Moon below the apparent horizon.
Context matters.
- Moon altitude is sampled and horizon crossings are refined locally with SunCalc; the window is planning-grade rather than an observatory ephemeris.
- Cloud, smoke, humidity, terrain and light pollution can matter more than the lunar window.
- Milky Way visibility also depends on season, galactic-core position and latitude; this result does not claim the Milky Way is visible.
Sources & scope
The calculation guide and this source trail show what the result means, where the rule came from and when it was last reviewed. This is source-and-calculation QA, not a claim of medical, legal, tax or other professional review; time-sensitive decisions still belong with the linked authority.
A visible path from input to answer.
Find astronomical darkness and the longest moon-below-horizon observing window for a location. The page keeps the calculation scope and its limits beside the result so the output can be checked, copied or revisited.
- Read the record. The calculator uses Observation evening, Latitude, Longitude and UTC offset on this date supplied in the form.
- Apply the scope. It applies the Global solar/lunar planning — user-provided coordinates and date-specific UTC offset reference and the explicit date, unit and counting conventions shown in the result.
- Expose the checkpoints. Intermediate rows, warnings and timeline events stay visible; the emphasized row identifies the primary checkpoint when one exists.
The primary window is the longest overlap between astronomical darkness (Sun below −18°) and the Moon below the apparent horizon.
- Longest moon-down windowNone
Calculation purpose: Find astronomical darkness and the longest moon-below-horizon observing window for a location. This is a general planning reference; local conditions, product instructions or personal records can override the estimate.
Calendar inputs use plain year-month-day semantics. Leap days, month ends, date-pair order and unsupported dates are checked before the result is shown.
Rule set 2.0.0 · Last reviewed 2026-08-22 · Next review due 2027-02-22. The source panel below contains the linked evidence.
What to know before using the result.
What counts as astronomical darkness?
CalcSpan uses the conventional boundary where the Sun is at least 18 degrees below the horizon.
What does the moon-down window mean?
It is the longest part of astronomical night when the Moon is calculated below the apparent horizon, reducing direct moonlight.
Does a moonless window guarantee Milky Way visibility?
No. Season, galactic-core position, latitude, clouds, smoke and local light pollution still determine whether the Milky Way is visible.