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CalcSpan/Photography, tides and astronomy/Stargazing Dark Sky Moon Window
Photography, tides and astronomy · Calculator

Dark Sky & Moon Window Calculator
without the guesswork.

Stargazing Dark Sky Moon Window — Find astronomical darkness and the longest moon-below-horizon observing window for a location. Uses a focused calculator with visible assumptions. Inputs stay in your browser.

Reference toolStargazing Dark Sky Moon WindowFind astronomical darkness and the longest moon-below-horizon observing window for a location. Inputs stay in your browser.
Reference calculation
Inputs

Enter your details.

Your inputs are processed locally and are not uploaded to CalcSpan.

Browser location stays on this device and is used only for this calculation.
Fri, Aug 21, 2026
Use my current location below, or paste coordinates from a map.
Include daylight-saving time for the observing location.

Plain-date semantics · start date is not counted unless the result explicitly says inclusive.

Your result
00:10 next day – 04:14 next day · 4 hrs 4 min

The primary window is the longest overlap between astronomical darkness (Sun below −18°) and the Moon below the apparent horizon.

Observation eveningAug 21, 2026
Sunset20:14
Astronomical dark window22:14 – 04:14 next day6 hrs
Longest moon-down window00:10 next day – 04:14 next day4 hrs 4 min
Moon illumination71%
Moon phaseWaxing gibbousWaxing
Coordinates47.6062, -122.3321 · UTC-7

Read before relying

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.
Rule set 2.0.0

Sources & scope

Verified

This page shows the source trail, jurisdiction and review date alongside the result. Always confirm time-sensitive rules with the linked authority.

CalcSpanPlain-date calculation method and input semanticsRetrieved 2026-08-20NOAA Global Monitoring LaboratoryGeneral Solar Position CalculationsRetrieved 2026-08-20U.S. Naval ObservatoryComplete Sun and Moon Data for One DayRetrieved 2026-08-20SunCalcSun and moon position calculation libraryRetrieved 2026-08-20
JurisdictionGlobal solar/lunar planning — user-provided coordinates and date-specific UTC offset
Review date2027-02-21
PrivacyNo input upload
Questions answered

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.

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