Sun below −18° · next 30 nights
True darkness at Mauna Kea
Astronomical darkness starts only when the Sun is 18° below the horizon. These bars are calculated for the coordinates, so high-latitude nights correctly show no true darkness when twilight never ends.
Longest night10.0 h5 Oct
Nights reaching −18°30 / 30Astronomical darkness occurs
Longest window19:20–05:20Moonlight is shown separately in the planner.
Solar geometry, not a clock rule
Hours of astronomical darkness
10.0 hours
Use this as the outer boundary for faint-object work. Moon altitude, cloud and transparency will narrow the practical window. Add the Moon →
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