Sun below −18° · next 30 nights
True darkness at Longueuil
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 night9.7 h9 Oct
Nights reaching −18°30 / 30Astronomical darkness occurs
Longest window20:00–05:40Moonlight is shown separately in the planner.
Solar geometry, not a clock rule
Hours of astronomical darkness
9.7 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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