StarCast · Dark Window Guide
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What Is Astronomical Twilight?

It's the last phase before the sky is truly dark. LightCast StarCast tracks the astronomical twilight end time for your location so you know exactly when to start shooting.

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The three phases

Civil, Nautical, and Astronomical Twilight

Twilight isn't one thing — it's three distinct phases defined by how far below the horizon the sun sits. Each has a different character for photography. Astronomical twilight ending is the signal astrophotographers wait for.

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Civil Twilight (0–6° below horizon)
Immediately after sunset. Enough ambient light to see without artificial lighting. The sky transitions from golden hour orange to deep blue. Blue hour photography window. Bright planets and the moon are visible but stars are not.
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Nautical Twilight (6–12° below horizon)
Sky is clearly dark, horizon still faintly visible. Bright stars are visible. Not suitable for Milky Way or deep sky photography — residual sky glow washes out faint objects in long exposures. Usable for star trails and bright planet photography.
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Astronomical Twilight (12–18° below horizon)
Sky is nearly fully dark. Faint objects become visible but residual solar illumination still slightly affects long exposures. The sky reaches true astronomical darkness when the sun drops to 18° below the horizon — this is when serious astrophotography begins.
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When does it end?

Timing Varies by Season and Latitude

At mid-latitudes in the US, astronomical twilight ends roughly 90 minutes to 2 hours after sunset in winter, and up to 2.5 hours after in summer when the sun's path is shallower. At high latitudes in summer — Alaska, northern Canada — astronomical twilight may never fully end and the sky never reaches true darkness. StarCast shows the precise dark window start and end for your location each night.

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StarCast Push Alerts
StarCast tracks the full dark window for your location — from astronomical twilight end to moonrise, or moonset to astronomical dawn. Push alerts notify you when a high-scoring window is forecast so you know when to be on location.
Common Questions
What is astronomical twilight?
The third and darkest twilight phase, when the sun is 12–18 degrees below the horizon. True astronomical darkness begins when the sun reaches 18 degrees below — the point at which no residual solar illumination affects the sky. StarCast shows this timing for any location in the iOS app and on the web.
What are the three types of twilight?
Civil (0–6° below horizon): blue hour, bright planets visible. Nautical (6–12°): clearly dark, horizon faintly visible, bright stars appear. Astronomical (12–18°): nearly fully dark, faint objects visible, Milky Way photography possible once the sun passes 18°.
How long after sunset can I start astrophotography?
Roughly 90 minutes to 2 hours after sunset at US mid-latitudes, when astronomical twilight ends. Longer in summer, shorter in winter. StarCast shows the exact time for your location each night.
Does astronomical twilight affect Milky Way photography?
Yes. Even during the early part of astronomical twilight, residual sky glow reduces contrast on faint objects like the Milky Way core in long exposures. Wait for full astronomical darkness — sun at 18° below or more — for the best results.
What is LightCast StarCast?
StarCast scores night sky conditions and tracks the dark window — astronomical twilight end, moon position, and moonrise/set — for any location. Push alerts notify you when conditions are worth the trip. Free on web at lightcastsuite.com/starcast, with push notifications in the LightCast iOS app. $2.99/month after a 7-day free trial.
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Know exactly when the sky goes dark.

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