Astronomical Twilight
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Sun crosses 18° below horizon
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Sun crosses 18° below horizon
Current Local Time
—This astronomical twilight calculator shows the exact astronomical dawn and dusk times (sun 12°–18° below horizon) for any location.
What Is Astronomical Twilight?
Astronomical twilight is the final phase of twilight when the sun sits between 12 and 18 degrees below the horizon. At this depth, the sky reaches near-total darkness, and even the faintest deep-sky objects become visible through a telescope. However, the sky is not yet fully dark — faint stars become visible partway through astronomical twilight, but true astronomical night (and full darkness for deep-sky observation) only begins once the sun drops past 18 degrees.
This distinction matters for astronomers and astrophotographers. During astronomical twilight, residual sky glow from the sun still contaminates long-exposure images of faint objects like galaxies and nebulae. The calculator above tells you exactly when astronomical twilight ends — and that's when the real observing session begins. For most mid-latitude locations, astronomical night lasts only a few hours in summer and extends to the full night in winter.
How Our Astronomical Twilight Calculator Works
Our astronomical twilight calculator determines the precise moments when the sun reaches 12 degrees and 18 degrees of depression below the horizon, marking the boundaries of astronomical dawn and dusk. It uses the SunCalc library with standard atmospheric refraction of 0.833° applied, and runs entirely in your browser.
The tool uses the IAU solar position model with precise atmospheric refraction and horizon dip corrections, giving astronomers the exact windows for deep-sky observation and astrophotography sessions. No data is sent to any server — the entire computation is local and instant.
FAQ — Astronomical Twilight
When does the sky become truly dark for stargazing?
True darkness begins when astronomical twilight ends — when the sun reaches 18° below the horizon. At this point, the sky is as dark as it gets (excluding moonlight and light pollution). Use the calculator above to find this time for your location. In my experience, the first 30 minutes after astronomical twilight ends produce the darkest, cleanest skies.
Does astronomical twilight exist near the equator?
Yes, but it's very short — sometimes just 10-15 minutes — because the sun descends steeply. Near the equator, the sun passes through the 12°-18° range quickly. In contrast, at high latitudes during summer, astronomical twilight can last all night, meaning the sky never gets truly dark. This is why polar regions have poor conditions for deep-sky astronomy in summer.
Can I do astrophotography during astronomical twilight?
You can photograph bright objects (the Moon, planets, bright star clusters) during astronomical twilight, but deep-sky objects (galaxies, nebulae, faint star clusters) require the sky to be fully dark — after astronomical twilight ends. I've found that the transition period produces interesting results for wide-field Milky Way shots, where the gradient from twilight glow to dark sky adds depth.
How does light pollution affect astronomical twilight?
Light pollution doesn't change the astronomical twilight timing (that's purely astronomical), but it determines how dark the sky actually becomes after twilight ends. In heavily light-polluted areas, the sky may never reach true darkness even after astronomical twilight. Use Bortle scale measurements alongside twilight data to assess actual observing conditions.
What's the difference between astronomical twilight and astronomical night?
Astronomical twilight is when the sun is between 12° and 18° below the horizon — the sky is transitioning to darkness. Astronomical night begins when the sun passes 18° below — the sky is fully dark with no residual solar glow. For serious astronomy, only astronomical night provides the conditions needed for deep-sky observation and long-exposure imaging.