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Twilight Calculator

Calculate the three twilight phases for a latitude and date.

About Twilight Calculator

The three twilight phases correspond to fixed solar depression angles, and their duration depends on how steeply the sun descends. At high latitudes in summer the sun never reaches 18 degrees below the horizon, so astronomical darkness never arrives. This is why astronomers at northern latitudes lose observing entirely for weeks around the summer solstice.

Formula

Twilight

Twilight phases end at 6, 12 and 18 degrees of solar depression below the horizon

civil twilight
To 6 degrees below the horizon, when outdoor activity is still possible
astronomical twilight
To 18 degrees, after which the sky is fully dark

Worked example

With the default values below, Twilight Calculator returns Civil Twilight Duration of 38.6. Change any input above and every figure here is recalculated in your browser.

Inputs used

Latitudeaccepts -66–66 degrees
51.5 degrees
Day of the yearaccepts 1–366
172
Sunset timeaccepts 0–23.75 h
21.25 h

Results

Civil Twilight Duration
38.6 min
Nautical Twilight Duration
48.2 min
Astronomical Twilight Duration
48.2 min
Full Darkness Begins
23:30
Total Twilight Duration
134.9 min

Step by step

  1. 1Values used
  2. 2Twilight
  3. 3Civil Twilight Duration= 38.6 min
  4. 4Nautical Twilight Duration= 48.2 min
  5. 5Astronomical Twilight Duration= 48.2 min
  6. 6Full Darkness Begins= 23:30
  7. 7Total Twilight Duration= 134.9 min

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Twilight calculated?

Twilight phases end at 6, 12 and 18 degrees of solar depression below the horizon The three twilight phases correspond to fixed solar depression angles, and their duration depends on how steeply the sun descends. At high latitudes in summer the sun never reaches 18 degrees below the horizon, so astronomical darkness never arrives.

Why does Twilight matter?

This is why astronomers at northern latitudes lose observing entirely for weeks around the summer solstice.

What values do I need to enter?

This calculator takes 3 inputs: Latitude, Day of the year, Sunset time. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Related Calculators

Twilight Calculator – Calculate Civil Twilight Duration