Teaching · For Teachers · Earth and space science

From sunset
to fully dark,
stage by stage.

Type a date and a place. The calculator gives each stage of dawn and dusk, how long it lasts and how many hours of true night are left, including the summer nights at high latitude when it never gets fully dark.

Time the twilightWhat you get

The twilight calculator

The worked example is Burlington, Vermont, on the June solstice. Nothing you type leaves this browser.

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What you get

Teaching · For TeachersFree

What you finish with
Start and end times of civil, nautical and astronomical twilight, sunrise and sunset, each twilight’s length and the hours of full darkness. It prints as a summary and downloads as a spreadsheet.
Who it is for
Earth and space science teachers and their students, families planning a stargazing night, and anyone working outdoors at dusk.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The For Teachers space in Caveat Teaching.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Find the place’s latitude and longitude on a map.
  2. Type the UTC offset in force that night.
  3. Choose the date.
  4. Read civil dusk for outdoor light and full darkness for faint stars.
  5. Try the same place in December to see twilight shrink.

Time the twilight

How the twilight times are found

Three depths below the horizon, the same geometry as sunset.

The worked example: Burlington on the June solsticeSunset8:41 PMcentre 0.833° downCivil dusk9:18 PMsun 6° downNautical dusk10:07 PMsun 12° downFully dark11:10 PMsun 18° downThe worked example: Burlington on the June solsticeSunset8:41 PMcentre 0.833° downCivil dusk9:18 PMsun 6° downNautical dusk10:07 PMsun 12° downFully dark11:10 PMsun 18° down
Each stage of dusk is the same sum as sunrise and sunset, solved for a deeper sun angle.

Three kinds of twilight

Civil twilight ends when the sun is 6° down; until then the brightest planets show but outdoor work still needs no lamp. Nautical twilight ends at 12°, when the horizon at sea fades from view. Astronomical twilight ends at 18°, when the last glow leaves the sky and the faintest stars appear.

Why it changes

Near the equator the sun drops almost straight down, so it reaches 6° below quickly. At higher latitude it slants along the horizon, and around the summer solstice it may never get 18° or even 12° below: above about 48.5° north there is no full darkness in late June. In winter the slant is less extreme and twilight is shorter.

Sources

  1. U.S. Naval Observatory, Astronomical Applications Department. Rise, set and twilight definitions (aa.usno.navy.mil).
  2. NOAA Global Monitoring Laboratory. Solar Calculator and its calculation details (gml.noaa.gov/grad/solcalc).
  3. Meeus, J. (1998). Astronomical Algorithms (2nd ed.). Willmann-Bell. Rising, transit and setting.
  4. NASA Science. Skywatching and observing conditions (science.nasa.gov).

The worked example uses Burlington’s published coordinates. Times are for a clear, level horizon. Your entries stay in this browser. Support: team@caveatacademic.com.