Sunrise, sunset & twilight calculator
Daily sun and twilight times for any position. Useful for:
- Switching navigation lights at sunset under COLREG Rule 20.
- Bridge watch handover at first/last light.
- Celestial fix planning — nautical twilight is the window when both stars and the horizon are visible.
- Cargo operations scheduling at ports with daylight-only restrictions.
- Polar voyage planning — periods of polar day or polar night.
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Twilight definitions
- Civil twilight — sun centre 6° below horizon. Brightest navigation aids visible; outdoor activities possible without artificial light.
- Nautical twilight — sun 12° below. Horizon still visible against a dark sky; the window for celestial sights with sextant.
- Astronomical twilight — sun 18° below. End of last residual sky brightness; a fully dark sky for astronomy.
- Sunrise / sunset — upper limb of the sun on the horizon, corrected for atmospheric refraction (-0.833°).
When to use this tool
Bridge watch planning around first/last light, celestial-fix window planning (nautical twilight is the only window when both stars + horizon are visible), polar-voyage sun-below-horizon assessment (some Arctic traffic experiences continuous polar night for weeks), COLREG Rule 20 navigation-light switching timing.
Worked example
Scenario: Bulker at 62°N, 15°W (Iceland-Faroes gap) on 21 December. Officer needs to plan nautical twilight window for celestial star sights.
Calculate: At 62°N in mid-winter, nautical twilight morning window ~08:00 LMT (only ~40 min); evening window ~15:00 LMT (~40 min). Sunrise 11:15, sunset 15:30. Sun altitude never exceeds ~4° — polar-adjacent short-day conditions.
Interpretation: Two very short celestial windows. Plan sextant sights accordingly + rely more on GNSS + inertial + radar-based fixes.
Regulatory basis
COLREG Rule 20: navigation lights exhibited from sunset to sunrise; also in restricted visibility during daytime. SOLAS V/34 passage-planning requires day/night differentiation. Watchkeeping under STCW VIII/1 + STCW A-VIII/2 references sunset/sunrise for bridge-team composition. No specific twilight regulation beyond these general lighting + planning requirements.