Sunrise, sunset, and twilight

Model: Geometric sun from the NREL Solar Position Algorithm; each crossing found by bisection between the day's highest and lowest points, which also decide polar states; sunrise and sunset at −0.833° (minus horizon dip when a height is given); solar noon from the NOAA equation of time

Show your work

1. Solar noon

   when the sun crosses this longitude's meridian

   at -104.9903° longitude = 2026-06-21 13:02 local (2026-06-21 1902Z)

2. Sunrise altitude

   −0.8333° for the sun's radius and refraction, less the dip from height

   -0.833° − 0° of dip = -0.833°

3. Sunrise

   the crossing of that altitude before solar noon

   at 39.7392°, -104.9903° = 2026-06-21 05:32 local (2026-06-21 1132Z)

The same steps an agent gets from the MCP server with explain: true.

Accuracy: Within 1 minute of USNO on 1,200 events (300 places and dates, 94% to the same minute), including twilights that graze their altitude

When to use this: Use this when you need the day's light: sunrise and sunset for a place and date, solar noon, the length of the day, and the three twilights, in local time and Zulu. It is the tool behind planning a flight, a survey, a shoot, or any outdoor work that has to finish before the light goes.

Limitations: The times are for a level horizon at sea level: hills, a valley, or a tall building move sunrise and sunset by minutes, and refraction near the horizon varies with the weather. Inside the polar circles the sun may not rise or set at all, which the result states rather than inventing a time for. Legal twilight comes from the Air Almanac.

Worked example: Denver on the June solstice (MDT, UTC−6). Source: add-practitioner-essentials scenario: evening events after 00:00Z are dated on the next UTC day. It is golden test vector v001, and every build checks the tool still gives its answer within its tolerance.

Review: Not yet independently reviewed by a geodesist.

Last verified: 2026-09-18, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.

Status: version 1.0.0, core 0.1.0. See this tool in the verification report.

Changes

• 2026-09-19, fixed: Short and legacy tzdb zone names such as CET, EET, and EST5EDT are now accepted wherever a time zone is asked for; before, they were refused as malformed UTC offsets. Changelog
• 2026-09-19, changed: Sunrise, sunset, and twilight is now stable, checked against 1,200 US Naval Observatory events at 300 places and dates (all within a minute, 94% to the same minute). Changelog
• 2026-09-19, fixed: Sun event times now use the NREL SPA geometric sun instead of the low-precision NOAA series, and each side of the day is decided on its own. A twilight that grazes its altitude is now placed correctly: at 67.19°S on February 1, 2026, evening civil twilight ends at 23:59 local (matching USNO) instead of never. Across 1,200 USNO events every time is within a minute; published vectors are unchanged. Changelog
• 2026-09-18, result change: Named IANA time zones like America/Denver are supported, so inputs that returned UNSUPPORTED now resolve to local times. Changelog

Checked against: 21 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.

Sources

• Solar Position Algorithm for Solar Radiation Applications (NREL/TP-560-34302), Reda, I., and Andreas, A., National Renewable Energy Laboratory, Revised January 2008. Sections 3.1 to 3.15 and Tables A4.2, A4.3; worked example in Table A5.1. Read free.
• NOAA Solar Calculator: General Solar Position Calculations, NOAA Global Monitoring Laboratory, Spreadsheet and equations after Meeus (1998). Declination, equation of time, hour angle, refraction, and sunrise at −0.833°. Read free.
• Astronomical Algorithms, Meeus, J., Willmann-Bell, 2nd edition. Chapters 25 (solar coordinates) and 28 (equation of time).

Terms

UTC: Coordinated Universal Time

The world's civil time standard, kept within 0.9 s of Earth's rotation by leap seconds. Aviation calls it Zulu time, written with a Z. Source: IERS Bulletin C (leap seconds)