The four aviation nights

Model: Sunrise and sunset (−0.833°) and civil twilight (−6°) from the NREL SPA geometric sun, for the evening of the date and the next morning

Show your work

1. Sunset and sunrise

   the sun's centre at −0.8333°, its radius and refraction allowed for

   at 39.7392°, -104.9903° = 2026-06-21 20:31 local (2026-06-22 0231Z) to 2026-06-22 05:32 local (2026-06-22 1132Z)

2. Civil twilight

   the sun 6° below the horizon, which is when logging night begins

   −6° at 39.7392°, -104.9903° = 2026-06-21 21:04 local (2026-06-22 0304Z) to 2026-06-22 05:00 local (2026-06-22 1100Z)

3. Passenger currency

   one hour after sunset to one hour before sunrise

   an hour inside 2026-06-21 20:31 local (2026-06-22 0231Z) to 2026-06-22 05:32 local (2026-06-22 1132Z) = 2026-06-21 21:31 local (2026-06-22 0331Z) to 2026-06-22 04:32 local (2026-06-22 1032Z)

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

Accuracy: Within 1 minute of the windows built from USNO times at 100 places (491 windows); the Air Almanac, the legal source for twilight, tabulates to the minute

When to use this: Use this when a regulation turns on which night it is: logging night flight, carrying passengers on night currency, when position lights are required, and the civil-twilight window a Part 107 operation works to. All four are computed for one place and date, each shown beside the regulation that defines it, because they start and end at different times.

Limitations: These are the US definitions for the place and date you enter. The Air Almanac is the legal source for twilight and tabulates to the minute; this computes the same events from the sun's geometry and agrees within a minute. It does not know your aircraft's equipment, your currency record, or any waiver, and it does not decide whether a flight is legal.

Worked example: Denver on 2026-06-21, landing at 21:20 local. Source: add-practitioner-essentials scenario: loggable night that does not count toward §61.57(b). 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, changed: The four aviation nights and local time to Zulu are now stable. The nights match 491 regulatory windows built from US Naval Observatory times at 100 places; Zulu matches Python zoneinfo on 2,388 instants and the Python documentation's Los Angeles examples. Results are unchanged. Changelog
• 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, 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

Checked against: 22 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.

Sources

• 14 CFR 1.1, General definitions (night), Federal Aviation Administration, Reviewed 2026-09-18. "Night": end of evening civil twilight to beginning of morning civil twilight, as published in the Air Almanac. Read free. Rules as of 2026-09-18.
• 14 CFR 61.57(b), Night takeoff and landing experience, Federal Aviation Administration, Reviewed 2026-09-18. Three takeoffs and three full-stop landings from 1 hour after sunset to 1 hour before sunrise, within the preceding 90 days. Read free. Rules as of 2026-09-18.
• 14 CFR 91.209, Aircraft lights, Federal Aviation Administration, Reviewed 2026-09-18. Lighted position lights from sunset to sunrise (in Alaska, during the period a prominent unlighted object cannot be seen from 3 statute miles). Read free. Rules as of 2026-09-22.
• 14 CFR 107.29, Operation at night, Federal Aviation Administration, Reviewed 2026-09-18. §107.29(c): civil twilight is 30 minutes before official sunrise and after official sunset, except in Alaska (the Air Almanac period). Read free. Rules as of 2026-09-18.

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)