How night time is legally defined (and how most apps get it wrong)
Civil twilight versus sunset, ICAO versus FAA, and why a serious pilot logbook has to compute night along the great-circle track.
Ask ten pilots how their logbook computes night time and you'll get ten answers. Most are wrong. Some are wrong by five minutes; some are wrong by an hour. Here's what the definition actually is, and why the way an app computes it is a good filter for whether the rest of the app is serious.
Civil twilight, not sunset
Under ICAO Annex 2, night is the period between the end of evening civil twilight and the beginning of morning civil twilight. Civil twilight is defined as the moment the geometric centre of the sun is 6° below the horizon. That's about 30 minutes after sunset at mid-latitudes — longer in summer at high latitudes.
EASA copies the ICAO definition. The FAA uses "the time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the American Air Almanac". Same concept, different table.
The mistake most apps make
Nine out of ten "logbook apps" compute night time using the sunset at either the departure or the arrival airport. That's wrong in three ways at once:
- It uses sunset, not civil twilight, so it underestimates night by 20 to 40 minutes.
- It uses a fixed point on the ground, ignoring that you're moving across a rotating planet at 450 knots.
- On east-west legs it can miss an entire crossing of the terminator.
What "computed along the track" actually means
To compute night correctly you sample points along the great-circle route between departure and arrival — every 60 seconds is a reasonable resolution — and at each point ask: was the sun's centre more than 6° below the horizon at that time and place? Sum the seconds that answered yes, round to the minute, and that's your night portion.
Edge cases that matter
- Polar routes. On a JFK-NRT polar sector you can cross civil twilight three times in one flight. A point-source calculation gets this catastrophically wrong.
- Westbound long-haul. LHR-YVR in northern summer spends nearly all its time in twilight but never fully dark. Any app that logs it as "all night" is guessing.
- Reroutes and holds. Great-circle assumes you flew the great circle. If you were vectored 200 nm around a system, override the value in the flight editor.
How Aerolog computes it
Aerolog samples the great-circle track at 60-second resolution, computes the sun's altitude at each point using the standard astronomical algorithm (Meeus, chapter 25), and sums the portion where the centre of the sun was more than 6° below the horizon. The result is displayed to the minute and is editable per flight.
Why this matters for your career
Night hours are a currency requirement (recent night landings), a licence upgrade currency (ATPL), and a hiring filter (many airlines list minimum night hours). Getting them wrong by 20% over 5,000 sectors is real money and real career impact. Use a logbook that does the geometry.