How accurate is NightLog Pro?

01 What “night” means here

• Night is the time with the centre of the sun more than 6° below the horizon — the end of evening civil twilight to the beginning of morning civil twilight. This is the definition in EASA Part-FCL and FAA 14 CFR 1.1, and it is the figure you log.
• Sunset to sunrise uses −0.833°, which allows for refraction and the sun’s radius, as almanacs do. It is shown for FAA 14 CFR 91.209 (position lights) and is not logged night.
• Both are measured at the aircraft’s position, at surface level. A Flight Level, if you enter one, is shown as information beside the figure and never changes it.
• The UK defines night differently (30 minutes after sunset to 30 minutes before sunrise). NightLog does not compute that definition yet, and says so on every result.

02 The sun, against the US Naval Observatory

The sun’s position comes from the equations behind NOAA’s solar calculator (after Meeus, Astronomical Algorithms), computed on your device. To check it we compare NightLog’s sunrise, sunset and civil-twilight times with those the US Naval Observatory publishes for the same airports and dates.

The USNO lists events by UT date, so an evening in Anchorage or a morning in Sydney can fall on the date before or after the one in the table. Each time is compared with the event on the local day it belongs to.

03 How the night is counted

• The aircraft holds at the departure during taxi-out, flies the great circle at constant ground speed, and holds at the arrival during taxi-in.
• The sun is checked every 10 seconds along that path, and each crossing of −6° and −0.833° is then pinned to the second.
• The night is the exact time between those crossings, and is rounded to the whole minute once, at the end — not minute by minute along the way.
• Everything runs on your device. In the Android app it works with no connection at all.

04 Test flights anyone can check

14 flights, version 1. Our test suite recounts each one second by second, separately from the calculator, and they agree to within 0.05 min. They cover the cases calculators get wrong: the date line, the midnight sun and the polar night, circuits at one airfield, two separate nights in one sector, flights across 00:00 UTC and over 16 hours.

Download the test flights (JSON) — each one carries the airports’ positions, the block times, taxi, the expected night in exact and whole minutes, and every crossing with its time and position.

05 What these numbers do not cover

• The route you actually flew. Airways, oceanic tracks, weather deviations, winds and the slower climb and descent all move the crossings. On a short sector the difference is usually small; on a long oceanic one it can be large. No calculator that assumes a great circle can know it.
• Times entered in local time. Typing local time where UTC is asked shifts both block times by the time-zone difference, and the night with them. NightLog shows the local time at each airport under each UTC field so you can see it at a glance.
• Your authority’s rule. The figure is the EASA and FAA civil-twilight definition. If you fly under another rule, check it.

NightLog Pro is an independent estimator. Your official logbook and your operator’s procedures remain the record of authority.