Day-Night Sequence

Night time must be recorded in the pilot's logbook. EASA FCL.050 states that each logbook must include the operational conditions time, of which night time is one. However, this can sometimes be difficult because during a flight, there may be several periods when the flight changes from day to night and vice versa, so that there are several periods of night. But how exactly is night defined, and what are some of the possible factors that can affect night time?


Embrear E190-E2 Cockpit during twilight

Definition of night time

EASA defines night in FCL.010 as follows:

"Night" means the period between the end of evening civil twilight and the beginning of morning civil twilight or such other period between sunset and sunrise as may be prescribed by the appropriate authority.

The FAA has a very similar definition of night in its Airplane Flying Handbook as the period between the end of evening civil twilight and the beginning of morning civil twilight.

This means that, contrary to what some people think, night is not simply defined as the period between sunset and sunrise. What this civil twilight actually refers to is discussed in the next section.

It is important to note that day and night may alternate during a flight, and that several sequences of day and night flight time may be part of the same flight. The night time to be recorded is the sum of all these sequences of night flight time. These sequences occur, for example, when flying from very high (polar) latitudes to lower latitudes.

Definition of civil twilight

The National Weather Service of the United States of America defines twilight as the time before sunrise and after sunset when the sky is partially illuminated by the sun, neither completely dark nor fully lit, and distinguishes between three types of twilight: civil, nautical and astronomical. Morning civil twilight begins when the geometric center of the sun is six degrees below the horizon and ends at sunrise. Evening civil twilight begins at sunset and ends when the geometric center of the sun is six degrees below the horizon. Both typically last about 30 minutes, but there are seasonal variations.

Illustration of the different types of twilight

The legitimate question now is which horizon to use, as this can vary considerably depending on position and altitude of the aircraft. In practice, for the sake of simplicity, a specific position and altitude are used to determine this twilight. In Switzerland, for example, Skyguide provides a list of all civil morning and civil evening twilight times in the VFR manual, which uses the Berne observatory as reference. This practice is useful for VFR flights within Switzerland. For longer flights, especially long-haul flights over the oceans, other simplifications or assumptions have to be made. One possible approach with good practical results is to treat the flight path as a great circle route and then to use a certain number of coordinates from this route to determine the corresponding position of the sun in relation to the horizon at a given time.


What is a great circle route?

A great circle route is the shortest path between two points on a sphere such as the Earth, often resulting in an arc-like path rather than a straight (rhumb) line on most commonly used chart projections.

Illustration of a great circle and rhumb line route between Zurich and New York

Polar twilight and polar night

As a significant number of flights, especially intercontinental ones between Europe and the US cross polar regions (see illustration below), the phenomenons of polar twilight and polar night are also a factor for night time calculation.

Screenshot from FlightRadar24 to illustrate the frequencies of Polar routes in transcontinental flights
Screenshot from FlightRadar24 (taken on February 22, 2024, at 12:13 UTC)

Polar twilight only happens in regions that are in the inner borders of the polar circle and are related to the winter solstice when the sun is below the horizon all day. During this time there is no true daylight and thus no true sunset and sunrise but only (polar) civil twilight. This phenomenon can be observed at latitudes between 67°24‘ and 72°34‘ North or South.

During a (civil) polar night, only a faint glow of light can be observed at noon. This phenomenon occurs when there is no civil twilight but only nautical twilight (geometric center of sun between 12° and 6° below the horizon). As a consequence, this is limited to latitudes above 72°34‘ (exactly 6° inside the polar circle).

Both phenomena add another level of complexity to calculate day and night sequences on routes which cross the polar circle at any time as both can either be very long or very short.

Day-night sequence changes

We have already mentioned several factors that can have a major influence on the day-night sequence. But other factors, such as seasons, also play an important role. In February in Europe, for example, flights with three or more changes between day and night can be observed when flying from the far north, such as Finnish Lapland (above the Polar Circle), to more southerly regions, such as Switzerland. There was one flight where this actually happened. It took off from EFKT at 14:33 UTC on 29 January 2022 and landed at LSZH at 17:58 UTC. It was still day when the plane took off. At 14:36 UTC it became night at its current position, at 16:03 UTC it became day again, and finally at 17:24 UTC it became night again. This means that a total of three changes were recorded on this single flight, despite it following a mostly north-south line.