First, Nautical Twilight — Page 426, Lesson 390

We’re moving into the third part of the Time chapter now, and this is where we get into twilight — the period when the Sun is below the horizon but the sky isn’t fully dark. Let’s start with the two named phases you’ll be tested on. First, Nautical Twilight. The centre of the Sun is between 6° and 12° below the sensible horizon. The sensible horizon is the horizon you actually see from your position — the line where sky meets Earth. During nautical twilight, general outlines are discernible and the brighter stars are visible. So you can make out the shape of the coastline or a ship, and you can see the bright stars for navigation. Second, Astronomical Twilight. The centre of the Sun is between 12° and 18° below the sensible horizon. Astronomically, this is often considered to be ‘complete darkness’ — the faintest stars become visible, and for most practical purposes it’s night. Now, the Air Almanac gives you the times for civil twilight. The LMT — that’s Local Mean Time — of civil twilight is tabulated on the intervening pages between the sunrise and sunset tables. There are two sets of tables. The Morning Civil Twilight (MCT) tables give the time when morning twilight starts. MCT ends at sunrise. So MCT is the beginning of dawn — the moment twilight begins in the morning, and it runs until the Sun actually rises. The Evening Civil Twilight (ECT) tables give the time when evening twilight ends. ECT starts at sunset. So ECT is the end of dusk — it begins at sunset and runs until twilight is over. All times in both tables are tabulated in LMT. Now, this matters because of the EASA definition of ‘night’. Night is defined as 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. That second part is the proviso. An example is the Air Navigation Order, which defines the period of night flying as from 30 minutes after sunset to 30 minutes before sunrise at the surface. So you have two different definitions of night in play — the EASA one based on civil twilight, and the ANO one based on sunset and sunrise with a 30-minute offset. Now let’s look at how you actually calculate the duration of twilight. To do this at a given latitude on a given date, you extract the time of sunrise or sunset for that date, and the time of MCT or ECT on the same date. The difference between the corresponding times gives you the duration of twilight. Let me walk you through the example. What is the duration of evening civil twilight in New York, USA — that’s 41°N, 074°W — on 19 July? From the Air Almanac, page 13, sunset on 19 July at 41°N is 1929 LMT, interpolated. From page 14, ECT on 19 July at 41°N is 2001 LMT, interpolated. So the duration of twilight is 2001 LMT minus 1929 LMT, which equals 32 minutes. So evening civil twilight lasts 32 minutes in New York on that date. Now, there are two extreme cases of twilight that are frequently examined. The first is the Equator. The second is high latitudes — say above 62°N or 62°S. In high latitudes, the Sun may be above or below the horizon all day, or civil twilight may last all night. The Air Almanac uses symbols to indicate these conditions. One symbol means the Sun remains continuously above the horizon — that’s the midnight sun. Another symbol means the Sun remains continuously below the horizon — that’s polar night. And there’s a third symbol, shown as four slashes — `////` — which means that in the period between sunset and sunrise, the Sun is never less than 6° below the sensible horizon. In plain English, twilight lasts from sunset all the way to sunrise the next morning. So you never get full darkness — the Sun never gets more than 6° below the horizon. Let me make sure you’ve got the key numbers. Nautical twilight: Sun centre 6° to 12° below the sensible horizon. Astronomical twilight: 12° to 18° below. Civil twilight is the one the Air Almanac tabulates, and it’s the one that defines EASA night. The duration calculation is simply the difference between sunset and ECT, or between MCT and sunrise. And the extreme cases — equator and high latitudes — are where the symbols come in, especially the `////` symbol meaning twilight lasts all night. That’s the core of this section