Start with nautical twilight — Page 426, Lesson 390

Let’s pick up with the twilight definitions, because they build straight into the tables you’ll use in the Air Almanac. I want to walk you through the two twilight types first, then the civil twilight tables, then the EASA night definition, then how to calculate twilight duration, and finally the extreme cases at the equator and high latitudes. Start with nautical twilight. The centre of the Sun is between 6° and 12° below the sensible horizon. During nautical twilight, general outlines are discernible and the brighter stars are visible. So it’s that stage where you can still make out shapes on the ground and pick out the bright stars, but it’s darker than civil twilight. Then astronomical twilight. The centre of the Sun is between 12° and 18° below the sensible horizon. Astronomical twilight is often considered to be ‘complete darkness’. So once the Sun’s centre drops below 18°, you’re into full astronomical night. Now, the civil twilight tables in the Air Almanac. The LMT of civil twilight is given in the Air Almanac on the intervening pages between the sunrise and sunset tables. LMT is Local Mean Time — the time based on the mean sun at your local meridian, not your zone time. The Morning Civil Twilight tables, abbreviated MCT, give the time when morning twilight starts. MCT ends at sunrise. All times are tabulated in LMT. The Evening Civil Twilight tables, abbreviated ECT, give the time when evening twilight ends. ECT starts at sunset. Again, all times are tabulated in LMT. Now the EASA definition of ‘night’. It’s 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. An example of that latter proviso 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 possible definitions — the EASA one based on civil twilight, and the ANO one based on sunset and sunrise with a 30-minute offset. Now, how to calculate the duration of twilight. To calculate the duration of twilight 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 in the corresponding times gives the duration of twilight. Let me run the example. What is the duration of evening civil twilight in New York, USA, at 41N 074W, on 19 July? From the Air Almanac, page 13, sunset on 19 July at 41N is 1929 LMT, interpolated. From page 14, ECT on 19 July at 41N is 2001 LMT, interpolated. Duration of twilight equals 2001 LMT minus 1929 LMT, which is 32 minutes. So evening civil twilight lasts 32 minutes in New York on that date. Now the extreme cases of twilight. Two extreme cases exist, and they’re frequently examined. The equator, and high latitudes, say above 62N or 62S. 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. A circle with a dot in the centre means the Sun remains continuously above the horizon. A circle with a horizontal line through it means the Sun remains continuously below the horizon. And the symbol with four slashes, ////, 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 to sunrise the next morning. So that’s the case where civil twilight never ends — it just runs all night. Let me show you the sunrise and sunset geometry so you can picture what these twilight angles mean. So to tie it together: you’ve got three twilight bands — civil from 0° to 6° below the horizon, nautical from 6° to 12°, astronomical from 12° to 18°. The Air Almanac gives you civil twilight times in LMT, and you use the difference between sunset and ECT, or sunrise and MCT, to get the duration. And at high latitudes, the symbols tell you whether the Sun never sets, never rises, or whether twilight just runs all night.