Let me first define the term precisely, because that’s the foundation — Page 444, Lesson 413

I want to walk you through what this page actually is, because at first glance it looks like a wall of numbers. This is the Evening Civil Twilight table from the General Navigation manual — the kind of table you’ll use in real flight planning to work out when civil twilight ends at a given latitude on a given date. Let me first define the term precisely, because that’s the foundation. Civil twilight is the period when the centre of the Sun is between 0° and 6° below the true horizon. In practical terms, it’s the time after sunset when there’s still enough natural light for normal outdoor activities without artificial lighting. For aviation, the end of evening civil twilight matters because it’s often the legal or operational boundary for certain flight operations — for example, when you need to have navigation lights on, or when you can no longer operate under visual flight rules without additional considerations. Now, how to read this table. The columns across the top are the months — I can see February, March, and April here, and the table continues across the page for the rest of the year. Each month is broken into five-day intervals — you’ll see the numbers 1, 2, 3, 4, 5, 6, 9, 12, 15, 18, 21, 24, 27, 30 running along the top. Those are the days of the month. So the table gives you the time of evening civil twilight for every fifth day. The rows down the left side are latitude, in degrees. You’ll see values like N 72, N 70, N 68, N 66, N 64, N 62, N 60, N 58, N 56, N 54, N 52, N 50, N 45, N 40, N 35, N 30, N 20, N 10, then 0, then S 10, S 20, S 30, S 35, S 40, S 45, S 50, S 52, S 54, S 56, S 58, and S 60. The N means north latitude, the S means south latitude, and 0 is the equator. So the table covers from 72° North down through the equator to 60° South. The times are given in hours and minutes — you’ll see the format like 17 36, 17 44, 17 51, meaning 17 hours 36 minutes, 17 hours 44 minutes, and so on. These are the local times of the end of evening civil twilight. Let me walk you through a specific example so you can see how the numbers fit together. Look at latitude N 60 — that’s 60° North. In February, the times run 17 36, 17 44, 17 51, 17 59, 18 06, 18 13, 18 21, 18 28, 18 35, 18 43, 18 50, 18 58, 19 05, 19 13, 19 21, 19 29. So early in February, evening civil twilight ends at about 17:36, and by the end of February it’s around 19:29. You can see the times getting later as the days lengthen. Now look at the equator — the row marked 0. The times there are much more constant: 18 39, 18 38, 18 38, 18 37, 18 37, 18 36, 18 36, 18 35, 18 34, 18 33, 18 32, 18 31, 18 30, 18 30, 18 29, 18 28. At the equator, the length of day barely changes through the year, so the end of civil twilight stays very close to 18:30 throughout February, March, and April. Now look at the southern latitudes — the S rows. At S 60, the times run 20 39, 20 29, 20 19, 20 10, 20 00, 19 50, 19 41, 19 31, 19 22, 19 12, 19 03, 18 54, 18 44, 18 35, 18 26, 18 17. So in the southern hemisphere during February and March — which is their late summer and autumn — the evenings are getting shorter, and the end of civil twilight gets earlier as the months progress. Here’s the key relationship to understand. The time of evening civil twilight depends on two things: your latitude and the date. At high latitudes, the variation through the year is large — you can see at N 72 the times swing from 16 58 up to 20 34. Near the equator, the variation is tiny. And in the southern hemisphere, the pattern is inverted relative to the north, because the seasons are opposite. One more thing to notice — the table is laid out so that for a given latitude row, you read across the columns for the date, and the number you find is the local time when evening civil twilight ends on that day. This is the kind of table you’d use in flight planning to determine, for example, the latest time you can complete a visual approach, or when you need to plan for night operations. So to summarise what we have here: this is the Evening Civil Twilight table, giving the time of the end of civil twilight for latitudes from 72° North to 60° South, for each month of the year in five-day intervals, with times in hours and minutes local time. The companion table — the Morning Civil Twilight table — would give you the start of civil twilight in the morning, and together they define the period of usable daylight for your flight planning.