First, let's be precise about what we're reading — Page 444, Lesson 413

I want to walk you through the Evening Civil Twilight table from the General Navigation manual. This is 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. First, let's be precise about what we're reading. This is a table of evening civil twilight times. Civil twilight is the period after sunset when the centre of the sun's disc is between 0° and 6° below the horizon — bright enough that terrestrial objects are still distinguishable and, in practice, you can still operate visually. The table gives you the time of the end of evening civil twilight in hours and minutes, in UTC, for a range of latitudes and dates. Now, how the table is laid out. The columns run across the months — here we have February, March, and April. Within each month, the columns are broken into three-day intervals: the 16th, 19th, 22nd, 25th, 28th, then the 3rd, 6th, 9th, 12th, 15th, 18th, 21st, 24th, 27th, 30th, and then the 2nd of the following month. So each vertical column is a specific date, and each horizontal row is a specific latitude. The rows are latitudes, labelled with N for north and S for south. The latitudes run from N 72° down through N 70, 68, 66, 64, 62, 60, 58, 56, 54, 52, 50, 45, 40, 35, 30, 20, 10, then the equator at 0°, and then into the southern hemisphere: 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. So you've got coverage from 72° North down to 60° South. Let me show you how to read a value. Take latitude N 60. On the 16th of February, the table gives 17 hours 36 minutes. On the 19th of February, 17 hours 44 minutes. You can see the times getting later as the month progresses — 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, and 19:29 by the 2nd of April. That's the key trend: as spring advances in the northern hemisphere, the days lengthen, so civil twilight ends later each day. Now look at what happens as you move north. At N 72 on the 16th of February, evening civil twilight ends at 16:58. At N 60 it's 17:36. At N 50 it's 17:53. At N 20 it's 18:22. So the higher the latitude in the northern hemisphere in late winter, the earlier the twilight ends — the sun sets earlier the further north you go at this time of year. Cross the equator. At latitude 0°, the times are nearly constant — around 18:39 on the 16th of February, drifting only slightly through the months. That's because at the equator day length barely changes through the year. Now go into the southern hemisphere. At S 10 on the 16th of February, evening civil twilight ends at 18:48. At S 30 it's 19:12. At S 40 it's 19:29. At S 52 it's 20:02. At S 60 it's 20:39. So in the southern hemisphere in February — which is late summer there — the higher the latitude, the later the twilight ends. The southern hemisphere is having its long summer days, so twilight extends well into the evening. Notice the contrast: in February, N 60 gives you 17:36 but S 60 gives you 20:39 — nearly three hours difference, purely because of the season in each hemisphere. One more thing to note. Look at the southern latitudes as the months progress. At S 60, the 16th of February is 20:39, but by the 2nd of April it's dropped to 18:17. The times are getting earlier — the southern autumn is advancing, days are shortening, so twilight ends earlier. The whole table is really a picture of the seasons working through the latitudes. So when you use this table in flight planning, you pick your latitude row, pick your date column, and read off the UTC time of the end of evening civil twilight. That tells you when visual conditions effectively end for that position on that day.