First, let's define what we're looking at — Page 457, Lesson 453

I want to walk you through the Evening Civil Twilight table from the General Navigation book. This is a table you'll use to find the time of evening civil twilight for any given latitude and date. First, let's define what we're looking at. Evening civil twilight is the period after sunset when the centre of the sun is between 0° and 6° below the horizon. In practical terms, it's the time when there's still enough natural light for most outdoor activities without artificial lighting — and for us in aviation, it's the time when the horizon is still clearly visible, which matters for visual navigation and for understanding when night flying restrictions begin. Now, let's look at the structure of this table. The columns across the top are dates. You'll see November, December, and January, with specific days listed: 19, 22, 25, 28, 1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31, and 3. So the table runs from 19 November through to 3 January, covering the period around the winter solstice. The rows down the left side are latitudes, given in degrees. You'll see N72, N70, N68, N66, N64, N62, and N60 — these are latitudes in the northern hemisphere, from 60°N up to 72°N. These are high latitudes, well into the Arctic region. The times in the body of the table are given in hours and minutes — the format is h m, so for example 14 38 means 14 hours 38 minutes. These times are the local time of evening civil twilight for that latitude on that date. Let me read you a few values so you can see how the table works. At latitude N72 on 19 November, evening civil twilight is at 14 hours 38 minutes. On 22 November at the same latitude, it's 14 hours 26 minutes. You'll notice the times are getting earlier as we move through November — the days are shortening as we approach the winter solstice. Now look at what happens around the solstice. At N72, on 1 January the time is 12 hours 59 minutes, and on 4 January it's also 12 hours 59 minutes. Then on 7 January it's 13 hours 02 minutes. So the times reach a minimum around the end of December and the very start of January, then start getting later again — the days are lengthening after the solstice. Let me also show you the latitude effect. Take 19 November: at N72 it's 14 38, at N70 it's 15 04, at N68 it's 15 24, at N66 it's 15 40, at N64 it's 15 53, at N62 it's 16 04, and at N60 it's 16 13. So the higher the latitude, the earlier evening civil twilight occurs — because at higher latitudes in winter, the sun sets earlier and gets below 6° below the horizon sooner. There's a figure in the book that shows this concept — shows how True North is defined by the line joining a point to the North Pole, which is the basis for understanding direction at these high latitudes. Now, one important thing to notice: at the very highest latitudes in this table, the times barely change across the whole period. Look at N72 — from 19 November at 14 38 to 1 January at 12 59, the change is about 1 hour 39 minutes. But compare that to N60, where 19 November is 16 13 and the values stay much closer to 15 50 to 16 13 across the whole period. The higher latitudes show a bigger swing because the sun's path changes more dramatically near the Arctic circle. Let me also point out the values at N60 more carefully. On 19 November it's 16 13, on 22 November 16 09, on 25 November 16 04, on 28 November 16 01, then 1 December 15 57, 4 December 15 55, 7 December 15 53, 10 December 15 51, 13 December 15 50, 16 December 15 50, and 19 December 15 51. So the minimum at N60 is around 13 to 16 December at 15 hours 50 minutes, and then it starts creeping back up. This table is part of the almanac data you'll use in navigation calculations. When you're planning a flight, you need to know when civil twilight ends because that defines the boundary between day and night operations. If you're flying under visual flight rules, you need to know when you must have your navigation lights on, and when night flying restrictions apply. One more thing to note about the format: the times are in local time, not UTC. So when you use this table, you need to know your local time zone and convert to UTC for your navigation calculations. Let me summarise what you need to remember. This table gives evening civil twilight times in local hours and minutes, for latitudes N60 to N72, for dates from 19 November to 3 January. The times get earlier as latitude increases, they reach a minimum around the winter solstice, and then they start getting later again. The higher the latitude, the more pronounced the seasonal variation. That's the Evening Civil Twilight table. When you're ready, we can move on to how this fits into the broader navigation picture.