Let me first define the term at the heart of this table — Page 440, Lesson 406

I want to walk you through what this page actually is, because at first glance it looks like a wall of numbers. This is a twilight table — specifically, the table for Evening Civil Twilight — and it's one of the tools you'll use in flight planning to work out when daylight conditions begin and end at a given latitude on a given date. Let me first define the term at the heart of this table. Civil twilight is the period when the centre of the Sun is between 0° and 6° below the horizon. During civil twilight there's enough natural light for most outdoor activities, and for aviation it's the period when the horizon is still clearly visible — which matters for visual navigation and for deciding when night flying rules kick in. The table you're looking at gives the time of evening civil twilight — that is, the moment when civil twilight ends and true night begins, as the Sun sinks past 6° below the horizon. Now, how do you read this table? Look at the structure. Across the top you have the months — January, February, and so on — and each month is divided into columns for days of the month: the numbers 2, 5, 8, 11, 14, 17, 20, 23, 26, 29, then 1, 4, 7, 10, 13, 16. So the table gives you the time of evening civil twilight for every third day or so, and you interpolate between them for the exact date you need. Down the left-hand side you have latitude, in degrees North. Look at the rows: N72, then 70, 68, 66, 64, 62, 60. So this particular table covers latitudes from 60°N up to 72°N. That's high northern latitudes — think of routes across the North Atlantic or northern Europe. Each cell in the table gives a time in hours and minutes — you'll see the columns headed h and m, for hours and minutes. So, for example, at latitude N72 in early January, the evening civil twilight time is around 13 hours 17 minutes — that's 13:17 in the afternoon. As you move down the table to lower latitudes, and as you move across the months toward summer, the times get later in the evening, because the days are longer. Let me point out something important about the latitude rows. Notice that the rows aren't evenly spaced — you have N72, then 70, 68, 66, 64, 62, 60. That's a 2° spacing between rows. And within each row, the times progress day by day. The table is designed so you read across a row for a fixed latitude, find the date column, and read the time of evening civil twilight. There's also a subtlety in how the times are laid out. Look at the way the numbers are grouped — you'll see times like 13 17, then 13 27, then 13 40, and so on. Each pair is an hours-and-minutes value. And you'll notice that within a row, as the days progress, the times increase — the twilight gets later as spring approaches. One more thing to notice: the table has a Dec. column at the far left, before January. That's December — the table actually starts in December and runs through the winter and spring months. So the full set of months covered is December, January, February, and so on across the top. Now, why does this matter for you as a pilot? In flight planning, you need to know when civil twilight occurs because it determines things like when you can operate under visual flight rules, when navigation lights must be displayed, and when night flying restrictions apply. This table gives you the evening end of that period — the moment the Sun drops below 6° and civil twilight gives way to night. Let me also point out the structure of the time values more carefully, because the layout can be confusing. In the N72 row, you see: 13 17, 13 27, 13 40, 13 53, 14 07, 14 21, 14 36, 14 50, 15 05, 15 19, 15 34, then 15 48, 16 02, 16 16, 16 30, 16 44, 16 58. So the times run from about 13:17 in mid-December up to about 16:58 by mid-February. That's a spread of nearly four hours across the winter-to-spring progression. For the lower latitudes, the times are later. Look at the N60 row: it starts around 15 59 and goes up to about 16 27. So at 60°N, evening civil twilight in December is around 16:00, whereas at 72°N it's around 13:17. That makes sense — at higher latitudes in winter, the Sun sets much earlier, so twilight ends earlier. There's one more detail in the layout worth noting. You'll see some rows have times that appear to be split across lines — for example, in the N70 row you have 14 13, 14 19, 14 27, 14 36, 14 46, 14 57, 15 08, 15 19, then 31, 15 43, 15 55, 16 07, 19, 31, 43, 16 55, 17 07. The numbers like 31, 19, 31, 43 that appear alone are actually the minutes part of the previous hour — so 15 31, 15 43, 15 55, 16 07, 16 19, 16 31, 16 43, 16 55, 17 07. The table compresses the layout to save space, so you have to read the hour from the preceding full time and then pick up the minutes from the standalone numbers. So the key skill here is: find your latitude row, find your date column, and read the time of evening civil twilight in hours and minutes. Then interpolate between the three-day intervals for your exact date. That's the essence of this page — it's a reference table for evening civil twilight at high northern latitudes, giving you the time in hours and minutes for each date across the winter and spring months. When you're planning a flight, you'd look up your latitude and date, read off this time, and use it to determine when civil twilight ends for your operation.