ATPL · Navigation · Oxford Atpl Book General Navigation 6th… lesson — Page 455, Lesson 446

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 Evening Civil Twilight table from the General Navigation manual. It's the kind of data you'd use in flight planning to work out, for a given date and latitude, what time civil twilight ends in the evening — in other words, when the sun has sunk far enough below the horizon that the sky is no longer considered "civil daylight." Let me set the scene. The table is organised by latitude, down the left-hand side, and by date, across the top. Look at the top of the page: the columns run from October through November, and within each month the days are broken into intervals — 2, 5, 8, 11, 14, 17, 20, 23, 26, 29, then 1, 4, 7, 10, 13, 16, 19. So each column is a specific calendar day, and each row is a specific latitude. The latitudes are given in degrees, and they run from N72 at the top, down through N70, N68, N66, N64, N62, N60, and then further down through 58, 56, 54, 52, and so on. Notice these are northern latitudes — this is a table for the northern hemisphere, in autumn, when the days are getting shorter. Now, the key thing to understand is the format of the times. Each entry is given in hours and minutes — you'll see pairs like "18 23", "18 08", "17 53". That's 18 hours 23 minutes, 18 hours 08 minutes, 17 hours 53 minutes. These are the local times at which evening civil twilight ends on that date at that latitude. Let me read you a few actual values so you can see the pattern. At N72, on October 2nd, the time is 18 hours 23 minutes. On October 5th at N72, it's 18 hours 08 minutes. On October 8th, 17 hours 53 minutes. Do you see what's happening? As the month progresses, the times are getting earlier — the sun is setting earlier, so twilight ends earlier. That's the seasonal trend. Now look down the latitude column. At N60, on October 2nd, the time is 18 hours 13 minutes. Compare that to N72 on the same date — 18 hours 23 minutes. So at higher latitude, twilight ends later on the same date. That's because at higher latitudes in autumn, the sun's path is more oblique, so it takes longer to sink below the horizon — the twilight lingers. That's a really important relationship to hold onto: for a fixed date, higher latitude means later evening civil twilight; for a fixed latitude, later in the season means earlier twilight. Let me also point out the structure of the table itself. You'll notice the numbers are arranged in a staggered, diagonal pattern rather than a simple grid. That's typical of these tables — the rows and columns are offset so that each latitude row is shifted relative to the date columns. The values are printed in a compact form, sometimes with the hour and minute split across lines, like "18" on one line and "23" on the next. You have to read them as pairs: the hour, then the minutes. Let me trace one more example so you're confident reading it. Take N64 on October 14th. Looking across the row, you'll find the entry 16 hours 06 minutes. And on October 26th at N64, it's 15 hours 55 minutes. Again — earlier as the month advances. One more thing to notice: the table extends down to lower latitudes — 58, 56, 54, 52, 50, 48, 46, 44, 42, 40, 38, 36, 34, 32, 30, 28 — and the times there are later in the evening than at the high latitudes. At N50 on October 2nd, for instance, you're looking at around 18 hours 10 minutes — later than the N72 value of 18 23? No, wait — let me re-check. At N72 on October 2nd it's 18 23, and at N50 it's about 18 10. So actually the higher latitude is later. That's the relationship I gave you: higher latitude, later twilight, for the same date. Good. So, to summarise what this page gives you: it's a lookup table — you enter with a latitude and a date, and you read out the local time of end of evening civil twilight. You'd use this in flight planning to know when daylight operations effectively cease, or when you need to consider lighting requirements, or when to plan a night approach. The two variables are the row (latitude) and the column (date), and the body of the table is the time in hours and minutes. That's the whole page — a reference table, not a concept to memorise, but a tool you need to be able to read quickly and accurately, because in the exam and in the aircraft you'll be pulling values out of tables like this under time pressure.