Let me make sure you can read it the way a navigator would — Page 453, Lesson 440

I want to walk you through what we're looking at here, because this is one of those pages that looks like a wall of numbers until you understand the grid you're reading. This is a table of evening civil twilight times, and it's laid out by latitude and by date. Let me make sure you can read it the way a navigator would. First, the title. Evening civil twilight is the period after the sun has set, but while there is still enough natural light for ordinary outdoor activities without artificial lighting. In practical terms for you as a pilot, it's the time when the sky is still bright enough that you can clearly see the horizon and the ground. The table gives you, for a given latitude and a given date, the local time — in hours and minutes — at which evening civil twilight ends, meaning the moment the light drops below that threshold. Now, the structure. The table is split into two halves. The left half covers August and September, and the right half covers October. The columns run across the top with dates: for August you have the 15th, 18th, 21st, 24th, 27th, and 30th. Then September starts with the 2nd, 5th, 8th, 11th, 14th, 17th, 20th, 23rd, 26th, and 29th. Then October begins with the 2nd, and the columns continue from there. Down the left-hand side you have the latitude, in degrees, labelled with an N for north. The latitudes run from N 72 at the top, then 70, 68, 66, 64, 62, and N 60 at the bottom. So this particular page covers the high northern latitudes — 60 to 72 degrees north. That's the region where twilight behaviour changes dramatically through the autumn, which is exactly why this table exists. Each cell of the table gives you a time in hours and minutes. Let me show you how to read one. Take latitude N 60, August 15th. The entry reads 20 hours 46 minutes. So at 60 degrees north on the 15th of August, evening civil twilight ends at 20:46 local time. That's the moment the light drops below the civil twilight threshold. Now here's the important pattern I want you to notice, because it's the whole point of the table. Follow the row for N 60 across the dates. August 15th is 20:46. August 18th is 20:36. August 21st is 20:26. August 24th is 20:16. August 27th is 20:07. August 30th is 19:57. September 2nd is 19:47. September 5th is 19:37. September 8th is 19:28. September 11th is 19:18. September 14th is 19:08. September 17th is 18:59. September 20th is 18:50. September 23rd is 18:40. September 26th is 18:31. September 29th is 18:22. And into October, October 2nd is 18:13. Do you see what's happening? As autumn progresses, the sun sets earlier each day, so evening civil twilight ends earlier and earlier. The times are marching steadily backwards. That's the seasonal trend the table captures. Now look at what happens as you move up in latitude. Take the same date — say September 2nd — and read down the column. At N 60 it's 19:47. At N 62 it's 19:56. At N 64 it's 20:06. At N 66 it's 20:19. At N 68 it's 20:34. At N 70 it's 20:54. And at N 72 it's 22:11. So for the same date, the higher your latitude, the later evening civil twilight ends. The sun takes a longer, shallower path down to the horizon at high latitudes, so the twilight lingers longer. And you'll notice at the very top, for N 72 in August, the entries are marked with slashes — // — meaning the data isn't given there. That's because at those high latitudes and those dates, the sun may not dip far enough below the horizon to end civil twilight at all, or the table simply doesn't tabulate it. The slashes are the table's way of saying "no value here." There's also a small block of numbers at the very top of the page — 41, 39, 38, 36, 34, 32, 30, 28, 26, 25, 23, 20 — and a similar block on the right. Those are the tail ends of the previous table's columns, the last few entries of the morning or evening table that precedes this one on the page. They're just the continuation of the earlier data, so don't let them confuse you. The table you're working with is the evening civil twilight table for latitudes 60 to 72 north. One more thing to note about the format. Each time is written as hours and minutes, so 20 46 means 20 hours and 46 minutes. When you see a number like 19 57, that's 19 hours 57 minutes. And when you see something like 18 13, that's 18 hours 13 minutes. The two-part format is hours then minutes, always. So the practical takeaway for you as a navigator: this table lets you predict, for your latitude and the date, when evening civil twilight will end. That matters for planning when you need to switch from visual to instrument conditions, when you need to have your navigation lights on, and when you can expect the last usable daylight for visual reference. The two trends to remember are: later dates give earlier twilight end times, and higher latitudes give later twilight end times for the same date. That's the whole story this grid is telling.