Now, look at the structure — Page 451, Lesson 433

I want to walk you through what's on this page, because it's one of the most practical tables you'll use in real flight planning. This is the Evening Civil Twilight table, and it's part of the almanac-style data you'll use to work out when daylight operations are possible. Let me first make sure you understand what "civil twilight" means, because the whole table hinges on it. Civil twilight is the period after the sun has set — or before it rises — when the sun's centre 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 you can still see the horizon clearly. The table gives you the time of the end of evening civil twilight — in other words, the moment when civil twilight finishes and it becomes properly dark. Now, look at the structure. The table is organised by latitude down the left-hand side, running from N 72° down through N 70°, N 68°, N 66°, N 64°, N 62°, and so on. And across the top, the columns are the dates — June 28th, then July 1st, 4th, 7th, 10th, 13th, 16th, 19th, 22nd, 25th, 28th, 31st, then August 3rd, 6th, 9th, 12th, 15th. So you read it like any table: pick your latitude, pick your date, and the number in the cell is the time of the end of evening civil twilight. The times are given in hours and minutes — you'll see the column headers marked "h" and "m" for hours and minutes. So a value like "23 51" means 23 hours 51 minutes, which is 23:51 in 24-hour time. Here's where it gets interesting, and I want you to notice the pattern. Look at the high latitudes — N 72°, N 70°, N 68°. You'll see the letter G in many of the cells. That G stands for a very important condition: it means there is no evening civil twilight on those dates at those latitudes. Why? Because at those high latitudes in midsummer, the sun never gets far enough below the horizon — it stays within 6° of the horizon all night, so civil twilight never actually ends. The sky never gets properly dark. That's what the G is telling you: the end of evening civil twilight doesn't occur. Now look at the cells marked with slashes — the "//" symbols. Those slashes indicate that the data isn't available or isn't applicable for that particular latitude and date combination. You'll see them scattered through the high-latitude rows, especially in the August columns as the season progresses. Let me read you a few actual values so you can see how the numbers behave. At N 68°, on June 28th, the end of evening civil twilight is at 23:32. By July 4th it's 22:50, and by July 7th it's 22:23. Notice how the times are getting earlier as the summer progresses — the days are shortening. At N 66°, on June 28th it's 23:15, then 22:46 on July 4th, 22:23 on July 7th, 22:04 on July 10th, and 21:47 on July 13th. Now here's a really important operational point, and I want you to see it clearly. Compare the same date across different latitudes. Take July 4th. At N 68° the end of evening civil twilight is 22:50. At N 66° it's 22:46. At N 64° it's 22:57. At N 62° it's 23:12. Do you see what's happening? The times are not simply getting later as you go further north — the relationship is more subtle, because the geometry of the sun's path relative to the horizon changes with latitude. At the very highest latitudes, the sun's path is so shallow that twilight lasts much longer, and eventually you hit the G condition where it never ends at all. Let me give you a couple more concrete readings. At N 64°, on June 28th, the value is 23:24. On July 4th it's 22:57, July 7th 22:37, July 10th 22:20, July 13th 22:04, July 16th 21:49, and July 19th 21:35. At N 62°, June 28th gives 23:36, July 4th 23:12, July 7th 22:55, July 10th 22:40, July 13th 22:26, July 16th 22:13, July 19th 22:01, July 22nd 21:48, and July 25th 21:36. So the whole table is a lookup tool. When you're planning a flight and you need to know whether you'll still have usable daylight — or specifically, when civil twilight ends — you find your latitude, find your date, and read off the time. This is exactly the kind of data you'd use to determine, for example, whether a flight can be completed before dark, or whether you need to plan for night operations. One more thing to note about the layout. You'll see the latitude values on the left — N 72, N 70, N 68, N 66, N 64, N 62 — and the corresponding time values are laid out in rows beneath each latitude label. The table is dense, but once you understand the grid — latitude down the side, date across the top, time in the cell — it becomes a straightforward reference. That's the essence of this page. It's a civil twilight table for the summer months, June through August, giving you the end of evening civil twilight in hours and minutes for latitudes from N 62° up to N 72°, with the G notation telling you when twilight never ends and the slashes marking unavailable data.