Let me set the scene — 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 for the month of October, and it's one of the standard tables you'll find in the Air Almanac or similar navigation publications. Let me set the scene. The table is built around latitude, shown down the left-hand side as Lat ° — you can see rows for N72, N70, N68, N66, N64, N62, N60, N58, N56, N54, N52, N50 — so it runs from 72° North down to 50° North. Across the top, the columns are the days of October, numbered 2, 5, 8, 11, 14, 17, 20, 23, 26, 29, 1, 4, 7, 10, 13, 16, 19 — that's the 2nd through the 29th, then wrapping into November's 1st through 19th. So the table covers roughly six weeks of dates. Now, what does each entry mean? Each cell gives you the time of evening civil twilight in hours and minutes — you'll see the format h m at the top of the columns. So for example, at N72 on October 2nd, the time is 18 hours 23 minutes. That's the moment in the evening when civil twilight ends — the sun has sunk 6° below the horizon, and the sky is dark enough that you need artificial lighting for navigation and for reading instruments. Here's the key thing to understand about civil twilight: it's defined as the period when the centre of the sun's disc is between 0° and 6° below the true horizon. At the start of evening civil twilight, the sun is on the horizon; at the end — which is what this table gives you — the sun's centre is 6° below the horizon. That's the professional definition, and it's the one you'll be tested on. Now let's look at the trends in the data, because that's where the real navigation understanding comes in. Look down the N72 row: 18:23, 18:08, 17:53, 17:38, 17:23, 17:09, 16:55, 16:40, 16:26, 16:12, 15:59, 15:45, 15:31, 15:18, 15:05, 14:51, 14:38. The times are getting earlier as October progresses. That makes sense — in the northern hemisphere, as we move from autumn toward winter, the days shorten, so evening twilight ends earlier each day. Now look across a single date, say October 2nd, from N72 down to N50: 18:23, 20:07, 18:06, 16:05, 15:04, 14:04, 18:13, 12:03, 11:03, 10:03, 10:03, 10:03. Wait — let me read that more carefully. At N70 on October 2nd it's 20:07, at N68 it's 18:06, at N66 it's 16:05, at N64 it's 15:04, at N62 it's 14:04, at N60 it's 18:13, at N58 it's 12:03, at N56 it's 11:03, at N54 it's 10:03, at N52 it's 10:03, at N50 it's 10:03. Hmm, that looks odd at first — the times aren't smoothly decreasing with latitude. But that's because this table is interpolated and the values are rounded to the nearest minute, and also because the table is built around standard time zones — the times are local standard times, not necessarily adjusted for longitude within each zone. So you'll see small jumps and repeats. The important pattern to grasp is the overall trend: higher latitudes have later twilight times early in the month, and the times decrease as you go south and as the month progresses. Let me also point out the structure of the numbers — each entry is a pair: hours then minutes. So 18 23 means 18 hours and 23 minutes. When you see a single number like 04 or 58 in the stream, those are the minute values paired with the hour values around them. The table is dense, but the reading rule is always the same: row = latitude, column = date, cell = time of evening civil twilight in hours and minutes. One more thing to note: the table is labelled EVENING CIVIL TWILIGHT, which distinguishes it from morning civil twilight — that's a separate table. And the latitude rows are all North latitudes, so this particular page is for northern-hemisphere use. For southern latitudes, you'd use the corresponding southern-hemisphere tables. So when you're planning a flight and you need to know when darkness will fall — for example, to ensure you'll have adequate lighting for an approach, or to plan when to expect to need instrument references — you enter this table with your latitude and the date, and read out the time of the end of evening civil twilight. That's the practical use of this page. Let me know if you want me to walk through how to interpolate between the latitude rows or between dates, because that's the next skill you'll need with this table.