Let me break down the structure, because the layout is the tricky part — Page 447, Lesson 418

Let’s start with what this table actually is, because it looks like a wall of numbers at first glance. This is the Morning Civil Twilight table from the General Navigation syllabus, and it’s one of the standard look-up tables you’ll use to solve time-of-day problems in flight planning. The table gives you the duration of morning civil twilight, in hours and minutes, for a given latitude and a given date (by month and day). The whole point is: if you know your latitude and the date, you can read straight off how long civil twilight lasts that morning — and that feeds directly into your sunrise/sunset and daylight calculations. Let me break down the structure, because the layout is the tricky part. The columns are the days of the month. Look at the top row: it starts with April, then columns labelled 2, 5, 8, 11, 14, 17, 20, 23, 26, 29, then May, then 2, 5, 8, 11, 14, 17. So the table is arranged in five-day intervals — you read the column for the nearest date to your actual date. The entries are in hours (h) and minutes (m). The rows are latitudes. Down the left edge you see N 72, N 70, N 68, N 66, N 64, N 62, N 60, then continuing down through N 50, N 40, N 30, N 20, N 10, 0 (the equator), S 10, S 20, S 30. So the table covers from 72° North down to 30° South. Now, the key thing to understand: the duration of morning civil twilight is not constant. It changes with both latitude and season. Let me show you the pattern, because that’s what makes this table useful. Look at the equator row (0°). The values there are all around 31 to 40 minutes — very short, and almost constant through the year. That’s because at the equator, the sun rises and sets almost vertically, so it passes through the twilight zone quickly. Now look at N 60. The values are much larger — around 4 hours in April, dropping to about 2 hours by late May. And at N 72, you see values like 3 hours 38 minutes in early April, dropping to 1 hour 04 minutes by mid-May. So the higher the latitude, the longer the twilight — and the more it varies with the season. Why? Because at high latitudes, the sun’s path is oblique — it slides along the horizon rather than dropping straight down — so it spends a long time in the twilight band. And as the season moves toward summer, the sun doesn’t go as far below the horizon, so twilight shortens. You’ll also notice some entries are marked with // — those are the dashes in the table. At the very high latitudes, like N 72 and N 70, you see these in the later columns. Those dashes mean no morning civil twilight occurs — the sun never gets far enough below the horizon for twilight to end before sunrise. In other words, it’s effectively continuous daylight — the sky never gets properly dark. That’s the midnight sun effect at high latitudes in summer. Now, how do you actually use this? The procedure is: you take your latitude, you take your date, you find the nearest column, and you read the duration in hours and minutes. That duration is the time from the start of morning civil twilight until sunrise. So if you know your sunrise time, you subtract this duration to get the time civil twilight begins — which matters for things like night flying restrictions and visual meteorological conditions at dawn. Let me walk one example so you see the mechanics. Take N 50 on April 2. Reading across the N 50 row to the April 2 column, the value is 5 hours 03 minutes. So at 50° North in early April, morning civil twilight lasts just over five hours. Compare that to the S 30 row — there the values are around 5 hours 47 to 5 hours 49 minutes in April and May. So the southern high latitudes have long twilight in their autumn, just as the northern ones do. One more thing to note: the table is symmetric in a sense — the same table, or its mirror, would apply to evening civil twilight, but this particular page is the morning version. The duration is the same for morning and evening at a given latitude and date; what changes is whether you add it to sunrise or subtract it from sunset. So, to summarise the core idea: morning civil twilight duration depends on latitude and date, this table lets you read it off directly in hours and minutes, the values grow with latitude, shrink toward summer, and the // dashes at high latitudes mean twilight never ends — continuous daylight. That’s the whole table. The skill is just reading it accurately — matching your latitude row, your date column, and pulling the hours and minutes without transposing digits.