First, let's define the term — Page 451, Lesson 432

I want to walk you through the Morning Civil Twilight table. This is a standard navigation table you'll use to find how long morning civil twilight lasts at a given latitude on a given date. First, let's define the term. Morning civil twilight is the period between the moment the sun's upper limb is 6 degrees below the horizon and the moment the sun's upper limb rises above the horizon. In plain terms, it's the time from when there's just enough light to see clearly outdoors — the start of civil twilight — until actual sunrise. The table gives you the duration of that period in hours and minutes. Now, how to read the table. The left-hand column is latitude, in degrees North, running from N 72 down to N 0. The top row is the months — June, July, August — and beneath each month, the days of the month: 1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31, and then 3, 6, 9, 12, 15, and so on. So the table covers roughly the middle of the year, from early June through late August. The body of the table gives the duration of morning civil twilight in hours and minutes. For example, at latitude N 60 on June 1st, the value is 00 hours 55 minutes. That means morning civil twilight lasts 55 minutes. At N 50 on June 1st, it's 03 hours 08 minutes — over three hours. So you can see the duration increases dramatically as you go north. Now, you'll notice some entries are marked with the letter G. At high latitudes — N 72, N 70, N 68, and parts of N 66 — the table shows G for many dates. G stands for "G" — it indicates that the sun does not go far enough below the horizon for civil twilight to end. In other words, at those high latitudes in midsummer, the sun never gets 6 degrees below the horizon, so there is no true morning civil twilight — it's effectively continuous daylight, or the twilight period runs into the next one. The G tells you that the normal definition of morning civil twilight doesn't apply on those dates. You'll also see entries marked with double slashes — //. Those indicate that the data is not applicable or not given for that latitude and date. For instance, at N 72, most of the table is slashes, because at that latitude in June and July the sun is so high that the conditions for the table simply don't hold. Let me walk you through a specific reading. Take latitude N 60, June 1st: 00 55. That's 55 minutes. Now look at N 60 on July 31st: the value is 03 21 — 3 hours 21 minutes. So over the course of the summer, the duration of morning civil twilight at N 60 grows from under an hour to over three hours. That's the key trend: the higher the latitude and the closer to midsummer, the longer the twilight. One more example to make sure the format is clear. At N 40, June 1st: 04 00 — 4 hours exactly. At N 20, June 1st: 04 59 — 4 hours 59 minutes. At N 10, June 1st: 05 19 — 5 hours 19 minutes. And at the equator, N 0, June 1st: 05 19 as well — wait, let me re-read. At N 0, the value is 05 19. So near the equator, morning civil twilight is roughly 5 hours 19 minutes, and it stays fairly constant through the summer — you can see the values at N 0 hover around 05 19 to 05 29 across the whole table. That's because at the equator, the sun's path is nearly perpendicular to the horizon, so twilight duration changes very little through the year. So the practical use: you're planning a flight, you know your latitude and the date, you look up the duration of morning civil twilight, and you add it to your calculations for sunrise and lighting conditions. That's the whole purpose of this table — it's one of the standard navigation references for time and light calculations. Now, one thing to note about the format: the values are given in hours and minutes, with the hours and minutes separated by a space. So "03 08" means 3 hours 8 minutes, not 3:08 in some other sense. And when you see a value like "01 10 01 39" — that's two separate entries side by side, one for one date and one for the next date in the column. Let me also point out the trend at the very high latitudes more carefully. At N 66, you see a mix: some dates have G, some have slashes, and some have actual times like 00 45, 01 19, 01 24, 02 01, 02 18. So at N 66, early June has G — no true twilight — but by late June and into July, the sun dips low enough that morning civil twilight reappears, and the table gives you the duration. At N 64, you see times like 00 36, 00 53, 01 12, 01 28, 01 43, 01 56, 02 09, 02 21, 02 32, 02 44, 02 54, 03 05 — so the duration grows from about 36 minutes in early June to over 3 hours by late August. So the overall picture: this table is a latitude-by-date lookup for the duration of morning civil twilight, with G marking dates where the sun never gets low enough for civil twilight to end, and slashes marking dates where the data isn't applicable. The higher your latitude, the longer the twilight, and the closer to midsummer, the longer it is as well. That's the core of what this table gives you.