Now, the layout — Page 449, Lesson 425

This is a table of evening civil twilight times. Let me explain what we're looking at, because this is a tool you'll use constantly in flight planning. Civil twilight is the period after sunset when the centre of the sun's disc is between 0° and 6° below the horizon. During civil twilight there's still enough natural light for most outdoor activities, and for aviation it's the period when you can still see the horizon clearly. The times in this table tell you, for a given latitude and date, what time evening civil twilight ends — that is, when the sun has sunk 6° below the horizon and full darkness begins. Now, the layout. The columns run across the top for the months May, June and July, and within each month there are columns for each day — the 17th, 20th, 23rd, 26th, 29th, then the 1st, 4th, 7th, 10th, 13th, 16th, 19th, 22nd, 25th and 28th of the following month. So this is a continuous run of dates from 17 May through to 28 July. The rows are latitude, in degrees north, from N 72 down through 70, 68, 66, 64, 62, 60, 58, 56, and so on. Each cell gives the time in hours and minutes at which evening civil twilight ends at that latitude on that date. Now here's the interesting part — the symbols. Where you see a "G", that stands for continuous daylight. At those high latitudes, in the middle of summer, the sun never gets 6° below the horizon, so civil twilight never ends — it stays light all night. That's the "G". Where you see "//", that's a different situation. Those slashes indicate that the sun doesn't even set — it's continuous daylight of a different kind, where the sun stays above the horizon the whole time. So at the very high latitudes in early summer, you get either continuous daylight or continuous twilight, and the table marks those with "G" or "//" instead of a time. Let me read you a few actual values so you can see how it works. At N 60 on 17 May, evening civil twilight ends at 21 hours 41 minutes. On 20 May it's 21 51, on 23 May 22 01, on 26 May 22 11, on 29 May 22 21. Then into June: 1 June 22 31, 4 June 22 40, 7 June 22 49, 10 June 22 57, 13 June 23 04, 16 June 23 10, 19 June 23 13, 22 June 23 14 — that's the longest day, the summer solstice — then 25 June 23 13, 28 June 23 09, and 1 July 23 03. Notice the pattern. The times build up to a maximum around 22 June, the solstice, and then start coming back down. That's the seasonal rhythm of the sun's declination. Now look at what happens as you go further north. At N 62, on 17 May the time is 22 12, then 22 26, 22 41, 22 58, and by 23 May it's 23 20 — and then the "//" takes over, because the sun doesn't set at all. At N 64, by 17 May it's already 23 05, and then it's all "//" — continuous daylight. So the key idea for you as a pilot: this table lets you determine, for your latitude and date, when evening civil twilight ends. That matters for night flying rules, for planning when you need to have your navigation lights on, and for understanding how much daylight you have available for a flight. At high latitudes in summer you may have no true night at all — the "G" and "//" entries tell you that. One more thing to note: the times are given in hours and minutes, and they're local times for the latitude band. When you use this in practice, you'll interpolate between the latitude rows and between the date columns to get the exact time for your position and your date. That's the essence of this table — a latitude-and-date grid giving you the end of evening civil twilight, with "G" marking continuous twilight and "//" marking continuous daylight at the high latitudes.