Let me show you how to read it — Page 449, Lesson 425

I want to walk you through what we're looking at here, because this is one of the most practical tables in the whole of General Navigation — the Evening Civil Twilight table. Now, before we touch the numbers, let's be absolutely clear on what civil twilight actually is. Civil twilight is the period of time after the sun has set, or before it rises, when the centre of the sun's disc is between 0° and 6° below the true horizon. In plain terms, it's that window when there's still enough natural light to carry out ordinary outdoor activities without artificial lighting — for a pilot, it's the period when the horizon remains clearly visible, which matters enormously for visual reference and for the legal definition of night flying. This particular table gives you the time of evening civil twilight — that is, the clock time at which civil twilight ends and true darkness begins — for a range of latitudes and dates. Let me show you how to read it. Look at the structure. The columns run across the top for the months May, June, and July, and within each month there's a row of dates — 17, 20, 23, 26, 29, then 1, 4, 7, 10, 13, 16, 19, 22, 25, 28, and then 1 again. So you've got a sweep of dates from mid-May through to the start of August. The rows down the left-hand side are latitude, in degrees North, running from N 72 down through 70, 68, 66, 64, 62, down to N 60 and below. The entries in the body of the table are times in hours and minutes — you'll see values like 23 05, 22 12, 21 41, and so on. That's the local time at which evening civil twilight ends for that latitude on that date. Now, here's the really interesting part, and it's where the table starts to tell you something important about high-latitude flying. Look at the top rows. For latitudes N 72 and N 70, the entire row is filled with the letter G. That G stands for the fact that at those latitudes, throughout this whole period, the sun never gets far enough below the horizon for civil twilight to end — you have continuous twilight, or effectively continuous daylight conditions, all through the night. There is no time of evening civil twilight to record, because darkness never truly falls. Then look at N 68 and N 66. You'll see a mixture of G entries and entries marked with //. The double slash indicates a period where the data is not applicable or not tabulated — in practice, at these latitudes you're transitioning between continuous twilight and a genuine end of twilight, and the table simply doesn't give you a time for those dates. Now drop down to N 64. You'll see a single entry of 23 05, and then a long run of // marks. And at N 62, you start to see actual times appearing — 22 12, 22 26, 22 41, 22 58, 23 20 — and then more // entries. By the time you get down to N 60 and below, you have a full set of times for every date. Look at the N 60 row: 21 41, 21 51, 22 01, 22 11, 22 21, 22 31, 22 40, 22 49, 22 57, 23 04, 23 10, 23 13, 23 14, 23 13, 23 09, 23 03. Notice how the times climb to a maximum around mid-June — that's the summer solstice, when the nights are shortest — and then start to come back down again as July progresses. The same pattern repeats in the rows below, at 58, 56, 54, and so on, with the times getting earlier as you go south, because the nights are longer at lower latitudes. So the key lesson here, and the thing I want you to take away: this table is your tool for determining, at a glance, when civil twilight ends at a given latitude on a given date in the northern summer. And the G and // symbols are not errors — they're deliberate annotations telling you that at high latitudes, either twilight never ends, or the data isn't tabulated for that combination. When you're planning a flight that will operate around sunset, this table tells you whether you'll still have daylight reference, or whether you're legally and practically into night conditions.