First, let's be clear about what civil twilight actually is — Page 447, Lesson 419

I want to walk you through the Evening Civil Twilight table. This is one of the tables you'll find in the almanac section of your navigation materials, and it's used to work out when civil twilight ends in the evening at a given latitude on a given date. First, let's be clear about what civil twilight actually is. Civil twilight is the period after sunset when the centre of the sun is between 0° and 6° below the horizon. In practical terms, it's the time when there's still enough natural light for most outdoor activities without artificial lighting. The table you're looking at gives you the time of the end of evening civil twilight — that is, the moment when the sun's centre reaches 6° below the horizon and civil twilight gives way to nautical twilight. Now, let's look at the structure of the table. The columns are arranged by date, and the rows are arranged by latitude. The latitude values run down the left-hand side, from N 72° at the top down to N 54° at the bottom. The dates run across the top, starting with April and then May, and within each month the days are listed in three-day intervals — 2, 5, 8, 11, 14, 17, 20, 23, 26, 29, and then 2, 5, 8, 11, 14, 17 for May. The times themselves are given in hours and minutes — you'll see the columns headed h and m. So, for example, at latitude N 60°, on April 2nd, the time of the end of evening civil twilight is 19 hours 29 minutes. On April 5th at the same latitude, it's 19 hours 36 minutes. You can see the times getting later as the days progress through April and into May, because the days are lengthening. Now, there's something very important in this table that you need to understand — the symbols. You'll see a series of slashes — // — appearing in the higher latitudes. Look at the N 72° row, for example. The times run from April 2nd through to about April 20th, and then you get a run of slashes. Those slashes mean that at that latitude, on those dates, evening civil twilight does not end — the sun never gets 6° below the horizon. This is the phenomenon of continuous daylight, or what we call the midnight sun effect. At very high latitudes in late spring, the sun stays close enough to the horizon that it never reaches 6° below it, so civil twilight lasts all night long. You'll also see the letter G appearing in the table, in the N 72° and N 70° rows. The G is a notation used in these almanac tables to indicate that the time given is for a specific reference — in this context, it marks entries where the value is given for Greenwich, meaning the time is quoted for the Greenwich meridian. When you're using this table for navigation, you need to be aware of which entries carry that G notation, because it affects how you apply the time to your own longitude. Let me show you how the times progress with latitude. Take April 2nd. At N 60°, the time is 19 hours 29 minutes. At N 58°, it's 19 hours 23 minutes. At N 56°, it's 19 hours 17 minutes. At N 54°, it's 19 hours 13 minutes. So you can see that as you go to lower latitudes, the end of evening civil twilight comes earlier. That makes sense — closer to the equator, the sun sets more steeply and gets below 6° more quickly. Now, let me point out something about how the table is laid out. You'll notice that the latitude rows are not all at the same spacing. The table gives values at 2° intervals — 72, 70, 68, 66, 64, 62, 60, 58, 56, 54 — but the rows for the lower latitudes are grouped together in a block, while the higher latitudes are spread out. This is just a formatting choice in the almanac to fit the data on the page, but the principle is the same: you read across the row for your latitude and down the column for your date, and where they intersect, that's your time. Let me also draw your attention to the fact that the times are given in whole minutes, and they're quoted in local mean time. When you're using this in flight planning, you'd take the time from the table for your latitude and date, and then you'd apply your longitude correction to convert from the meridian the table is referenced to, to your own position. One more thing to notice — the way the times increase as you go north. Look at May 2nd. At N 60°, it's 20 hours 53 minutes. At N 62°, it's 21 hours 12 minutes. At N 64°, it's 21 hours 35 minutes. At N 66°, it's 22 hours 07 minutes. And at N 68°, it's 23 hours 05 minutes. You can see the times getting dramatically later as you go north, and then at N 70° and N 72° you start hitting those slashes where twilight never ends. So, to summarise what you need to take away from this table: it gives you the time of the end of evening civil twilight for a given latitude and date, in hours and minutes of local mean time. The slashes indicate no end of twilight — continuous daylight. The G notation marks entries referenced to Greenwich. And the times get later with increasing latitude and with progression through the spring months. That's the Evening Civil Twilight table. When you're ready, we can move on to how this feeds into your overall twilight and daylight calculations for flight planning.