First, the title: Evening Civil Twilight — Page 451, Lesson 433

This is a page from the Air Almanac — specifically, the Evening Civil Twilight table. Let me walk you through exactly what you're looking at, because this is a working navigation table, not a reading passage. First, the title: Evening Civil Twilight. Civil twilight is the period after sunset when the centre of the sun is between 0° and 6° below the horizon — that's the time when there's still enough light for most outdoor activities without artificial light. This table gives you, for a given latitude and date, the time of evening civil twilight — the moment when civil twilight ends, i.e., when the sun's centre reaches 6° below the horizon. Now, the structure. The table is organised by latitude down the left-hand side, and by date across the top. Look at the latitude column: you have N 72, N 70, N 68, N 66, N 64, N 62, then N 10, S 10, S 30, S 40, S 45, S 50, S 52, S 56, S 60. So it covers both northern and southern latitudes. The dates across the top run from June 28 through August 15 — so this particular page covers the summer months. The times are given in hours and minutes — you'll see columns headed h and m. For example, look at the row for N 10: the times run from 05 19, 05 20, and so on. That's 05 hours 19 minutes, 05 hours 20 minutes — these are the times of evening civil twilight in UTC, expressed as hours and minutes. Now, here's the critical part — the G entries. Look at the rows for N 72, N 70, N 68, N 66. You'll see a lot of G symbols. That G stands for continuous daylight — at these high latitudes in summer, the sun never gets 6° below the horizon, so there is no evening civil twilight at all. The sun stays above the twilight limit all night. So instead of a time, the table prints G to tell you: no civil twilight occurs on this date at this latitude. And you'll also see // symbols — those are dashes or slashes. These indicate that the value is not tabulated, or that the entry is not applicable for that particular latitude and date combination. For example, at N 66 you'll see a run of // before the times begin — meaning for those early dates, the data isn't given, often because the condition is marginal or the twilight is continuous. Now let me show you how the times behave. Look at the N 10 row: 05 19, 05 20, 05 21, 05 22, 05 23, 05 24, 05 24, 05 25, 05 26, 05 27, 05 27, 05 28, 05 28, 05 28, 05 29, 05 29, then 05 30. So the time of evening civil twilight is getting later as the days progress through June into July — the days are lengthening, so twilight ends later. Then look further down the row: 05 54, 05 54, 05 55, 05 55, 05 55, 05 56, 05 56, 05 56, 05 55, 05 55, 05 55, 05 54, 05 53, 05 53, 05 52, 05 51, 05 50. Now the times are decreasing — after mid-July, the days start shortening, so twilight ends earlier. That's the seasonal trend you'll see across the table. Now look at the S 10 row: 06 11, 06 11, 06 12, 06 12, 06 12, 06 12, 06 11, 06 11, 06 10, 06 09, 06 08, 06 07, 06 06, 06 04, 06 03, 06 01, 05 59. Notice the times are earlier here — because it's winter in the southern hemisphere during June and July, so the days are short and twilight ends earlier. And look at the S 60 row: 08 08, 08 07, 08 06, 08 04, 08 01, 07 58, 07 54, 07 50, 07 45, 07 41, 07 35, 07 29, 07 23, 07 17, 07 10, 07 03, 06 56. The times are getting earlier as you go through the dates — again, southern winter, days shortening. So the key relationships here: latitude determines whether twilight occurs at all (the G entries at high northern latitudes), and date determines whether the twilight time is increasing or decreasing through the season. The times are in UTC, in hours and minutes, and the table is read by finding your latitude row and your date column, then reading the intersection. One more thing to note — the times are given to the minute, and you'll see the values change by one or two minutes between adjacent dates. That's the daily rate of change of the twilight time, which is useful for interpolation if you need a date that falls between the tabulated dates. That's the whole structure of this page. It's a straightforward look-up table, but the G and // entries are the ones you need to interpret carefully — G means continuous daylight, no twilight, and // means the value isn't tabulated for that combination.