Let me set the scene — Page 453, Lesson 440

I want to walk you through what this page actually is, because at first glance it looks like a wall of numbers. This is a twilight table — specifically, the Evening Civil Twilight table from the almanac section of your General Navigation manual. It's the kind of table you'll use in flight planning to know when civil twilight ends at a given latitude on a given date. Let me set the scene. The table is organized by latitude down the left-hand side, running from N 72 degrees down to N 52 degrees — that's 72° North down to 52° North. And across the top, the columns are dates, running from August through September and into October. The dates are broken into three-day intervals: 15, 18, 21, 24, 27, 30 for August, then 2, 5, 8, 11, 14, 17, 20, 23, 26, 29 for September, and then 2 for October. Now, what is civil twilight? It's the period after sunset when the sun is between 0° and 6° below the horizon — the time when there's still enough natural light for most outdoor activities without artificial lighting. The evening civil twilight is the end of that period, the moment when civil twilight ends and it becomes too dark. That's the time this table gives you. The numbers in the body of the table are local mean times, expressed in hours and minutes — you'll see values like 22 45, 21 44, 20 19, and so on. So for a given latitude and date, you read across and down to find the time at which evening civil twilight ends. Let me walk you through a few examples so you can see the pattern. Take N 72 — the highest latitude on the table. For August 15, the table shows 22 45 — that's 22:45, or 10:45 PM. As you move through the dates, the times get earlier: August 18 shows 22 11, August 21 shows 21 44, and by September 2 it's 20 19. Notice the trend — as autumn approaches, twilight ends earlier each day. By October 2, at N 72, the time has come down to 18 23 — that's 6:23 PM. Now compare that with N 52, the lowest latitude on the table. For August 15, the time is 20 46 — that's 8:46 PM. And for October 2, it's 18 13 — 6:13 PM. So you can see the latitude effect: at higher latitudes, twilight lasts longer in summer, and the seasonal change is more dramatic. At N 72, the change from August to October is over four hours; at N 52, it's about two and a half hours. Let me also point out the S 30, S 52, and S 60 entries you'll see scattered in the table. Those are southern latitudes — 30° South, 52° South, and 60° South. The table includes them so you can look up twilight times for southern-hemisphere operations too. For example, at S 30, the times run from 06 09 down through 05 57, 05 44, and so on — those are evening times in the southern winter, which corresponds to August and September. And at S 60, you'll see times like 06 56, 06 48, 06 41 — again, evening civil twilight times for those southern latitudes. Now, one thing I want you to notice: the table uses // symbols for some entries at the highest latitudes. Look at N 72 for August 15, 18, 21, 24, 27, and 30 — those are all marked //. That's the table's way of telling you that at those latitudes and dates, there is no evening civil twilight — the sun never gets far enough below the horizon for civil twilight to end. This happens in high latitudes during summer, when you get the phenomenon of continuous daylight or very long twilight. The // is a flag that says "this entry doesn't apply — there's no end to civil twilight on this date at this latitude." So the practical use of this table in flight planning: you pick your latitude, you pick your date, and you read off the local mean time when evening civil twilight ends. That tells you when natural light conditions change — which matters for things like planning your arrival, your fuel, and your night-flying considerations. Let me give you one more read-through of the structure so it's locked in. Left column: latitude, from N 72 down to N 52, plus the southern entries S 30, S 52, S 60. Top row: dates from August 15 through October 2. Body: local mean times in hours and minutes. And the // marks at high latitudes for summer dates, meaning no evening civil twilight exists on those days. That's the whole table. It's a reference tool — you don't compute anything, you just look it up. The skill is knowing how to enter it: latitude on the left, date across the top, and read the time where they intersect.