First, the title: Morning Civil Twilight — Page 453, Lesson 439

I want to walk you through the Morning Civil Twilight table. This is the kind of data you'll use in real flight planning, so let's make sure you understand exactly what you're looking at and how to read it. First, the title: Morning Civil Twilight. Civil twilight is the period when the sun is between 0° and 6° below the horizon. In the morning, it begins at sunrise and ends when the sun reaches 6° below the horizon. This table gives you the time of morning civil twilight for various latitudes and dates. Now, the structure. The columns are dates, running from August 15 through September, then October 2 through October 29. The rows are latitudes, from N 72 down through N 60, N 50, N 20, N 10, 0, S 10, S 30. So you have both northern and southern latitudes here. The values in the table are times, given in hours and minutes. For example, at N 60 on August 15, the time is 03 21 — that's 03 hours 21 minutes. These are the times of morning civil twilight at those latitudes on those dates. Let me point out something important. Look at the N 72 row. The first several entries are marked with // — double slashes. That means there is no morning civil twilight at that latitude on those dates. Why? Because at very high latitudes in late summer, the sun never gets low enough to produce civil twilight — it stays above 6° below the horizon all night. So the table uses // to indicate that condition. Now, watch how the times change as you move down the latitudes. At N 72, the times are early — around 01 07 to 05 14 — because at high latitudes, twilight comes very early in the morning. As you move south to N 60, the times shift later, around 03 21 to 05 25. At N 50, they're later still, around 04 13 to 05 29. And at N 20, they're around 05 17 to 05 29. Now look at the 0 row — the equator. The times are around 40 minutes — that's 00 40 — meaning morning civil twilight at the equator is very early, just after midnight. And as you go into the southern latitudes, S 10 gives times around 50 minutes, S 30 gives times around 06 09 down to 05 54. Here's the key pattern to notice. At northern latitudes, the times get later as you move from August into October — the days are getting shorter, so twilight comes later in the morning. But at southern latitudes, the times get earlier — because the southern hemisphere is moving into spring, so days are lengthening. Let me also point out the N 60 row in detail, because it shows the full progression. On August 15 it's 03 21, then 03 30, 03 38, 03 47, 03 55, 04 03 — and it keeps climbing through September and into October, reaching 05 25 by October 29. That's a clear demonstration of how the time shifts with the season. One more thing to note. The table has multiple sub-columns within each month. For example, under August you have 15, 18, 21, 24, 27, 30 — those are the specific dates. Then under September you have 2, 5, 8, 11, 14, 17, 20, 23, 26, 29. And under October you have 2, 5, 8, 11, 14, 17, 20, 23, 26, 29. So each column is a specific date, and each row is a specific latitude. So when you're planning a flight, you find your latitude, find the date, and read the time of morning civil twilight. That tells you when twilight begins at your position on that day. That's the Morning Civil Twilight table. Do you have any questions about how to read it, or would you like to move on?