First, the title: Morning Civil Twilight — Page 440, Lesson 405

This is the start of the "Time (3)" chapter, and what we have in front of us is the Morning Civil Twilight table. This is a standard navigation data table you'll use in flight planning, so let's make sure you can read it properly. First, the title: Morning Civil Twilight. Twilight is the period when the sun is below the horizon but its light still illuminates the sky. Civil twilight specifically is the period when the sun's centre is between 0° and 6° below the horizon — bright enough for normal outdoor activities and, importantly for us, for visual navigation. The "morning" version is the one that ends at sunrise. Now, the table itself. Look at the columns. The left-hand column lists latitudes, from N72 down to 20 — that's 72° North down to 20° North. The top row lists the months, from December through January and February. So this table gives you the time of morning civil twilight for a range of northern latitudes across the winter months. The numbers in the body of the table are in hours and minutes — the h and m headings at the top tell you that. Each entry is the local time at which morning civil twilight begins for that latitude on that date. Let me show you how to read one entry. Take the top row, N72. The first entry under December is 10 49 — that means 10 hours 49 minutes. So at 72° North in early December, morning civil twilight begins at 10:49 local time. As you move across the row into January and February, the times get earlier — 10:40, 10:31, and so on — because the days are lengthening. Now watch what happens as you move down the latitude column. At N60, the December entry is 08 06 — 8 hours 6 minutes. At N50, it's 07 20. At N40, it's 06 51. At N30, it's 06 29 — wait, let me check that. At N30 the entry is 29 30 — no, that's not right. Let me re-read. At N30, the entry reads 29 30 — hmm, that doesn't fit the pattern. Let me look again. Actually, at N30 the entry is 06 29 — no. Let me be careful. The row for 30 shows 29 30 30 31 31 31 31 30 29 28 27 25 24 22 20 17 15 — those are minutes only, with the hour implied. So at N30, the time is 06 hours and 29 minutes in early December, then 06:30, 06:30, 06:31, and so on. Here's the key point: the lower the latitude, the earlier the twilight time. At N72 it's nearly 11 in the morning; at N20 it's around 5 in the morning. That makes sense — closer to the equator, the sun rises earlier, so civil twilight begins earlier. And notice the bottom row, latitude 0 — the equator. The entries there are 36 38 39 40 42 43 44 05 45 05 46 05 47 05 48 05 48 05 49 05 49 05 49 05 50 05 50 — so around 5:36 to 5:50 in the morning. At the equator, twilight times are nearly constant throughout the year because day length doesn't vary much there. One more thing to notice: the table is not perfectly smooth. Look at the N50 row — it reads 07 20 07 20 07 20 07 19 07 18 07 17 07 15 07 12 07 10 07 07 07 03 06 59 06 55 06 51 06 46 06 41 06 36. The times decrease steadily but not by the same amount each day — some days drop by 2 minutes, some by 3, some by 4. That's because the rate of change of day length varies through the season. So in practice, when you're planning a flight, you'd find your latitude on the left, your date across the top, and read off the time of morning civil twilight. That tells you when you'll have enough daylight for visual navigation. Now, there's a figure that goes with this — — which shows the geometry of True North and longitude. That's the underlying concept: the table is built on the relationship between your longitude, your latitude, and the sun's position relative to the Earth's rotation. That's the Morning Civil Twilight table. When you're ready, we can move on to the next part of the chapter.