Now, what exactly is civil twilight — Page 457, Lesson 452

This is the start of a new topic in your navigation studies — Time, Part 3 — and what I want to walk you through first is the Morning Civil Twilight table. This is a real working table you'll use in flight planning, so let's make sure you can read it properly. The table gives you the time of morning civil twilight for a range of latitudes, from N72 down through the equator at 0, and on to S20. The columns run across the months from November through December and into January — you'll see the day numbers along the top: 19, 22, 25, 28, 1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31, 3. So this table spans the end of November, all of December, and the first few days of January. Now, what exactly is civil twilight? It's the period when the sun is between 0° and 6° below the horizon — that's the definition you need to hold onto. Morning civil twilight begins at the moment the sun's centre is 6° below the horizon and ends at sunrise, when the sun's centre crosses the horizon. During civil twilight there's enough light for normal outdoor activities without artificial lighting — that's why it matters for flight planning, because it defines when you can operate without needing lights, and it's a legal limit for certain operations. Let me show you how to read the table. Take the row for N60 — that's 60° North. Look across to the column for, say, December 1. The time given is 07 45 — that's 07 hours 45 minutes. That's the local time of morning civil twilight at that latitude on that date. So at 60° North on the 1st of December, morning civil twilight begins at 07:45. Now watch what happens as you move north. Look at N72 — the top row. On the 1st of December the time is 10 02. That's much later in the morning than at N60. And look at the pattern across the month: at N72 the times get later and later through December — 10 15, 10 27, 10 38, 10 48, 10 54, 10 57 — peaking around the 28th of December at 10 57, then starting to come back down in January: 10 57, 10 53, 10 46, 10 38. That's the seasonal swing — the latest morning twilight occurs around the winter solstice, and the table captures that beautifully. Compare that with the equator. Look at the 0 row — the equator. The times barely change at all. On the 19th of November it's 05 20, and through the whole table it stays around 05 20 to 05 38. At the equator there's almost no seasonal variation in twilight times — the sun rises and sets at nearly the same time all year. That's a key contrast to hold onto: the higher your latitude, the more the twilight times swing through the year; at the equator, they're essentially constant. Now look at the southern hemisphere rows — S10 and S20. At S10, the times are around 05 05 to 05 20 — early morning. And at S20, look at the values: 04 48, 04 48, 04 48, 04 47 — they're even earlier, and they're almost flat across the whole period. So in the southern hemisphere in December, which is their summer, morning twilight comes very early — before 5 a.m. — and stays early because the days are long. Let me also point out the intermediate rows so you can see the progression. At N50, the 1st of December gives 07 02. At N40, it's 06 47. At N30, 06 14. At N20, 05 56. At N10, 05 40. So as you step down in latitude from N72 to the equator, the morning twilight time gets progressively earlier — from 10:02 at N72 down to about 05:20 at the equator. That's the latitude effect: higher latitude, later morning twilight in winter. One more thing to notice — the table is laid out with latitude in the left column and the dates across the top, and each cell gives hours and minutes. When you use this in practice, you'll interpolate between latitudes and between dates to get the exact time for your position and your day of operation. That's the skill — reading the table is the first step, and interpolating within it is what you'll do in the exam and in real flight planning. So to summarise what we've got: this is the Morning Civil Twilight table, covering N72 to S20, from 19 November to 3 January. Civil twilight is the sun between 0° and 6° below the horizon, and morning civil twilight is when that period begins. The times are local times of that beginning. High northern latitudes in December get very late morning twilight — past 10 a.m. at N72 — while the equator stays near 05:20 all year, and southern latitudes in their summer get twilight before 5 a.m. That's the whole picture of this table.