First, the title: this is the table for morning civil twilight — Page 444, Lesson 412

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: this is the table for morning civil twilight. Civil twilight is the period when the sun is between 0° and 6° below the horizon — enough light for most outdoor activities, and for aviation it’s the time when the horizon is still clearly visible. The table gives you the local mean time of the start of morning civil twilight, in hours and minutes, for a range of latitudes and dates. Look at the layout. The top row is the Latitude in degrees, running from N 72 down through N 60, N 50, N 20, N 10, 0, S 10, S 20, S 30, S 35, S 40, S 45, S 46. So the table covers both northern and southern latitudes, and the equator is the 0 row. The columns are dates, starting with February — the 16th, 19th, 22nd, 25th, 28th — then March — the 3rd, 6th, 9th, 12th, 15th, 18th, 21st, 24th, 27th, 30th — and then April — the 2nd. So this is a seasonal table, covering late winter into early spring. Each cell gives the time in hours and minutes — the format is hours, then minutes, so a value like 07 32 means 07 hours 32 minutes. That’s the local mean time of the start of morning civil twilight for that latitude on that date. Let me walk you through a couple of examples so you can see the pattern. Take the N 72 row, the highest northern latitude in the table. On February 16th, the time is 07 32 — that’s 7:32 in the morning. Now watch what happens as the dates progress: February 19th is 07 18, February 22nd is 07 04, February 25th is 06 50, February 28th is 06 35. The times are getting earlier. By March 3rd it’s 06 20, and it keeps decreasing through March and into April — April 2nd is 03 38. So at high northern latitudes, morning civil twilight starts progressively earlier as spring approaches. That makes sense — the days are lengthening. Now compare that with the N 60 row. February 16th is 06 53, and April 2nd is 04 41. Earlier than N 72 on the same dates, but the same trend — times decreasing through the season. Now look at the N 50 row: February 16th is 06 36, April 2nd is 05 03. And the N 20 row: February 16th is 06 06, April 2nd is 05 31. Notice how the change across the season is much smaller at lower latitudes. At N 20, the difference from February to April is only about half an hour, whereas at N 72 it’s nearly four hours. That’s the latitude effect — the higher your latitude, the more the twilight times shift with the seasons. Now the equator, the 0 row. February 16th is 05 50, and April 2nd is 05 36. Very little change — the equator has nearly constant day length year-round, so twilight times barely move. Now go to the southern latitudes. The S 10 row: February 16th is 05 40, and April 2nd is 05 43. Notice the times are increasing slightly — they get later as the season progresses. That’s because in the southern hemisphere, this period is moving toward autumn and shorter days. The S 20 row: February 16th is 05 29, April 2nd is 05 45. The S 30 row: February 16th is 05 16, April 2nd is 05 47. And the S 46 row, the highest southern latitude in the table: February 16th is 04 51, April 2nd is 05 38. So in the south, the trend is reversed — twilight starts later as spring approaches in the north. There’s also a figure that goes with this — — which shows the sunrise and sunset geometry. That’s the visual companion to this table, showing how the sun’s position relative to the horizon defines these twilight periods. So the key takeaways: this table gives you the local mean time of the start of morning civil twilight for a grid of latitudes and dates. Northern latitudes show times decreasing through the season; southern latitudes show times increasing. The equator is nearly constant. And the higher the latitude, the greater the seasonal change. That’s the core of reading this table — you’ll use it to know when civil twilight begins at your position, which matters for planning operations that depend on daylight.