The rows are latitudes, from 72° North all the way down to 30° South — Page 447, Lesson 418

I want to walk you through this table, because it's the kind of thing you'll actually use in flight planning. This is the Morning Civil Twilight table, and it's laid out for the months of April and May, with columns for each day — the 2nd, 5th, 8th, 11th, 14th, 17th, 20th, 23rd, 26th, and 29th of each month. The rows are latitudes, from 72° North all the way down to 30° South. The values in the table are in hours and minutes — that's the `h` and `m` at the top of each column. So when you read a value like `03 38`, that means 3 hours and 38 minutes. Now, what does this number actually represent? It's the duration of morning civil twilight — the time from when the sun is 6 degrees below the horizon until sunrise. That's the definition of civil twilight: the period when there's enough light for normal outdoor activities, and for our purposes, when you can still see the horizon clearly for visual navigation. Let me show you how to read it. Look at 60° North on April 2nd — the value is `04 41`, so 4 hours and 41 minutes of morning civil twilight. Now look at the same latitude on May 29th — it's `02 14`, so 2 hours and 14 minutes. You can see the duration is getting shorter as we move from April into May, because the sun is rising earlier and the nights are getting shorter in the Northern Hemisphere. Now here's the key pattern I want you to notice. Look at the high latitudes — 72° North. The values are very long, like `03 38` on April 2nd, and then they drop off rapidly. By April 26th, you see `01 04`, and then the table shows `//` — those slashes mean the data is not applicable. At those high latitudes, you get into the period of continuous daylight, where the sun never gets far enough below the horizon for civil twilight to occur. The sun stays within 6 degrees of the horizon all night, so there's no true darkness and no defined twilight period. Now compare that to the low latitudes. Look at 10° North — the values are remarkably consistent, around `05 36` to `05 16` across the whole period. And at the equator, 0°, the values are almost constant — about `05 40` to `05 39`. That's because at the equator, the sun rises and sets at nearly the same time all year, so the twilight duration barely changes. The seasonal variation is minimal. Now look at the Southern Hemisphere rows — 10° South, 20° South, 30° South. Here the pattern reverses. At 30° South, the value on April 2nd is `05 47`, and by May 29th it's `05 49` — it's actually getting slightly longer. That's because in the Southern Hemisphere, we're moving into autumn and winter, so the nights are getting longer and twilight lasts longer. So the big picture is this: latitude determines the seasonal variation in twilight duration. High latitudes have dramatic swings — long twilights in spring, then continuous daylight. Low latitudes have almost no variation. And the hemispheres are opposite — when the Northern Hemisphere's twilight is shortening, the Southern Hemisphere's is lengthening. This table is your reference for planning. If you're flying at dawn and you need to know how long you'll have usable light for visual reference, you look up your latitude, find the date, and read off the duration of morning civil twilight. It's a practical tool, and understanding how it varies with latitude and season is what makes you a professional navigator rather than someone who just reads numbers off a page.