Let me define the terms first, because everything on this page depends on them — Page 451, Lesson 432

I want to walk you through what this page actually is, because at first glance it looks like a wall of numbers. This is a Morning Civil Twilight table — a standard navigation reference table used in flight planning. Let me define the terms first, because everything on this page depends on them. Civil twilight is the period when the sun is between 0° and 6° below the horizon. During civil twilight there's enough natural light to see clearly — you can read, you can see the horizon, and you can generally conduct normal outdoor activities without artificial light. In aviation terms, this is the period when you can still see the horizon clearly, which matters for visual flight. Morning civil twilight is the period that begins at dawn — when the sun is 6° below the horizon — and ends at sunrise, when the sun reaches the horizon. So the table is telling you, for a given latitude and date, how long that morning twilight period lasts. Now let me show you how to read the table itself. The left-hand column is latitude, marked in degrees North — you'll see N 72, N 70, N 68, all the way down to N 10 and then 0, which is the Equator. The top row is the months — June, July, August — and then the days of the month running across: 1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31, then 3, 6, 9, 12, 15. Each cell in the table gives you the duration of morning civil twilight in hours and minutes — that's what the "h" and "m" column headers mean. Let me read you a few actual values so you can see the pattern. At latitude N 60, on June 1st, the duration is 00 hours 55 minutes. As the month progresses — June 4th, 7th, 10th — the values climb: 01 02, 01 09, 01 18. By late June it's over two hours. Then into July and August it keeps climbing — 02 25, 02 35, 02 45, 02 54, 03 03, 03 12, and by August 15th it's 03 hours 21 minutes. Now here's the critical thing I want you to notice — the G entries. Look at the high latitudes: N 72, N 70, N 68. Most of their cells are filled with the letter G. That G stands for "no twilight" — or more precisely, continuous daylight. At those high latitudes in summer, the sun never gets far enough below the horizon to produce a distinct twilight period. The sun stays within 6° of the horizon all night, so there's no defined morning civil twilight. The G is the table's way of saying "this doesn't apply here." And you'll also see the // symbols — those are dashes or placeholders. They appear at the high latitudes too, and they indicate that the value is not applicable or the data isn't given for that combination of latitude and date. At N 72, for instance, you see a long run of // across most of the summer. Let me trace one more latitude so you can see how the values behave. At N 50, June 1st gives 03 hours 08 minutes. It climbs steadily — 03 10, 03 13, 03 15, 03 19 — and by mid-June it's over three and a half hours. By late June it's approaching four hours: 03 58, 04 03, 04 08, 04 13. Now watch what happens as you go south. At N 20, June 1st is 04 hours 59 minutes — nearly five hours. At N 10, it's 05 hours 19 minutes. And at the Equator — latitude 0 — the value is 00 hours 37 minutes on June 1st. Wait — did you catch that? The Equator has a much shorter twilight than the higher latitudes. That's the key relationship this table demonstrates: the duration of morning civil twilight increases with latitude. The further you are from the Equator, the longer the twilight lasts. At the Equator, the sun crosses the horizon almost vertically, so it passes through the 6° twilight band very quickly. At high latitudes, the sun's path is much more oblique, so it takes far longer to climb through that same 6° band. And there's a second relationship: the duration increases as you move from June toward August — that is, as the northern summer progresses toward the autumn equinox. Look at N 60 again: 00 55 in early June, 03 21 by mid-August. The twilight lengthens through the season. Let me also point out the N 40 row, because it shows the pattern very cleanly. June 1st is 04 hours 00 minutes, and it climbs: 04 02, 04 03, 04 05, 04 07, 04 10, 04 12, 04 15, 04 17, 04 20, 04 23, 04 26, 04 29, 04 32, 04 36, 04 39, 04 42. By mid-August it's 04 hours 53 minutes. So to summarize what this table gives you as a pilot: for any latitude and any date in June, July, or August, you can look up how long morning civil twilight lasts. That tells you when dawn begins — when the sun is 6° below the horizon — and when sunrise occurs. That's directly relevant to flight planning, because it defines when you have sufficient daylight for visual operations, and it helps you compute times of sunrise and sunset for your route. One more thing to note — the values are given in hours and minutes, and you'll see the format is always two digits for hours, two for minutes, like 01 39 or 02 18. When you see a value like 01 10 01 at N 68, that's the table's way of laying out the numbers across the columns — the first two digits are the hours, the next two are the minutes. That's the whole page. It's a reference table, not a concept to memorize — the skill is knowing how to enter it with your latitude and date, and read out the twilight duration.