We’re looking at the Morning Civil Twilight table now — Page 449, Lesson 424

We’re looking at the Morning Civil Twilight table now. This is the kind of data you’ll use to work out when civil twilight begins at a given latitude on a given date — and it’s laid out as a grid, so let me walk you through how to read it. The table is organised by latitude down the left-hand side, from N 72° at the top, all the way down through the equator at 0°, and then into the southern hemisphere to S 20° at the bottom. Across the top, the columns are dates, running from May 17th, through June, into July 1st. So each row is a latitude, each column is a date, and the number sitting at the intersection is the time of morning civil twilight, expressed in hours and minutes. Now, before we read any numbers, I need to explain the two symbols you’ll see scattered through the table. The first is “G”. Wherever you see a G, it means there is no morning civil twilight at all on that date at that latitude. The sun never gets far enough below the horizon for civil twilight to begin — it’s effectively daylight all through the night, or the twilight condition simply doesn’t occur. The second symbol is “//”, a double slash. That marks a period where the data isn’t given — the table simply doesn’t cover that combination of latitude and date, so you treat it as a blank, not as a time. Let me give you a concrete example so you can see how the grid works. Take latitude N 60°. On May 17th, the table gives 02 14 — that’s 2 hours 14 minutes. So morning civil twilight begins at 0214 local time on that date at N 60°. Now follow that same row across the dates. On May 20th it’s 02 05, on May 23rd it’s 01 55, and it keeps getting earlier as you move through June — down to 00 49 around mid-June, then it starts climbing back up again. Notice the pattern: the times get smaller towards the summer solstice, then increase again afterwards. That’s the seasonal swing in twilight times. Now look at what happens as you move north. At N 64°, the row starts with 00 57 on May 17th and 00 23 on May 20th — very short twilight durations — and then it drops into “//” for most of June and July, before returning to numbers later. And at N 72°, the whole row is G. That’s the key contrast: the further north you go in summer, the less morning civil twilight there is, until at the highest latitudes it vanishes entirely — that’s your G. Let me also show you the southern hemisphere rows, because they behave the opposite way. At S 10°, on May 17th the time is 05 16, and it gets later through June — 05 15, 05 17, 05 20 — because in the southern winter the twilight is lengthening. At S 20° you see 05 58, 05 59, 06 01, 06 02 — the times are climbing. So the table is symmetric in logic but inverted in season: northern summer shortens twilight, southern winter lengthens it. One more thing to note about the units. Every entry is in hours and minutes — the first two digits are hours, the next two are minutes. So 02 14 is 2 hours 14 minutes, 05 58 is 5 hours 58 minutes. There are no seconds in this table, and no time-zone correction built in — these are local times for the given latitude and date. So when you use this table operationally, you pick your latitude, pick your date, and read off the time. If you hit a G, there’s no morning civil twilight that day. If you hit a “//”, the table doesn’t give you that value. That’s the whole structure of the Morning Civil Twilight table.