Let me first tell you exactly what we're looking at — Page 449, Lesson 424

I want to walk you through this table, because it's one of the most practical tools you'll use in real flight planning. This is the Morning Civil Twilight table, and it's part of the time section of your General Navigation syllabus. Let me first tell you exactly what we're looking at. The table gives you the duration of morning civil twilight in hours and minutes, for any latitude from 72° North down to 20° South, across the months of May, June, and July. Now, what is civil twilight? It's the period when the sun is between 0° and 6° below the horizon. In practical terms, it's the time when there's enough light to see the horizon clearly and conduct normal ground operations, but the sun hasn't actually risen yet. For a pilot, this matters because it defines when you can legally and practically operate without artificial lighting, and it's a factor in your pre-flight planning. Let me show you how to read this table, because the layout is a bit unusual at first. The left-hand column is your latitude, running from 72° North at the top, down through 60° North, 50° North, 40° North, 30° North, 20° North, 10° North, then the Equator marked as 0, and continuing into the Southern Hemisphere at 10° South and 20° South. Across the top, you have the dates, running from May 17th, through May 20th, 23rd, 26th, 29th, then June 1st, 4th, 7th, 10th, 13th, 16th, 19th, 22nd, 25th, 28th, and finally July 1st. Each date column gives you the duration of morning civil twilight in hours and minutes. Now, here's the critical part — the G symbols. When you see a G in the table, it stands for continuous daylight. At high latitudes in summer, the sun never actually sets, so there is no twilight at all. You'll see this at 72° North across the entire period, and at 70° North for most of it. At 68° North, you start to see the G disappear in the later dates, and at 66° North, the G only appears in the middle of June. That's the effect of the Earth's axial tilt — the higher your latitude, the longer the period of continuous daylight around the summer solstice. The // symbols mean the data is not applicable or not given for that latitude and date. You'll see these at the higher latitudes where the twilight duration is either continuous daylight or the table simply doesn't provide a value. Now let's look at the actual numbers. At 60° North, on May 17th, morning civil twilight lasts 2 hours 14 minutes. As you move through the dates, it gradually decreases — by June 1st it's down to 1 hour 27 minutes, and it reaches its minimum around the solstice in late June at about 49 minutes. Then it starts increasing again. At 50° North, the values are longer — 3 hours 32 minutes on May 17th, decreasing to about 3 hours 6 minutes around the solstice. At 40° North, you're looking at 4 hours 13 minutes on May 17th, dropping to about 3 hours 58 minutes in mid-June. At 30° North, it's 4 hours 39 minutes early on, settling around 4 hours 31 minutes at the solstice. At 20° North, you get 5 hours 00 minutes on May 17th, and it stays fairly steady around 4 hours 55 minutes through June. At 10° North, it's about 5 hours 16 minutes throughout. At the Equator (0°), the duration is remarkably constant — 31 to 38 minutes across the entire period. That's because at the Equator, the sun rises and sets at nearly the same rate all year round. Now here's the fascinating part — as you cross into the Southern Hemisphere, the values start to increase again. At 10° South, morning civil twilight is about 5 hours 44 minutes on May 17th, and it grows through the period. At 20° South, it's 5 hours 58 minutes on May 17th, rising to 6 hours 2 minutes by early July. So the pattern is clear: twilight duration is longest at high latitudes and shortest near the Equator, and it varies with the season because of the sun's declination. In the Northern Hemisphere summer, the high northern latitudes get continuous daylight, while the Southern Hemisphere, which is in winter, gets longer twilight periods. For your flight planning, this table is used to determine when civil daylight begins at your departure or destination. You take the duration from this table and apply it to the time of sunrise, and that gives you the start of morning civil twilight. It's a standard reference you'll use in navigation calculations, and knowing how to extract the correct value for your latitude and date is a skill you'll be tested on. That's the complete picture of this table — how to read the latitude and date axes, what the G and // symbols mean, and how the values change with latitude and season.