The table does the work for you — Page 440, Lesson 402

This is the start of a new topic — the sunset tables in the General Navigation manual. Let me walk you through what you're actually looking at, because these tables are a classic exam trap if you don't read them properly. What you have here is a sunset table — a pre-computed listing that gives you the local mean time of sunset for a range of latitudes and dates. The whole point of this table is that you, as a navigator, can look up when the sun goes down without doing any spherical trigonometry. The table does the work for you. Let me decode the layout, because that's where most of the confusion happens. First, the latitude rows. Look at the left-hand column. You'll see entries like N72, N70, N68, N66, N64, N62 — those are north latitudes in degrees. Then further down you'll see S10, S30, S52, S60 — those are south latitudes. So the table covers both hemispheres. The N stands for north, the S stands for south, and the number is the latitude in degrees. Now the date columns. Across the top you'll see the months — December, January, February — and then a row of day numbers: 30, 2, 5, 8, 11, 14, 17, 20, 23, 26, 29, 1, 4, 7, 10, 13, 16. So the columns are individual dates. December 30th, then January 2nd, 5th, 8th, and so on through February. Each column gives you the sunset time for that specific date at that specific latitude. Now here's the critical part — the time format. Look at the numbers in the body of the table. You'll see things like 12 36, 13 27, 13 57, 14 22. These are hours and minutes — local mean time. So 12 36 means 12 hours 36 minutes. 13 27 means 13 hours 27 minutes, which is 1:27 PM in civilian time. The table gives you sunset in local mean time, not UTC, not zone time — local mean time. That's a distinction you need to hold onto. Now, look at the N72 row — the highest latitude shown here. You'll see a row of symbols — those are the degree symbol, the little circle. What that tells you is that at 72 degrees north, on those particular dates, there is no sunset — the sun never goes below the horizon. That's the midnight sun phenomenon. At very high latitudes in summer, the sun stays up around the clock, so the table can't give you a sunset time — there isn't one. The symbol marks those cells as "no sunset." Now look at the N70 row — 12 30, 13 14, 13 41, 14 03, 14 22, 14 40, 14 58, 15 14, then 29, 15 44, 15 59. Notice how the times increase as you move from December into January and February. That's the key trend — as winter progresses toward spring, the days get longer, so sunset gets later. The sun sets later and later each day. Now compare N70 with N62 — look at the N62 row: 14 41, and then the numbers climb. At N62, the sunset times are later than at N70 for the same date. That's the latitude effect — the further south you go, the later the sunset is in winter. At higher latitudes in winter, the sun sets earlier because the days are shorter. So the table captures both the date trend and the latitude trend. Now let me show you the south latitude rows, because they work the opposite way. Look at S10, S30, S52, S60. At S60 — 02 40, 02 44, 02 49, 02 55, 03 01, 03 08 — those times are increasing as you go from December into February. But wait — in the southern hemisphere, December is summer. So at S60, the days are getting shorter as you move toward February, which is autumn down there. Yet the times are increasing. Let me make sure you see this correctly. Actually, look more carefully at the S60 row: 02 40, 02 44, 02 49, 02 55, 03 01, 03 08, 03 15, 03 23, 03 31, 03 39, 03 47, 03 55, 04 03, 04 12, 04 20, 04 28, 04 36. The times are getting later — sunset is happening later in the day as February approaches. In the southern summer, the sun sets later as you move through the season. So the trend is the same direction — later sunsets — but the seasonal context is opposite. Now here's the critical reading skill — interpolation. The table gives you sunset times at specific latitudes — N72, N70, N68, N66, N64, N62 — and at specific dates. But your actual position won't be exactly on one of those latitudes, and your actual date won't be exactly on one of those dates. So you have to interpolate — estimate the value between the tabulated entries. If you're at N63, you look between the N64 and N62 rows. If your date is January 3rd, you look between the January 2nd and January 5th columns. The table gives you the framework; you do the fine adjustment. Let me also point out the time progression within a row — look at the N66 row: 13 34, 13 41, 13 50, 14 00, 14 10, 14 21, 14 32, 14 44, 14 56, 15 07, 15 19, then 30, 42, 15 53, 16 04, then 15, 26. Notice the increments — they're not uniform. Early in the season the sunsets change by a few minutes per day; later they change by larger amounts. That's because the rate of change of day length isn't constant through the year. The table preserves those real, irregular increments — you don't smooth them out. One more thing — the units. Everything in the body is hours and minutes of local mean time. There's no seconds column, no AM/PM — it's a 24-hour clock. 16 14 means 16 hours 14 minutes, which is 4:14 PM. You need to be comfortable converting between the 24-hour format the table uses and the 12-hour format you might use in conversation. So to summarise what you've got: a sunset table covering north and south latitudes from N72 down to S60, for dates from December 30th through mid-February, giving local mean time of sunset in hours and minutes. The symbol cells at high north latitude mean no sunset — the sun never sets. The times increase as the season progresses, and they vary with latitude. And when your position or date falls between the tabulated values, you interpolate. That's the structure. When you're ready, we can move on to how you actually use this in a navigation problem — how you combine the sunset time with your longitude to get a UTC time, and why that matters for your flight planning.