The latitude rows — Page 440, Lesson 402

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 sunset table — a standard page from the Air Almanac style of data that you'll use in General Navigation. It gives you, for a range of latitudes and for each day of the year, the local mean time of sunset at Greenwich meridian. Let me orient you on the structure, because once you see the layout, the numbers stop being noise. The latitude rows. Down the left-hand side you'll see latitude values: N72, N70, N68, N66, N64, N62, and so on down through N10, then the equator at 0, then S10, S30, S52, S60. So the table covers from 72° North down to 60° South. Each row is a band of latitude, and the times in that row apply to that latitude. The date columns. Across the top you have the months — January, February, and so on — and under each month a set of day numbers: 30, 2, 5, 8, 11, 14, 17, 20, 23, 26, 29, 1, 4, 7, 10, 13, 16. These are the dates you read the table against. The header also carries the label SUNSET and the word Dec. — that's the declination of the sun, which is the underlying parameter the table is built around. The time values. Each cell contains a time in hours and minutes — you'll see pairs like 12 36, 13 27, 14 22, and so on. Those are the local mean times of sunset for that latitude and that date. Notice the pattern: as you go from January toward mid-year, the times get later — sunset gets later in the evening — and then they come back earlier. That's the seasonal swing you'd expect. The special marker. You'll see a symbol — a circle, a degree sign — appearing in the high-latitude rows, N72, N70, N68. Where that symbol appears, the table is telling you something important: at those latitudes and those dates, the sun does not set — it's the period of continuous daylight, the midnight sun. So instead of a time, you get that marker, meaning "no sunset occurs." Now, the key professional point I want you to take away: this is a lookup table for planning. In navigation, you use sunset and sunrise times to plan things like night operations, fuel planning for flights that will continue after dark, and crew duty considerations. You enter the table with your latitude and the date, and you read out the local mean time of sunset. That time is referenced to the Greenwich meridian — the table is built for the meridian of Greenwich — so when you're operating at a different longitude, you'd apply your longitude correction to convert that Greenwich mean time into your own local mean time. Let me also point out the declination column again, because it's the engine behind the whole table. The sun's declination — its angular distance north or south of the celestial equator — changes through the year. In January it's far south, which is why high northern latitudes get very short days or no daylight at all. As declination moves north through the year, the northern latitudes get longer days and later sunsets. The table is essentially a pre-computed answer to that declination-driven geometry, so you don't have to do spherical trigonometry in the cockpit — you just read the row and column. One more practical note on reading the numbers themselves. The times are given in hours and minutes, like 12 36 meaning 12 hours 36 minutes. And you'll notice the values are continuous across the page — the row for a given latitude runs all the way across the months, and the columns for a given date run down through the latitudes. So you interpolate naturally: if your exact latitude or date falls between two printed values, you estimate between them. That's the whole page in essence: a latitude-by-date grid of local mean sunset times at Greenwich, with the midnight-sun marker for high latitudes, all driven by the sun's declination. When you're planning a flight that will cross into darkness, this is the table you'd pull to know when the sun goes down at your position.