Let’s pick this up right where the sunrise/sunset picture leaves off — Page 426, Lesson 385

Let’s pick this up right where the sunrise/sunset picture leaves off. I want to walk you through the December 21st case, because that’s the mirror image of what we just saw for June. On December 21st, the Northern hemisphere is tilted away from the Sun. So now the observer at R — that’s a position in the Northern hemisphere, in mid or high northern latitudes — experiences a short period of day and a long period of night. The Sun rises later than 0600 and sets earlier than 1800 hours. That’s northern winter. Now go to the observer at S, in the Southern hemisphere. S experiences a short night and a long day. The Sun rises earlier than 0600 and sets later than 1800. That’s southern summer. The observer at E, on the Equator, again experiences equal periods of day and night. Now the key contrast for this date. The observer at X experiences constant night — and so do all observers north of X, which is the Arctic Circle. That’s the Arctic winter, when the Sun never rises. And the observer at Y experiences constant day — and so do all observers south of Y, the Antarctic Circle. That’s the Antarctic summer, when the Sun never sets. So here’s the general rule I want you to hold onto: the length of day and night, and the timings of sunrise and sunset, vary with two things — latitude, and the declination of the Sun, which is just the calendar date. At the Equator, periods of day and night may be considered equal at all times of the year. But there’s a small correction: because of atmospheric effects, the length of the day at the Equator is approximately 6 minutes longer than the length of the night. We’ll come back to that later. Now let’s move to the practical side — how we actually get these times. The Air Almanac provides tables from which you can extract the LMT of sunrise and sunset. LMT is Local Mean Time — the time at your particular meridian, not the zone time. The tables show times for every third day, and for various latitudes between 72°N and 60°S. Here’s the interpolation rule, and it’s important for the exam. If the latitude you need is not one of the listed latitudes, you must interpolate to the nearest minute of time. And note how the listed latitudes are spaced: they’re 10° apart near the Equator, 5° apart in mid latitudes, and 2° apart in high latitudes. So the spacing gets finer as you move away from the Equator — because that’s where the sunrise and sunset times change fastest with latitude. So to summarise the whole picture: on any given date, day length and sunrise/sunset times depend on your latitude and the Sun’s declination. The Equator is the constant — roughly equal day and night all year, with that 6-minute atmospheric correction. And when you need a time for a latitude that isn’t in the Air Almanac table, you interpolate to the nearest minute, using the appropriate spacing for that latitude band.