Let's track the same observer positions — Page 426, Lesson 385

We're picking up right where we left off with the seasons, but now I want to take you to the opposite side of the year. We just looked at the June solstice, when the Northern hemisphere tilts toward the Sun. Now let's look at December 21st, the corresponding situation when the Northern hemisphere is tilted away from the Sun. This is the northern winter and southern summer. Let's track the same observer positions. First, the observer at R. R 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 the signature of northern winter — the daylight window is squeezed, and it's shifted later in the morning and earlier in the evening. Now the observer at S. S experiences the exact opposite: 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. And here's the key contrast with the June case. In June, the observer at X — north of the Arctic Circle — had constant day. But in December, the observer at X experiences constant night, as do all observers north of X. That's the Arctic winter, when the Sun never rises. And the observer at Y, at the Antarctic Circle or further south, experiences constant day, as do all observers south of Y. So the roles of the two polar circles have completely swapped. Now, stepping back, I want to give you the general rule that ties all of this together. The length of day and night, and the timings of sunrise and sunset, vary with two things: latitude and the declination of the Sun. Declination is just the calendar date — the Sun's apparent position north or south of the Equator through the year. At the Equator itself, periods of day and night may be considered equal at all times of the year. But there's a subtlety I want you to note: 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 atmospheric effect later. Now let's move to the practical side — how we actually get these times. This is the methodology for sunrise and sunset tables. 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, as opposed to zone time. The times are shown for every third day, and for various latitudes between 72 North and 60 South. Here's the catch you'll face in practice. If the latitude for which you need the time of sunrise or sunset is not one of the listed latitudes, you must interpolate to the nearest minute of time. And the spacing of those listed latitudes is not uniform — that's important. The listed latitudes are 10 degrees apart near the Equator, 5 degrees apart in mid latitudes, and 2 degrees apart in high latitudes. So the closer you get to the poles, the finer the tabulation, because that's where the day length changes most rapidly with latitude. So to summarise the whole picture: the December solstice gives you short days and long nights in the north, long days and short nights in the south, constant night above the Arctic Circle, constant day below the Antarctic Circle, and equal day and night at the Equator — with that 6-minute atmospheric correction. And when you need actual times, you go to the Air Almanac, extract LMT for your latitude, and interpolate to the nearest minute when your latitude falls between the tabulated values.