Up here, the Sun's behaviour changes completely — Page 432, Lesson 394

Let's look at twilight in high latitudes. Up here, the Sun's behaviour changes completely. On or about June 21, at latitudes higher than 66°N, the Sun stays above the horizon all day — it never sets. And remember the mirror image: in the Southern Hemisphere, further south than 66°S, the Sun will not rise at all on that date. That illustration is based on a 360° time-lapse photograph taken in Lapland in midsummer. Now, the same diagram lets us see what happens earlier or later than June 21, when the Sun sits lower in the sky. And the Sun also sits lower as the observer's latitude increases — so the effect grows the further north you go. Here's the key case. In Figure 26.7, the Sun sets at position A and rises at position B. Between those two times — from sunset to sunrise — the Sun never goes below 6° below the sensible horizon. The sensible horizon is the horizon you actually see from your position. So because the Sun stays within 6° of that horizon all night, the entire period between sunset and sunrise counts as twilight. The Air Almanac identifies this situation with the symbol //// — four slashes. You can check this yourself in your Air Almanac by looking at the time for evening civil twilight at 64°N on June 28. Now, the effect of altitude on all of this. The times published in the Air Almanac are in Local Mean Time — LMT — and they're published for sea level at the appropriate position. So they're a baseline. Increase your altitude, and sunrise happens earlier than published, and sunset happens later. Why? Because at altitude your visual horizon is increased — you can see further over the curve of the Earth, so you see the Sun sooner and keep it in view longer. But here's the counter-intuitive part: increasing altitude actually decreases the total duration of civil twilight. Twilight is caused by refraction of the Sun's rays from the atmosphere. The higher the aircraft, the less light is refracted from the reduced upper atmosphere. For observers in the Space Shuttle, the duration of twilight is negligible — essentially zero. Now let's work through the worked example. We have 1833 LMT, minus 0429, converted arc to time, giving 1404 UTC. Then we add the ECT — evening civil twilight — of 1857, and subtract the sunset of 1833. The duration of ECT is 24 minutes. Then Question 4: 14 OCT ECT at 1738 LMT, 15 OCT MCT at 0556 LMT. The time between is 1218. And Question 5 has three parts. Part a: sunrise at 0458 LMT. Part b: sunset at 1910 LMT. Then 0744 arc to time gives sunset at 1126 UTC. Part c: ECT at 1938 LMT, again 0744 arc to time, giving 1154 UTC, then add the +0800 ST correction, and ECT becomes 1954 ST. So the pattern throughout: the Air Almanac gives you LMT at sea level, and you apply the altitude effects — earlier sunrise, later sunset, shorter twilight — and you convert between time scales using arc to time and the standard time correction.