First, the high-latitude case — Page 432, Lesson 394

I want to walk you through what happens to twilight when you climb into the high latitudes, and then what altitude does to all of these times. This is the part of the book where the simple sunrise/sunset picture starts to bend. First, the high-latitude case. Up near the poles, the Sun can stay above the horizon for the entire day. On or about June 21 — the summer solstice — the Sun remains above the horizon all day at latitudes higher than 66°N. That's the Arctic Circle. And remember the mirror image: in the Southern hemisphere, further south than 66°S, the Sun will not rise at all on that date. So at the solstice, one pole has a full day of Sun, the other has no Sun at all. The book illustrates this with a 360° time-lapse photograph taken in Lapland in midsummer. That's the key image — the Sun circles the sky without ever dipping below the horizon. Now, using that same diagram, we can picture what happens earlier or later than June 21, when the Sun is lower in the sky. And similarly, the Sun will be lower in the sky as the latitude of the observer increases. So the further north you go, or the further you get from the solstice, the lower the Sun's path becomes. Now here's the interesting twist. In Figure 26.7, the Sun sets at position A and rises at position B. Between the times of sunset at A and sunrise at B, the Sun never goes below 6° below the sensible horizon. Let me unpack that. The sensible horizon is the horizon you actually see from your position. When the Sun is less than 6° below that horizon, its light still reaches you through the atmosphere. So even though the Sun has set, you still have twilight — the whole night long. The book calls this situation "Twilight between sunset and sunrise." And this situation is identified in the Air Almanac 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. That's the practical confirmation of what we just described. Now let's move to the second big idea: the effect of altitude on sunrise, sunset, and twilight. The times published in the Air Almanac are in Local Mean Time — LMT — and they are published for sea level at the appropriate position. So those are your baseline values. Here's what altitude does. First, an increase in altitude results in sunrise occurring earlier and sunset later than published. Why? Because of the increased visual horizon at altitude. When you're high up, you can see further over the curve of the Earth, so you see the Sun appear earlier in the morning and keep it in view later in the evening. Second, and this is the counterintuitive one: an increase in altitude results overall in a decrease in the duration of civil twilight. That seems backwards — you'd think being higher would give you more twilight. But here's the reason. 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. You're above more of the atmosphere that does the refracting, so less light gets bent down to you. For observers in the Space Shuttle, the duration of twilight is negligible. Essentially zero — because there's almost no atmosphere left above them to refract the Sun's light. Now let's look at the worked examples the book gives, because they show you the arithmetic. The first one is an evening civil twilight calculation. We have 1833 LMT as a starting time, then minus 0429 — that's an arc to time conversion, converting a longitude difference into a time difference. That gives us 1404 UTC. Then we add the ECT — that's Evening Civil Twilight — of 1857, and subtract the sunset time of 1833. The duration of ECT is 24 minutes. So the evening civil twilight lasts 24 minutes in this case. Then there's Question 4. We have 14 OCT ECT at 1738 LMT, and 15 OCT MCT — that's Morning Civil Twilight — at 0556 LMT. The time between them is 1218. That's the total night duration between evening and morning civil twilight. And Question 5 walks through a fuller calculation. Part a: sunrise at 0458 LMT. Part b: sunset at 1910 LMT. Then we have 0744 as an arc to time conversion, giving a sunset time of 1126 UTC. Part c: evening civil twilight at 1938 LMT, again with 0744 arc to time, giving 1154 UTC. Then we add 0800 as the ST correction — that's the Standard Time correction, converting from UTC to the local standard time zone. That gives us an ECT of 1954 ST. So the evening civil twilight, expressed in standard time, is 1954. Let me make sure the big picture is clear. The Air Almanac gives you times at sea level in Local Mean Time. If you're flying high, sunrise comes earlier and sunset later because of your extended visual horizon. But twilight itself gets shorter with altitude, because twilight depends on atmospheric refraction, and there's less atmosphere above you to do the refracting. And in the high latitudes, near the solstice, you can have twilight that never ends — the Sun stays within 6° of the horizon all night, and the Air Almanac flags that with the //// symbol. That's the full picture of how twilight behaves at the extremes of latitude and altitude.