Question: Night, as defined in CFR14, FAR Part 1, is the time between the end of evening civil twilight and morning civil twilight, as published in the American Air Almanac, converted to local time. But what exactly IS the meaning of twilight?
- Twilight is when the last chord of the setting sun can no longer be seen below a perfectly flat horizon.
- Twilight is when the first nightingale sings.
- Twilight, in this case civil twilight, is when the center of the sun’s disc goes one solar diameter below the horizon.
- Civil twilight means when the center of the sun’s disc is six degrees below an ideal horizon.
The answer is number 4. And what’s more, nautical twilight is 12 degrees, and astronomical is 18!
Subject: The Longest Day
Question: One of the results of how accurately we can now measure time with atomic clocks has been the realization that the rate of the earth’s rotation is actually slowing! True, is doesn’t slow down a great deal, but it IS slowing down—but by how much?
- one hour each year
- one minute a year
- about a second every year
- one second a century
The answer is number 4. It’s mostly due to tidal friction, believe it or not. It’s about one hour every 450,000 years. So our brachiating ancestors lived 22, 20, maybe 18 hour days, depending on how far back you go. Also, the moon was a tiny bit closer in then (a few dozen miles), which actually would shorten the days a bit more. Starting in 1972 with Coordinated Universal time, we’ve been (the astronomical weenie technocrats, that is) inserting (or removing) ‘leap seconds’ to keep with International Atomic Time. (They usually do it, if it’s needed, on June 30 or December 31.)
Subject: Duh! Which Way Do We Go?
Question: Finally! There is now a way to determine left or right traffic at unfamiliar airports, without leafing through your AFD, overflying and looking for a wind sock or tetrahedron, and without bugging folks on the CTAF! It’s been staring us in the face for (depending on where you live) about two years! What IS that?
- new LARGE TYPE airport signage, and ‘L’ or ‘R’ subscript on untowered airport runways
- (J. Edgar would love this) pending legislation making NO RIGHT TURNS at all airports
- change in VFR charts during the last two years
- new AWOS feature
The answer is number 3. Yup, just pull out your sectional. About two years ago, VFR charts started revealing this choice morsel of information at the bottom of the information area for each airport, for any pattern that is NOT left-handed. For example, for Frederick, MD, it now says ‘RP 5,12’ (meaning of course, a right-hand pattern for runways 5 and 12).