4 ms·

What seemed odder to me is GP's talking about speed as if it's the same as distance: > a knot is "exactly equal to 1/60th of a degree of latitude. ...Therefore

What seemed odder to me is GP's talking about speed as if it's the same as distance: > a knot is "exactly equal to 1/60th of a degree of latitude. ...Therefore, a knot is equal to one minute of distance." I'm not sure what that quote is from, but it's is akin to saying "A metre per second is equal to one metre." Um, no. The units don't match.

jcrawfordor 5y ago
Oh, I think I didn't even read that correctly since it seems like it's just a mistake. A knot is, for historic reasons, a term for NM per hour. An NM is defined by latitude. I believe that the term knots originates from a method of measuring speed at sea by use of a twisted rope and a sea anchor, but perhaps that's apocryphal. Presumably though the definition in terms of NM was a later improvement. While I don't have any real evidence for this, I would speculate that part of the reason for the enduring use of knots in aviation is that, much like the old twisted rope, it links the unit to the instrument. The speeds usually quoted for aircraft are in KIAS, Knots Indicated Airspeed, a measurement which is basically defined by what the instrument shows. It is intentionally uncorrected for various errors in the instrument design. This happens to be convenient because many of the same factors that affect the instrument calibration also affect the aircraft handling, so handling at 100 KIAS tends to feel similar even though the actual speed varies by altitude etc. KCAS and KTAS, Calibrated and True Airspeed, are both KIAs after correction for various errors and are useful for certain calculations but not nearly as often as KIAS. My point is that it might be helpful to use Kts rather than MPH or KPH because it helps keep these factors in mind and helps to reduce ambiguity about whether a number is indicated or true, although people very frequently just write knots when meaning KIAS. Of course for ground speed none of this matters but ground speed isn't really a "flight" concern per se, more just used for navigation calculations.

jcrawfordor 5y ago

Oh, I think I didn't even read that correctly since it seems like it's just a mistake. A knot is, for historic reasons, a term for NM per hour. An NM is defined by latitude. I believe that the term knots originates from a method of measuring speed at sea by use of a twisted rope and a sea anchor, but perhaps that's apocryphal. Presumably though the definition in terms of NM was a later improvement. While I don't have any real evidence for this, I would speculate that part of the reason for the enduring use of knots in aviation is that, much like the old twisted rope, it links the unit to the instrument. The speeds usually quoted for aircraft are in KIAS, Knots Indicated Airspeed, a measurement which is basically defined by what the instrument shows. It is intentionally uncorrected for various errors in the instrument design. This happens to be convenient because many of the same factors that affect the instrument calibration also affect the aircraft handling, so handling at 100 KIAS tends to feel similar even though the actual speed varies by altitude etc. KCAS and KTAS, Calibrated and True Airspeed, are both KIAs after correction for various errors and are useful for certain calculations but not nearly as often as KIAS. My point is that it might be helpful to use Kts rather than MPH or KPH because it helps keep these factors in mind and helps to reduce ambiguity about whether a number is indicated or true, although people very frequently just write knots when meaning KIAS. Of course for ground speed none of this matters but ground speed isn't really a "flight" concern per se, more just used for navigation calculations.