Knots and Machs

Knots and Machs As we all know, speed describes the distance that an object travels per time unit. Therefore speed is measured in kilometers per hour, miles per hour or meters per second. However, there exist several arbitrary units that do not exactly meet this definition. The one arbitrary speed unit, that is understandable for the majority of us, is that used by sailors. Since the commonly accepted distance unit for sailors is the nautical mile, which equals to 1,852 meters, the speed would quite logically be measured in nautical miles per hour. Among sailors one nautical mile per hour is referred to as the "knot". So if you are told that a vessel travels at the speed of 8 knots, this means that it moves at the speed of 8 nautical miles per hour. If, by mistake, you happen to tell a sailor that a vessel travels at the speed of 8 knots per hour, you at least risk to be labeled an incompetent person. Another arbitrary speed unit is the speed indicator used in aviation. Since an airplane travels in the air, it is logical, that the speed unit here would be the speed of sound in the air. This measurement unit is referred to as the M or Mach Number. And this is how it works - if an object travels at the speed that exceeds the sound speed by 1.2 times, they say that the object's speed is 1.2 machs. Why would we need this? The thing is, the speed of sound in the air depends on many parameters, such as air temperature, pressure, density, humidity, etc. One plane can be tested in the northern latitudes, another -- in the tropical ones. But if the speed expressed in machs is the same for both of them, it would mean that the second plane would have the same speed as the first one under similar conditions.