Ask an engineer what time it is and he (or she) will tell you how a watch is made.  Ask a person who has mastered the art of assembly language computer programming what time it is and you will get the exact time in binary, thus every bit usable.

 

METER

 

An international standard unit of length, approximately equivalent to 39.37 inches. It was redefined in 1983 as the distance traveled by light in a vacuum in 1/299,792,458 of a second (.0000000033356409519815204957557671447492 of a second).  Now there's some accuracy.

 

But, in 1960, the length was determined by using a definition based upon a wavelength of krypton-86 radiation, thanks to Superman.

 

Earlier, the French thought it should equal 10^-7 or one ten-millionth of the length of the meridian through Paris from pole to the equator.  This was later shown to be at least .2 mm in error. Ahhhh, "Le plume de ma tante" and other French lessons.

 

KILOMETER or KLICK

 

1000 METERs all over the world.  Don't ask about the Ole Rømer mile (Scandinavia) or what the Danes and Germans did with Kms 2 centuries ago.  

 

STATUTE MILE

 

Queen Elizabeth I of England redefined the mile from 5000 feet to 8 furlongs (5280 feet) by statute in 1593 and, currently, a foot is the imperial foot now defined to be exactly 0.3048 meters.  Don't ask any more about the basis for feet or iambic pentameters.

 

The Roman mile was 1000 double 2.4 foot (whose foot?) paces - apparently made while wearing armor and represents the distance between milestones to Rome.  Better yet, a league is the distance a horse or man could walk in 1 hour (however that hour was then measured) - usually about 3 miles. 

 

KNOT = Nautical Miles per Hour

 

The nautical mile was historically defined as a minute of arc along a great circle of the Earth. It can therefore be measured on a meridan as change of latitude on a nautical chart. Slight variations in national definitions were settled in 1929, when the IEHC in Monaco adopted a definition of 1 nm = 1852 meters. The US adopted it in 1954. The Royal Hydrographic Office of the UK converted in 1970. I don't know about other places like France, Australia or Utah.

 

A nautical league is about 3 nm - not measured by how long you've been walking on water.

 

SECOND

 

A fundamental unit of the hour frequently referenced herein.  The time needed for a cesium-133 atom to perform 9,192,631,770 complete oscillations.  Just ask any artificial-star GPS satellite user, "How important is that amount of accuracy?"

 

Hmmmmm, a second used to be 1/86400 of a mean solar day until 1960.  Because of variances, in 1967 atomic clocks were shown to be more accurate and the Cesium clock, in its ground state at 0 degrees Kelvin, was affirmed in 1997.

 

SUMMARY

 

Let's see, 200 Kts = 230.156 mph = 102.8888888888888889 Benz (meters/sec) - Use http://www.onlineconversion.com/

 

Hey, these and millions of other units of measurement have changed over time - currently we rely on the speed of light in a vacuum and the twittering of a specific Cesium atom to keep us honest.

 

How about this report to ATC: "N1234 is now 15 DME on the RFD 180 radial."  How far laterally? That depended on the altitude. 

 

When observing closing aircraft, there are only 2 distances that matter: No problem and Yikes!