knot
Speedexact by definition
The knot is one nautical mile per hour, exactly 1,852/3,600 m/s, which is 0.5144... m/s. Since both the nautical mile and the hour are exactly defined, the relation is exact, though the decimal repeats. It is the standard speed unit for ships, aircraft and wind in marine and aviation forecasts.
Watch out: "Knots per hour" is wrong: a knot already contains the per-hour. The knot is also about 15% faster than a mile per hour, so treating the two as equal understates a vessel's speed.
| 1 kn | 0.51444444 m/s |
The name comes from the chip log, a wooden board thrown astern on a line with knots tied at regular intervals. A sailor counted the knots that ran out while a sandglass emptied, and that count was the ship's speed. The knot spacing and glass interval commonly cited are about 47 ft 3 in and 28 seconds, a ratio that reproduces one nautical mile per hour closely.
About the knot
The knot exists because of the nautical mile, and the nautical mile exists because of latitude. One nautical mile is one minute of arc of latitude, so a vessel making 10 knots northward changes its latitude by 10 minutes every hour, and a navigator can convert speed to position with mental arithmetic. That property is why the unit survived the arrival of GPS: charts still carry latitude scales, and dead reckoning still works when the electronics do not.
The name comes from the measuring method. A chip log was a weighted wooden quadrant tossed over the stern, attached to a line with knots tied along it at a fixed spacing. The log stayed roughly stationary in the water while the ship sailed away, paying out line, and a sailor counted knots passing through his hand for the duration of a sandglass. The spacing and the glass were chosen together so the count read directly in nautical miles per hour, no arithmetic required. Aviation inherited the unit from marine navigation along with the nautical mile and the compass rose, which is why an airliner's airspeed indicator reads in knots while its altimeter reads in feet.