George Moore of Queenstown asks :-

Why is speed at sea measured in knots and what are they?

John Campbell, a physicist at the University of Canterbury and an old sea cadet, responded.

Originally speed at sea was measured by dropping a wooden chip overboard near the bow, and timing how long it took to reach a known distance towards the stern. For this the unit of speed could be whatever anyone liked.

Eventually position at sea could be determined by astronomical sightings, for which the sky must be clear and, for accuracy, preferably the sea calm. An accurate measurement of speed was needed to estimate the ship's position inbetween the accurate measurements.

The name mile came from the latin "mille passes" ie "thousand paces". It was a quite variable distance and different towns let alone countries had different values for the mile and these changed over the ages. Eventually the geographic mile was defined as 1609 and a bit metres.

The shortest distance between two ports is on a great circle, an arc centered on the center of the Earth. For navigation purposes the distance used at sea was the nautical mile, defined as the distance along a great circle which subtended an angle of one minute of arc. There are 360 degrees in a full circle, ie once round the Earth, and 60 minutes in each degree, so a nautical mile is the great circle distance around the world divided by 360 x 60 = 21600.

Unfortunately for navigation, the Earth isn't exactly a perfect sphere so the nautical mile measured around the equator would differ from that measured through the poles. In 1929 the international nautical mile was defined as 1852 metres so is 243 metres longer than a geographic mile.

The knot is a speed of one nautical mile per hour. The term arose from a device used to measure speed. A weighted board was lowered from the stern and a light line allowed to run out. A sand glass which emptied itself in 28 seconds was used for timing. The distance travelled in that time was measured by the number of equally-spaced knots in the rope run out. These knots were 47 feet 3 inches (ie 14.4 metres) apart so that the number of knots running out in the 28 seconds gave the speed in nautical miles per hour, or put another way, minutes of arc per hour.

Hence the name knots for speed at sea. Note that the above device didn't allow for ocean currents.