The nautical mile "was defined as the meridian arc length corresponding to one minute (1/60 of a degree) of latitude at the equator, so that Earth's polar circumference is very near to 21,600 nautical miles (that is 60 minutes × 360 degrees). Today the international nautical mile is defined as 1,852 metres (about 6,076 ft; 1.151 mi). The derived unit of speed is the knot, one nautical mile per hour."
I always thought it had something to do with ropes with knots hanging off a ship moving in the water. Maybe that was how they estimated it when traveling, but it wasn't the (somewhat arbitrary) definition as I thought. Good to know.
The original method was to drop a piece of wood known as a log off the bow of a boat and count the seconds until the stern passed the log. Knowing the length of the boat and the time, you could calculate the speed.
ReplyDeleteThis evolved into the chip log. The chip was a quarter circle of wood sheet weighted to float upright in the water. It was tied to a knotted rope. You dropped the chip over the stern and the boat sailed away from the chip which pulled the rope off a reel. The number of knots pulled off the reel in a set time gave the speed in nautical miles per hour, colloquially knots.
Our chip log has knots spaced 48 feet apart and a 30 second sand glass as a timer.
Every half hour the compass course and the speed measured by the log were recorded on a pegboard. At the end of each four hour watch the navigator recorded the peg positions in his logbook and calculated a new position. Thus if the course was West and the speed averaged four knots, the boat had travelled sixteen miles Westwards during the watch.
The proper name for this was deduced reckoning, but became known as dead reckoning, perhaps because of what happened if you got it wrong.
Unfortunately, even we use knots when we sail, and this is Europe with a long history of using metric.
ReplyDeleteLatitude and longitude derive from the Babylonian idea that a circle has 360 degrees. If you are going to navigate using latitude, longitude and spherical trigonometry it makes sense to use arcminutes of Earth's circumference as your unit of distance.
DeleteRevolutionary France defined the metre as 1/10,000,000 of the meridian distance from the North Pole to the Equator passing through Paris. That makes a nautical mile 1852 metres.
While the metric system has become the standard for scientific measurement, I do not recall a direct metric system of navigation.
Oh, I didn't know this Babylonian thing /s I'm pretty sure we can navigate using metric units even if we use latitude, longitude, and spherical trigonometry. There's a thing called "conversion" that is readily done by computers that we use anyway for navigation nowadays. Furthermore only the difference in latitude is meaningful as a multiple of the nautical mile and it is not a big help anyway in any calculation.
DeleteYou could design a metric coordinate system.
ReplyDeleteThe distance from pole to Equator is 10,000 kilometres. You could define that as 100 units of latitude, each unit of 100 kilometres instead of 90 degrees, each of 60 nautical miles. The circumference of the Equator is 40,000 kilometres which would be 400 units of longitude instead of 360 degrees.
It would work, but the changeover would take a lot of effort.
The Equator would be 40,000 kilometres or 400 units.
Yeah, we can design a metric coordinate system. But we can use metric without that.
DeleteThanks for the erudite comments....
ReplyDelete