Mile, Nautical and Statute
Mile, Nautical and Statute
The statute mile has it origins in Roman times where a measure of a thousand paces, mille passum, was used for large distances. For the Romans, a pace was the distance between the same foot touching the ground—that is the distance covered in two steps. The pace was taken as being equal to five Roman feet, a length that historians have calculated to be 11.65 inches.
The study of Anglo-Saxon texts reveals that the early English used a measure for long distances called a mil (plural: mila), which was also equal to 5,000 feet. However, the foot used by the Saxons was measured by using an adult's actual foot, and so was about 80 percent of the length of the Roman foot.
Although the confusion between Roman and Saxon feet was resolved by the Statute for Measuring Land of 1305, making the foot close to our modern measure, this did not get translated into resolving the conflict between the two miles for measuring long distances. It was nearly 200 years later, in 1593, that Elizabeth I signed into law a statute titled An Acte againste newe Buyldinges. The act prohibited any new construction within three miles of the gates of the City of London. In this statute the mile was declared to be 8 furlongs, a furlong being 40 rods, which was itself 16½ feet long. Thus the statute mile was a total of 5,280 feet, the length it is today.
The Nautical Mile
A nautical mile is a distance of 1,852 meters, as recommended by the International Hydrographic Conference of 1929. Before the sixteenth century, navigation at sea used landmarks, seamarks, and seabed samples to estimate the depth of an area of water. It was rare for ships to sail in waters deeper than 100 fathoms (600 feet), and if they did, it was for short distances. Navigational information was exchanged between ships' pilots, each of whom kept his own set of notes called a Rutter.
There were a variety of measures of distance in different areas of the world. In the Mediterranean, Roman miles, or leagues equal to 1½ Roman miles, were used. In waters off the western European coast, distances were in kennings, about 20 miles, which was the farthest one could be from a coastline before losing sight of it.
A full understanding of the spherical nature of Earth became widely accepted at the end of the fifteenth century and led to an improvement in map making. Pilots began showing meridians of longitudes and parallels of latitudes on the maps that accompanied their Rutters. As sailors began to venture farther from land, they began to use a new measure, which they called a league, but which was twice its counterpart on land, being three Roman miles in length.
In 1484, King John II of Portugal formed a commission to tackle the problem of finding latitude in the Southern Hemisphere where the North Star was not available for navigation. The solution was to use the solar tables of the astronomer Zacuto of Salamanca, which allowed navigators to use the Sun as a means of determining latitude. It should also be noted that the Portuguese had their own version of the league, which was four Roman miles in length.
Compounding the confusion over which league was more accurate, there was disagreement over the size of Earth, and consequently the distance between each degree of latitude. The Portuguese used a measurement of 25,200 Roman miles for the circumference of Earth given by the ancient Greek astronomer Eratosthenes. For political reasons, the Spanish adopted the circumference measurement, given by Ptolemy, of 18,000 Roman miles. This allowed the Spanish to claim the Moluccas in the East Indies in the early part of the sixteenth century. In 1617 the Dutch scientist Snell completed a new assessment of the circumference of Earth and found it to be 24,630 Roman miles (24,024 statute miles).
The will of Sir Thomas Gresham established a college in London in 1596, with one of the principal aims being the teaching of astronomy in relation to navigation. Gresham College soon attracted the best mathematical minds of the day, either as teachers or researchers. Edmund Gunter (1581–1626) was a mathematician and also a church minister whose church was across the Thames river from Gresham College. Gunter reasoned that the navigator was concerned with two principle problems: the ship's position, and the distance it had sailed, or had to sail. Gunter believed that the most acceptable unit of distance for a navigator would be one in which the angle measurements of latitude could be related to the distance traveled. The result of this reasoning was to take one minute of a meridian as being equal to the unit of distance. In accepting Snell's measurement of the circumference of Earth, Gunter defined his "nautical mile" to be equal to 6,080 feet, the length of one minute of arc at 48°. Because Earth is not a perfect sphere, the minute of circumference measurement actually varies between 6,046 feet at the equator to 6,108 feet at the pole. Other countries used a measurement for the nautical mile at 45° of latitude, which put the nautical mile at a length of 6,076 feet. Finally, in 1929 the nautical mile was agreed internationally as being equal to 1,852 meters.
see also Distance, Measuring; Measurement, English System of; Navigation.
Conner, R. D. The Weights and Measures of England. London: Her Majesty's Stationery Office, 1987.
A DIFFERENT KIND OF KNOT
The knot is a measurement of speed equal to a rate of one nautical mile per hour. The term knot comes from the original method of measuring speed at sea, which was to throw a shaped log overboard attached to a rope with equally spaced knots. The log was weighted so that the shaped face gave the maximum resistance to towing, and was practically stationary in the sea. By counting the number of knots that passed overboard during a period fixed by a sand timer, navigators were able to calculate the speed of the ship.
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