# What's the difference between a nautical and a statute mile?

A Dawson

Until the 18th century, the mile used by sailors was the same as a mile on land. Based on the Roman mile Which was designed to represent 1,000 double steps (two strides of a soldier), the statute mile was redefined several times until Elizabeth I decided to set it at 5,280 feet (1,609 metres).

The measure used by sailors changed when they found a more convenient way to measure how far the, had travelled using me shin and the stars. "Sailors were still using leagues and miles in the 16th century," says Dr Gloria Clifton, navigational-instruments curator at the National Maritime Museum In London. "The nautical mile really came in with the establishment of scientific and astronomical navigation in the 18th century. There was no single date for the changeover. It was a fairly gradual process. When navigating around a coast with a chart was often easier to use the old system. But by the 19th century they all seemed to be using nautical miles."

Instead of being a largely arbitrary measure based on the strides of an ancient soldier, the nautical mile was based on the circumference of the Earth. The measure is defined as the distance travelled in one minute of arc--1/60th of a degree--in a great circle around the Earth. "It's fairly close to a statute mile--about 1.15 times the length," says Clifton.

There is only one problem with this definition: the Earth isn't a perfect sphere--its slightly flattened at the poles. As a result, one minute of arc can mean 6,092 feet at 60[degrees] and 6,046 feet at the equator. So, at several stages since its first use authorities have set a compromise length for the nautical mile. The latest change, In 1929, converted the measurement from exactly 6,080 feet to being based on the metric system. As a result, the nautical mile was shortened by about three feet to 1,852 metres.