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A furlong is a measure of distance in imperial units and U.S. customary units equal to one-eighth of a mile, equivalent to 220 yards, 660 feet, 40 rods, or 10 chains. The exact value of the furlong varies slightly among English-speaking countries.
Five furlongs are approximately 1 kilometre (1.0058 km is a closer approximation). Since the original definition of the metre was one-quarter of one ten-millionth of the circumference of the Earth (along the great circle coincident with the meridian of longitude passing through Paris), the circumference of the Earth is about 40,000 km or about 200,000 furlongs.
The name furlong derives from the Old English words furh (furrow) and lang (long). Dating back at least to early Anglo-Saxon times, it originally referred to the length of the furrow in one acre of a ploughed open field (a medieval communal field which was divided into strips). The system of long furrows arose because turning a team of oxen pulling a heavy plough was difficult. This offset the drainage advantages of short furrows and meant furrows were made as long as possible. An acre is an area that is one furlong long and one chain (66 feet or 22 yards) wide. For this reason, the furlong was once also called an acre's length, though in modern usage an area of one acre can be any shape.
Among the early Saxons, the rod was the fundamental unit of land measurement. A furlong was forty rods, an acre 4 × 40 rods, or 4 rods by 1 furlong. and thus 160 square rods. At the time, the Saxons used the North German foot, which was 10% longer than the foot of today. When England switched to the shorter foot in the late 13th century, rods and furlongs remained unchanged, since property boundaries were already defined in rods and furlongs. The only thing that changed was the number of feet and yards in a rod or a furlong, and the number of square feet and square yards in an acre. The definition of the rod went from 15 old feet to 16 1/2 new feet, or from 5 old yards to 5 1/2 new yards. The furlong went from 600 old feet to 660 new feet, or from 200 old yards to 220 new yards. The acre went from 36,000 old square feet to 43,560 new square feet, or from 4000 old square yards to 4840 new square yards.
The furlong was historically viewed as equivalent to the Roman stade (stadium), which in turn derived from the Greek system. For example, the King James Bible uses the term "furlong" in place of the Greek stadion, whereas modern translations translate into miles in the main text and relate the original numbers in footnotes.
In the Roman system, there were 625 feet to the stade, eight stade to the mile, and three miles to the league. A league was considered to be the distance a man could walk in one hour, and the mile (mille, meaning one thousand) consisted of 1000 passus (paces, 5 feet, or double-step).
After the fall of Rome, Medieval Europe continued with the Roman system, which proceeded to diversify, leading to serious complications in trade, taxation, etc. Around the turn of the century of 1300, England by decree standardized a long list of measures. Among the important units of distance and length at the time were foot, yard, rod(or pole), furlong, and mile. The rod was 5½ yards or 16½ feet (= 3 feet/yard × 5½ yards), and the mile was 8 furlongs, so the definition of the furlong became 40 rods and that of the mile became 5280 feet (= 8 furlongs × 40 rods/furlong × 16½ feet/rod).
A 1675 description states, "Dimensurator or Measuring Instrument whereof the mosts usual has been the Chain, and the common length for English Measures 4 Poles, as answering indifferently to the Englishs Mile and Acre, 10 such Chains in length making a Furlong, and 10 single square Chains an Acre, so that a square Mile contains 640 square Acres." —John Ogilby, Britannia, 1675
The official use of furlong was abolished in the United Kingdom under the Weights and Measures Act of 1985, which also abolished from official use many other traditional units of measurement.
In Myanmar, furlongs are currently used in conjunction with miles to indicate distances on highway signs.
In the rest of the world, the unit enjoys very limited use, with the notable exception of horse racing. Distances for thoroughbred horse races in Australia were metricated in 1972  but in the United Kingdom, Ireland, Canada, and the United States, races are still given in miles and furlongs.
The city of Chicago's street numbering system allots a measure of 800 address units to each mile, in keeping with the city's system of eight blocks per mile. This means that every block in a typical Chicago neighbourhood (in either North/South or East/West direction but rarely both) is approximately one furlong in length. Salt Lake City's blocks are also each a square furlong in the downtown area. The blocks become less regular in shape, further out from the centre, but the numbering system (800 units to each mile) remains the same everywhere in Salt Lake County. Bus stops in Ann Arbor, Michigan, are about a furlong apart. Blocks in central Logan, Utah, and in large sections of Phoenix, Arizona, are similarly a square furlong in extent (eight to a mile, which explains the series of freeway exits: 19th Ave, 27th, 35th, 43rd, 51st, 59th ...). City blocks in Melbourne's Hoddle Grid are also one furlong in length.
The furlong is also a base unit of the humorous FFF system of units.
The exact conversion of the furlong to SI units varies slightly among English-speaking countries. In Canada and the United Kingdom, which define the furlong in terms the international yard of exactly 0.9144 metre, a furlong is 201.168 m. Australia does not formally define the furlong, but does define the chain and link in terms of the international yard. In the United States, which defines the furlong, chain, rod, and link in terms of the U.S. survey foot of exactly 1200⁄3937 metre, a furlong is approximately 201.1684 m; the US does not formally define a "survey yard". The difference of approximately 2 parts per million between the US value and the "international" value is insignificant for most practical measurements.