| Furlongs (fur) | Astronomical units (au) |
|---|---|
| 1 Furlong | 0.00000000134472502221 au |
| 2 Furlongs | 0.00000000268945004442 au |
| 3 Furlongs | 0.00000000403417506664 au |
| 4 Furlongs | 0.00000000537890008885 au |
| 5 Furlongs | 0.00000000672362511106 au |
| 10 Furlongs | 0.0000000134472502221 au |
| 20 Furlongs | 0.0000000268945004442 au |
| 25 Furlongs | 0.0000000336181255553 au |
| 50 Furlongs | 0.0000000672362511106 au |
| 100 Furlongs | 0.000000134472502221 au |
| Reference | Furlongs (fur) | Astronomical units (au) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.00147638 fur | 1.98532 × 10-12 au |
| Average adult human height | 0.00845065 fur | 1.13638 × 10-11 au |
| A football pitch (length) | 0.521952 fur | 7.01882 × 10-10 au |
| A marathon | 209.75 fur | 0.000000282056 au |
| Height of Mount Everest | 43.9881 fur | 0.0000000591519 au |
The furlong is a unit of length equal to 220 yards, 660 feet, or exactly 201.168 metres. It is one eighth of a mile and ten chains.
The name is a contraction of the Old English furh lang, meaning furrow long, and describes the distance a team of oxen could plough before needing rest. That practical origin fixed the length of a medieval strip field, and the furlong therefore predates the mile's English definition. When the Roman mile of 5000 feet was reconciled with English land measurement, the mile was lengthened to 5280 feet so that it would contain a whole number of furlongs. The awkward figure that generations of schoolchildren have memorised is a direct consequence of preserving the older agricultural unit.
Horse racing is where the furlong remains in active daily use. Race distances in Britain, Ireland, the United States and much of the racing world are stated in miles and furlongs, so a race described as one mile two furlongs covers 2010 metres. Sectional times are recorded furlong by furlong, and commentary is built around the unit. Distance markers along the rail count down the remaining furlongs to the finish.
The furlong's relationship to area explains its persistence in older records. An acre was originally defined as one furlong long by one chain wide, the area a team could plough in a day. This is why an acre is a long thin rectangle rather than a square, and why its modern value of 43,560 square feet looks arbitrary.
Outside racing the unit is largely archaic, though it appears in some historical land deeds and in the occasional legal description. Computing folklore keeps it alive in the furlong per fortnight, a deliberately absurd unit of speed used to make a point about dimensional analysis.
It is the reason the acre has the shape it does. An acre was defined as one furlong long by one chain wide, 220 yards by 22, and that long thin rectangle is exactly what a team of oxen could plough in a day without turning more often than necessary. The proportion of ten to one is preserved in the ridge and furrow patterns still visible from the air across the English Midlands, and in field boundaries that have outlasted every plough that made them. The furlong therefore survives less as a measurement than as a shape written into the landscape.
One furlong equals 220 yards, 10 chains, one eighth of a mile, or 201.168 metres.
The astronomical unit is a length equal to exactly 149,597,870,700 metres, or very nearly 150 million kilometres. The symbol is au. It approximates the mean distance between the Earth and the Sun and is the standard measure for distances within the solar system.
The unit was originally defined by that orbital relationship rather than by a fixed number. Earlier definitions tied it to the properties of a hypothetical body orbiting the Sun, which meant its value depended on the gravitational constant and was subject to revision as measurements improved. The International Astronomical Union ended that dependence in 2012 by fixing the astronomical unit as an exact number of metres. The change simplified calculations and removed an inconvenience: a unit whose length shifted whenever a physical constant was refined.
Determining its value was one of the great problems of observational astronomy. Transits of Venus across the Sun's disc, observed from widely separated points on Earth, allowed the distance to be triangulated. Expeditions were mounted for the transits of 1761, 1769, 1874 and 1882, and James Cook's first Pacific voyage was organised around the 1769 event. Radar ranging to Venus in the 1960s eventually settled the figure far more precisely than any optical method.
The unit makes solar system distances legible. Mercury orbits at 0.39 au, Mars at 1.52 au, Jupiter at 5.2 au and Neptune at 30.1 au. The Kuiper Belt extends to roughly 50 au. Voyager 1, the most distant human-made object, has passed 165 au. Light takes about 499 seconds to cover one astronomical unit, so the Sun is a little over eight light minutes away.
Beyond the solar system the unit becomes unwieldy, and astronomers switch to light years and parsecs. One parsec is 206,265 au.
Spacecraft navigation depends on the unit being exact rather than approximate. Trajectories to the outer planets are computed over distances of tens of astronomical units, and an error in the length of the unit itself would propagate into every position calculation. Fixing the value in 2012 removed that source of drift from the ephemerides used for mission planning.
One astronomical unit equals 149,597,870.7 kilometres, or about 92.956 million miles.