| Nautical miles (nmi) | Hands (hh) |
|---|---|
| 1 Nautical mile | 18228.3464567 hh |
| 2 Nautical miles | 36456.6929134 hh |
| 3 Nautical miles | 54685.0393701 hh |
| 4 Nautical miles | 72913.3858268 hh |
| 5 Nautical miles | 91141.7322835 hh |
| 10 Nautical miles | 182283.464567 hh |
| 20 Nautical miles | 364566.929134 hh |
| 25 Nautical miles | 455708.661417 hh |
| 50 Nautical miles | 911417.322835 hh |
| 100 Nautical miles | 1822834.64567 hh |
| Reference | Nautical miles (nmi) | Hands (hh) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.000160367 nmi | 2.92323 hh |
| Average adult human height | 0.000917927 nmi | 16.7323 hh |
| A football pitch (length) | 0.0566955 nmi | 1033.46 hh |
| A marathon | 22.7835 nmi | 415305 hh |
| Height of Mount Everest | 4.77808 nmi | 87096.5 hh |
The nautical mile is a unit of length equal to exactly 1852 metres. The symbol is nmi, though NM and M also appear in maritime and aviation practice. It is longer than the statute mile by about 15 per cent.
Its definition is geographical rather than arbitrary. One nautical mile corresponds to one minute of latitude, so sixty nautical miles span one degree. This makes navigation on a chart remarkably direct: a navigator can measure a distance against the latitude scale printed on the chart's edge without any conversion, because the two are the same thing. No other length unit has this property, which is why the nautical mile has resisted metrication where almost every other traditional unit has given way.
Because the Earth is slightly flattened, a minute of latitude is not constant, varying from about 1843 metres at the equator to 1862 metres at the poles. The First International Extraordinary Hydrographic Conference in Monaco adopted the round figure of 1852 metres in 1929 to settle the discrepancy. The United States accepted this value in 1954 and the United Kingdom in 1970, both having previously used slightly different definitions.
Speed at sea and in the air is measured in knots, one knot being one nautical mile per hour. The name records the method: a log line knotted at regular intervals was paid out behind a ship and the knots counted against a sand glass. Aviation adopted both units from maritime practice, and aircraft airspeed indicators worldwide read in knots.
Maritime law is written in the same unit. Territorial waters extend twelve nautical miles from the baseline and exclusive economic zones two hundred, figures set by the United Nations Convention on the Law of the Sea.
A related unit occasionally appears in older texts. The geographical mile is one minute of arc along the equator, about 1855 metres, and differs slightly from the nautical mile because it is measured on a great circle rather than a meridian. The distinction rarely matters in practice but explains small discrepancies between historical sources.
One nautical mile equals 1852 metres, about 1.15078 statute miles, or 6076.12 feet.
The hand is a unit of length equal to exactly four inches, or 101.6 millimetres. It survives in one specialised application: measuring the height of horses and ponies. Almost nowhere else is it still in use.
The measurement is taken from the ground to the withers, the ridge between the shoulder blades where the neck meets the back. The withers are chosen because they are a fixed skeletal point that does not move as the horse raises or lowers its head, so the figure is reproducible in a way that measuring to the top of the head would not be.
Notation is unusual and frequently misread. Heights are written with a decimal point that does not indicate a decimal fraction. A horse of 15.2 hands stands fifteen hands and two inches, which is 62 inches or 157.5 centimetres. Because a hand is four inches, the digit after the point runs only from 0 to 3, and 15.4 is not a valid height. The abbreviation hh, for hands high, is common.
The boundary between a pony and a horse is conventionally set at 14.2 hands, though the exact threshold varies between breed registries and competitive disciplines. Draught breeds such as the Shire commonly exceed 17 hands. The tallest horses on record have stood over 20 hands.
The unit derives from the breadth of a human palm, an origin shared with several ancient measures. Egyptian and Roman systems both used a palm of comparable size. Its standardisation at four inches in England is usually credited to a statute of Henry VIII. The persistence of the hand in equestrian use is largely conservatism, and metric heights in centimetres are now given alongside it in international competition rules.
Measuring accurately is harder than it appears. The horse must stand square on level ground with its head in a natural position, and a purpose-made measuring stick with a spirit level and sliding crossbar is used rather than a tape. Shoeing adds height, so official measurements specify whether the horse is shod, and competition rules that impose height limits require certification by an appointed measurer.
One hand equals 4 inches, 101.6 millimetres, or 0.1016 metres.