| Nautical miles (nmi) | Light years (ly) |
|---|---|
| 1 Nautical mile | 1.95756554461 × 10-13 ly |
| 2 Nautical miles | 3.91513108923 × 10-13 ly |
| 3 Nautical miles | 5.87269663384 × 10-13 ly |
| 4 Nautical miles | 7.83026217845 × 10-13 ly |
| 5 Nautical miles | 9.78782772307 × 10-13 ly |
| 10 Nautical miles | 1.95756554461 × 10-12 ly |
| 20 Nautical miles | 3.91513108923 × 10-12 ly |
| 25 Nautical miles | 4.89391386153 × 10-12 ly |
| 50 Nautical miles | 9.78782772307 × 10-12 ly |
| 100 Nautical miles | 1.95756554461 × 10-11 ly |
| Reference | Nautical miles (nmi) | Light years (ly) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.000160367 nmi | 3.13929 × 10-17 ly |
| Average adult human height | 0.000917927 nmi | 1.7969 × 10-16 ly |
| A football pitch (length) | 0.0566955 nmi | 1.10985 × 10-14 ly |
| A marathon | 22.7835 nmi | 4.46002 × 10-12 ly |
| Height of Mount Everest | 4.77808 nmi | 9.3534 × 10-13 ly |
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 light year is the distance light travels in vacuum during one Julian year, equal to exactly 9,460,730,472,580,800 metres, or about 9.46 trillion kilometres. The symbol is ly.
It is a unit of distance, not of time, a point the name obscures and which causes persistent confusion. Its exactness follows from two defined quantities: the speed of light is fixed at 299,792,458 metres per second, and a Julian year is defined as exactly 365.25 days of 86,400 seconds. Multiplying the two gives a figure with no measurement uncertainty at all.
The unit's appeal is that it makes the finite speed of light explicit. Looking at an object twelve light years away means seeing it as it was twelve years ago, and this delay is not a curiosity but the basis of observational cosmology. The most distant galaxies observed lie billions of light years away, so telescopes are effectively instruments for looking into the past.
Nearby distances are modest by the standard of the unit. Proxima Centauri, the closest star to the Sun, is 4.25 light years away. Sirius is 8.6. The centre of the Milky Way lies about 26,000 light years from Earth, and the galaxy spans roughly 100,000. The Andromeda Galaxy is some 2.5 million light years distant and is visible to the unaided eye from a dark site, which makes it the most remote object most people will ever see directly.
Professional astronomers generally prefer the parsec, which arises naturally from parallax measurement, and journals report distances in parsecs, kiloparsecs and megaparsecs. The light year dominates popular writing because it needs no explanation beyond the speed of light.
Related units follow the same principle at smaller scales. The light second, about 300,000 kilometres, is close to the Earth-Moon distance, and radio engineers use the light nanosecond, roughly 30 centimetres, when reasoning about signal propagation along a cable. The light minute and light hour occasionally appear in descriptions of the outer solar system.
One light year equals 63,241 astronomical units, about 0.3066 parsecs, or 9.4607 trillion kilometres.