| Meters (m) | Light years (ly) |
|---|---|
| 1 Meter | 1.05700083402 × 10-16 ly |
| 2 Meters | 2.11400166805 × 10-16 ly |
| 3 Meters | 3.17100250207 × 10-16 ly |
| 4 Meters | 4.2280033361 × 10-16 ly |
| 5 Meters | 5.28500417012 × 10-16 ly |
| 10 Meters | 1.05700083402 × 10-15 ly |
| 20 Meters | 2.11400166805 × 10-15 ly |
| 25 Meters | 2.64250208506 × 10-15 ly |
| 50 Meters | 5.28500417012 × 10-15 ly |
| 100 Meters | 1.05700083402 × 10-14 ly |
| Reference | Meters (m) | Light years (ly) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.297 m | 3.13929 × 10-17 ly |
| Average adult human height | 1.7 m | 1.7969 × 10-16 ly |
| A football pitch (length) | 105 m | 1.10985 × 10-14 ly |
| A marathon | 42195 m | 4.46002 × 10-12 ly |
| Height of Mount Everest | 8849 m | 9.3534 × 10-13 ly |
The metre is the base unit of length in the International System of Units, written m. Every other SI length unit is defined as a multiple or fraction of it, and units of area, volume, speed and many others are built from it in turn.
Its definition has been revised four times, each revision replacing a physical object with something more reproducible. The French Academy of Sciences proposed in 1791 that the metre should be one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris. Surveying that arc took six years and produced a small error, since the Earth is not a perfect sphere. In 1889 the definition moved to a platinum-iridium bar held at Sèvres, near Paris, with copies distributed to signatory nations. In 1960 it was redefined as 1,650,763.73 wavelengths of orange-red light emitted by krypton-86.
The current definition dates from 1983 and is unusual in that it fixes a different quantity. The speed of light in vacuum is defined as exactly 299,792,458 metres per second, and the metre follows as the distance light travels in 1/299,792,458 of a second. Light speed is therefore no longer measured; it is a defined constant, and improvements in measurement now refine the metre rather than the speed.
The unit is used worldwide for building dimensions, athletics tracks, swimming pools, water depth, fabric and rope. Only a small number of countries retain imperial units for general purposes, and even there the metre dominates science and medicine.
Everyday reference points help fix the scale. A standard interior door is roughly two metres tall. A single stride for an adult is close to three quarters of a metre, which is why pacing is a workable rough measure of distance. The width of a single traffic lane is between three and three and a half metres in most countries.
One metre equals 100 centimetres, 1000 millimetres, or 0.001 kilometres. It is approximately 3.28084 feet, or 39.3701 inches.
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.