| Centimeters (cm) | Light years (ly) |
|---|---|
| 1 Centimeter | 1.05700083402 × 10-18 ly |
| 2 Centimeters | 2.11400166805 × 10-18 ly |
| 3 Centimeters | 3.17100250207 × 10-18 ly |
| 4 Centimeters | 4.2280033361 × 10-18 ly |
| 5 Centimeters | 5.28500417012 × 10-18 ly |
| 10 Centimeters | 1.05700083402 × 10-17 ly |
| 20 Centimeters | 2.11400166805 × 10-17 ly |
| 25 Centimeters | 2.64250208506 × 10-17 ly |
| 50 Centimeters | 5.28500417012 × 10-17 ly |
| 100 Centimeters | 1.05700083402 × 10-16 ly |
| Reference | Centimeters (cm) | Light years (ly) |
|---|---|---|
| A sheet of A4 paper (long side) | 29.7 cm | 3.13929 × 10-17 ly |
| Average adult human height | 170 cm | 1.7969 × 10-16 ly |
| A football pitch (length) | 10500 cm | 1.10985 × 10-14 ly |
| A marathon | 4219500 cm | 4.46002 × 10-12 ly |
| Height of Mount Everest | 884900 cm | 9.3534 × 10-13 ly |
The centimetre is one hundredth of a metre, written cm. Among the metric subdivisions it is the one people reach for most readily in daily life, occupying the range where objects are small enough to hold but too large to measure comfortably in millimetres.
Body measurement is its most common application. Height, waist, chest and inside leg are recorded in centimetres across most of the world, and clothing is sized accordingly. Paediatric growth charts plot height and head circumference in centimetres against age. Rainfall over long periods, snow depth and the dimensions of furniture and luggage are all reported the same way.
The unit had a formal role in science for nearly a century. The centimetre-gram-second system, adopted by the British Association for the Advancement of Science in 1874, took the centimetre as its base unit of length. CGS units such as the erg, the dyne and the gauss were standard in physics until the metre-kilogram-second system displaced them, and SI formally superseded CGS in 1960. Some CGS units persist in astronomy and in parts of electromagnetism.
Volume follows naturally. A cubic centimetre, written cm3 or cc, equals exactly one millilitre. Engine displacement is often quoted in cubic centimetres, particularly for motorcycles, and medical syringes are marked the same way.
Despite its usefulness, the centimetre sits awkwardly in engineering practice. Technical drawings prefer millimetres precisely to avoid mixing units that differ by a factor of ten, since a misplaced decimal point between the two is a plausible and expensive error.
Map scales often make the unit explicit. A 1:25,000 map means one centimetre on the paper represents 25,000 centimetres on the ground, or 250 metres, so four centimetres cover a kilometre. Walkers and orienteers use this relationship constantly, and it is one of the clearer illustrations of why a decimal system is convenient: converting between the two scales requires only moving a decimal point.
Two well-known wavelengths fall in this range and are named by it. Neutral hydrogen radiates at 21 centimetres, a line predicted in 1944 and detected seven years later, and because hydrogen fills the galaxy that single wavelength has mapped the spiral arms of the Milky Way and the rotation curves that first indicated dark matter. Domestic microwave ovens work at 12.2 centimetres, chosen from a band set aside for industrial and medical use rather than for any special resonance with water. In both cases the centimetre is the natural unit because the wave is the size of a hand.
One centimetre equals 10 millimetres or 0.01 metres, and is very close to 0.3937 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.