| Light years (ly) | Inches (in) |
|---|---|
| 1 Light year | 3.72469703645 × 1017 in |
| 2 Light years | 7.4493940729 × 1017 in |
| 3 Light years | 1.11740911093 × 1018 in |
| 4 Light years | 1.48987881458 × 1018 in |
| 5 Light years | 1.86234851822 × 1018 in |
| 10 Light years | 3.72469703645 × 1018 in |
| 20 Light years | 7.4493940729 × 1018 in |
| 25 Light years | 9.31174259112 × 1018 in |
| 50 Light years | 1.86234851822 × 1019 in |
| 100 Light years | 3.72469703645 × 1019 in |
| Reference | Light years (ly) | Inches (in) |
|---|---|---|
| A sheet of A4 paper (long side) | 3.13929 × 10-17 ly | 11.6929 in |
| Average adult human height | 1.7969 × 10-16 ly | 66.9291 in |
| A football pitch (length) | 1.10985 × 10-14 ly | 4133.86 in |
| A marathon | 4.46002 × 10-12 ly | 1661220 in |
| Height of Mount Everest | 9.3534 × 10-13 ly | 348386 in |
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.
The inch is a unit of length equal to exactly 25.4 millimetres. It is one twelfth of a foot and one thirty-sixth of a yard. The symbol is in, though a double prime is common in technical contexts and a straight double quote is often substituted informally.
The name comes from the Latin uncia, meaning a twelfth part, which also gave English the word ounce. Early definitions were anatomical or agricultural. A statute of Edward II defined the inch as three barleycorns laid end to end, and the width of a man's thumb served as a working approximation in several European languages. Dutch and Swedish still use words for inch that mean thumb.
Its modern precision dates from 1959, when the International Yard and Pound Agreement fixed the yard at exactly 0.9144 metres across the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa. Before that agreement the American and British inches differed very slightly, which mattered in precision manufacturing. The United States retained a separate survey inch for land measurement until it was formally retired at the end of 2022.
The inch remains dominant in several international industries regardless of local measurement systems. Screen sizes are quoted in inches worldwide, measured diagonally. Tyre rim diameters, bicycle wheel sizes, plumbing pipe bores and rack-mounted equipment all use inch designations, and the standard 19-inch rack is a global fixture. Photographic sensor sizes use inch fractions that no longer correspond to any real dimension, a survival from television camera tube measurements.
Subdivision is traditionally binary rather than decimal. An inch is halved repeatedly into eighths, sixteenths and thirty-seconds, which suits ruler markings and fits the fractional sizing of drill bits and spanners. Decimal inches are used in machining, where thousandths of an inch are called thou or mils.
Screens and printing keep it in daily use far outside the countries that use it for anything else. Televisions, monitors and phones are sold by the diagonal in inches everywhere, so a 55-inch television has the same name in Paris as in Chicago. Typography rests on the same base: there are 72 points to the inch, a convention inherited from the printing trade and written into every computer font system, and image resolution is quoted in dots or pixels per inch. Anyone laying out a page in a country that has been metric for a century is therefore working, without particular thought, in twelfths and seventy-seconds of an inch.
One inch equals 25.4 millimetres, 2.54 centimetres, or 0.0254 metres exactly.