| Light years (ly) | Micrometers (μm) |
|---|---|
| 1 Light year | 9.46073047258 × 1021 μm |
| 2 Light years | 1.89214609452 × 1022 μm |
| 3 Light years | 2.83821914177 × 1022 μm |
| 4 Light years | 3.78429218903 × 1022 μm |
| 5 Light years | 4.73036523629 × 1022 μm |
| 10 Light years | 9.46073047258 × 1022 μm |
| 20 Light years | 1.89214609452 × 1023 μm |
| 25 Light years | 2.36518261815 × 1023 μm |
| 50 Light years | 4.73036523629 × 1023 μm |
| 100 Light years | 9.46073047258 × 1023 μm |
| Reference | Light years (ly) | Micrometers (μm) |
|---|---|---|
| A sheet of A4 paper (long side) | 3.13929 × 10-17 ly | 297000 μm |
| Average adult human height | 1.7969 × 10-16 ly | 1700000 μm |
| A football pitch (length) | 1.10985 × 10-14 ly | 105000000 μm |
| A marathon | 4.46002 × 10-12 ly | 4.2195 × 1010 μm |
| Height of Mount Everest | 9.3534 × 10-13 ly | 8.849 × 109 μm |
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 micrometre is one millionth of a metre, written µm using the Greek letter mu. It is also widely called the micron, a name the General Conference on Weights and Measures formally abolished in 1967 but which remains in everyday use across manufacturing and materials science.
The unit covers the scale of individual cells and fine particles. A human red blood cell is about 7 µm across. Bacteria typically measure 1 to 5 µm. Human hair ranges from roughly 17 to 180 µm in diameter, which is why hair is visible while cells are not. The limit of unaided human vision falls near 50 µm.
Air quality regulation relies on the micrometre. PM10 and PM2.5 refer to particulate matter smaller than 10 µm and 2.5 µm respectively. The distinction matters medically: larger particles are filtered by the nose and throat, while PM2.5 penetrates deep into the lungs and can enter the bloodstream.
Precision engineering works in the same range. Machining tolerances are commonly specified in micrometres, and surface roughness is quoted in the same unit. Bearing clearances, paint film thickness and the flatness of optical components are all measured this way. The measuring instrument called a micrometer, or micrometer screw gauge, is named for the precision it offers rather than for a fixed relationship to the unit.
Wavelengths in the infrared are usually given in micrometres rather than nanometres. Thermal imaging cameras typically operate between 8 and 14 µm, the band where objects at everyday temperatures emit most strongly.
Filtration is specified almost entirely in this unit. Water filters are rated by the smallest particle they retain, commonly between 0.2 and 50 µm, and a 0.2 µm filter is fine enough to remove most bacteria. Surgical masks and respirators are tested against particles in the same range. The N95 designation refers to a filter that captures at least 95 per cent of airborne particles at the hardest size to trap, around 0.3 µm.
Its older name still causes confusion. Until 1967 the unit was officially called the micron and written with a bare Greek mu, and the word remains common in industry even though the conference of that year removed it from the SI. Semiconductor manufacturing carried the name for a generation: chips were described by their process node in microns, falling from ten in the 1970s to below one by 1990, at which point the industry switched to nanometres and kept going. The modern node names no longer correspond to any physical dimension on the chip, but the earlier ones did, and they were quoted in exactly this unit.
One micrometre equals 1000 nanometres, 0.001 millimetres, or 10-6 metres.