ConversionFrom.com > Unit dictionary > Light year

Light year (ly)

Length unit · Astronomical units

All units Length converter

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.

Light year conversion table

UnitSymbol1 Light year equals1 of these equalsConverter
ÅngströmÅ9.46073047258 × 1025 Å1.05700083402 × 10-26 lyLight years to Ångströms
Nanometernm9.46073047258 × 1024 nm1.05700083402 × 10-25 lyLight years to Nanometers
Micrometerμm9.46073047258 × 1021 μm1.05700083402 × 10-22 lyLight years to Micrometers
Millimetermm9.46073047258 × 1018 mm1.05700083402 × 10-19 lyLight years to Millimeters
Centimetercm9.46073047258 × 1017 cm1.05700083402 × 10-18 lyLight years to Centimeters
Decimeterdm9.46073047258 × 1016 dm1.05700083402 × 10-17 lyLight years to Decimeters
Meterm9.46073047258 × 1015 m1.05700083402 × 10-16 lyLight years to Meters
Decameterdam946073047258080 dam1.05700083402 × 10-15 lyLight years to Decameters
Hectometerhm94607304725808 hm1.05700083402 × 10-14 lyLight years to Hectometers
Kilometerkm9460730472581 km1.05700083402 × 10-13 lyLight years to Kilometers
Inchin3.72469703645 × 1017 in2.68478211842 × 10-18 lyLight years to Inches
Handhh9.31174259112 × 1016 hh1.07391284737 × 10-17 lyLight years to Hands
Footft3.10391419704 × 1016 ft3.22173854211 × 10-17 lyLight years to Feet
Yardyd1.03463806568 × 1016 yd9.66521562632 × 10-17 lyLight years to Yards
Chainch470290029854689 ch2.12634743779 × 10-15 lyLight years to Chains
Furlongfur47029002985469 fur2.12634743779 × 10-14 lyLight years to Furlongs
Milemi5878625373184 mi1.70107795023 × 10-13 lyLight years to Miles
Nautical milenmi5108385784331 nmi1.95756554461 × 10-13 lyLight years to Nautical miles
Leaguelea1959541791061 lea5.1032338507 × 10-13 lyLight years to Leagues
Nautical leaguenlea1702795261444 nlea5.87269663384 × 10-13 lyLight years to Nautical leagues
Astronomical unitau63241.0770843 au0.0000158125074098 lyLight years to Astronomical units
Parsecpc0.306601393786 pc3.26156377717 lyLight years to Parsecs