The light year is a measure of length outside the International System of Units.


The light-year is an astronomical unit of length. It is often used to identify cosmic distances in astronomical outreach. In the scientific environment, however, is expected in parsecs.


A light year is the distance traveled by an electromagnetic wave such as light travels in one Julian year in a vacuum. These are 9,460 trillion kilometers ( 9,460 · 1012 km ).

There are a plurality of definitions for a year. So there is the tropical year, the Gregorian year, the Julian year and the sidereal year. These soft up to 0.005 percent from each other, which at more precise information leads to divergences. Therefore, the International Astronomical Union ( IAU) has recommended to design a " year " without specifying the julian year ( = exactly 365.25 days ). This, together with the speed of light in vacuum, the light year is exactly defined. Since the meter in the SI is defined by the speed of light ( 299,792,458 - the-art part of a light-second ) corresponds to a light-year an exact integer number of meters.

Similar to the light-year, the units of light seconds, light minutes, light-hour light-day and defined. These are defined exactly on the speed of light ( the definition of the meter in the International System of Units respectively).

This corresponds to

  • A light-second (1 Ls): 299 792.458 km ≈ 300,000 km = 300 million meters
  • A light-minute ( ch 1 ): 17 987 547.48 km ≈ 18 million km = 18 billion meters
  • A light-hour (1 Lh): 1,079,252 848.8 km ≈ km = 1.08 trillion meters
  • A light day (1 Ld): 25,902,068 371.2 km ≈ km = 26,000,000,000,000 meters
  • One light year (1 Lj ): 9,460,730,472,580.8 km ≈ 9.5 trillion km = 9.5 quadrillion meters

A light year is the addition as:

The unit light-year is different decimal multiples (SI - prefixes ) in use, for example KLJ or MLJ.


  • The average distance from Earth and the Moon is about 1.3 light-seconds.
  • The Earth is from the Sun on average about 500 light-seconds, or 8.3 light-minutes.
  • The mean distance between the Sun and Neptune is about 4.17 light-hours.
  • The sun nearest star, Proxima Centauri, is about 4.2 light years away.
  • The diameter of our galaxy, the Milky Way is about 100,000 light-years.
  • The distance to the nearest large galaxy, the Andromeda galaxy, is approximately 2.4-2.7 million light years.
  • The Voyager 1 spacecraft, which launched in 1977, 2013, reached a distance from the Earth of about 18 billion kilometers, which is about 18 light-hours (or about 1/ 500 light- years ) and left the heliosphere.