The light year (symbol: ly) is the distance that light travels in a vacuum in one year. Although one might think that one year is defined as the time for the Earth to complete one revolution of its orbit of the Sun, there are still several definitions depending on the frame of reference. According to the recommendations of the International Astronomical Union (IAU), the Julian year, which has a length of 365.25 days or 31,557,600 seconds, shall be used.
The light year is a unit of distance larger than an astronomical unit (AU). It is commonly used to measure interstellar distances, where other units of measure are too small. It is rarely used in interplanetary distances, because it is too large to be useful.
Also used are the light second, light minute, light hour, light day, light week and light month. As opposed to the light year, whose value more or less depends on the definition of year, the values of all these units are defined exactly.
- velocity of light in vacuum: 299,792,458 m/s  (meters/second)
- one Julian year = 365.25 days = 31,557,600 s
- 1 ly = 9,460,730,472,580,800 m
Comparison to other Units
- 1 parsec = 3.2616 ly
- 1 AU = 0.0000158 ly = 15.8×10-6 ly
- 1 ly = 0.30660 parsecs
- 1 ly = 63,241 AU
- 1 ly = 5,878,625,373,183.607 mi = 5.878625 * 1012 mi
- 1 ly = 9,460,730,472,580.800 km = 9.4607304 * 1012 km
- 1 ly = 10,346,380,656,803,149.606 yd = 10.346381 * 1015 yd
- 1 ly = 31,039,141,970,409,448.818 ft = 31.039142 * 1015 ft
Distances in light-years (or other light-time units)
- The maximum distance between the Earth and the Moon is about 1.3 light seconds.
- The maximum distance between the Earth and the Sun is about 499 light seconds or 8.3 light minutes.
- The diameter of our Solar System is about 150 light hours or about 6 light days.
- The hypothetical Oort cloud is approximately 2 light years in diameter.
- The nearest star, Proxima Centauri, is a distance of about 4.2 light years from the Sun.
- The diameter of our Milky Way Galaxy is about 100,000 light years.
- The distance to the naked-eye Andromeda Galaxy is about 2.4 to 2.7 million light years.
- Review of Particle Physics Particle Data Group: W.-M. Yao et al., J. Phys. G 33, 1 (2006).
- 1 mi = 1609.344 m
- 1 yd = 0.9144 m
- 1 ft = 0.3048 m