Light year

The light year (symbol: ly) is the distance that the light travels in 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 there are still several definitions dependig on the frame of reference. According to the recommendations of the International Astronomical Union (IAU) the siderial year (the time between two succeeding, identical positions of the sun) shall be used.

numerical value

• velocity of light in vacuum: 2.99792456 * 10⁸ m/s
• one siderial year = 365.25 days = 31557600 s
• 1ly = 9.460730409 * 10¹⁵m