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 of the Sun, 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.

The light year is a unit of distance larger than an astronomical unit. 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.

numerical value

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