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 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. 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 m/s (meters/second)
• one julian year = 365.25 days = 31557600 s
• 1ly = 9.460730409 * 10¹⁵m

Distances in Light-Years

• the average distance between earth and moon are about 1.3 light seconds
• the average distance between earth and sun are about 500 light seconds or 8.3 light minutes
• the diameter of out solar system is about 150 light hours
• the nearest star, Proxima Centauri, has a distance of about 4.2 light years
• the diameter of our galaxy is about 100,000 light years
• the distance to a galaxy called Andrromeda nebular is about 2.4 to 2.7 million light years