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 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. 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.

Similar units are the light second, light minute, light hour, light day, light week, 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.

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

Comparison to other Units

• 1 parsec = 3.2616 ly
• 1 AU = 15.8×10^-6 ly
• 1 ly = 9.460730409 * 10¹² km
• 1 ly = 5.8786253 * 10¹⁵ mi
• 1 ly = 31.039141 * 10¹⁵ ft
• 1 ly = 10.346380 * 10¹⁵ yd

Distances in Light-Years

• the average distance between earth and moon is about 1.3 light seconds
• the average distance between earth and sun is about 500 light seconds or 8.3 light minutes
• the diameter of our solar system is about 150 light hours
• the Oort cloud is approximately 2 light years in diameter.
• 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 Andromeda nebular is about 2.4 to 2.7 million light years