Light year/Talk

Article said light year was defined using the time the Earth takes to orbit the sun. It isn't -- its defined in terms of the Julian year (365.25 days of 86400 SI seconds each). The reason for this is that the time it takes for the Earth to orbit the sun can be known only to limited accuracy, and varies over time. Thus if the light year was defined based on that, it would not be a stable nor accurate measurement. (Although arguably the amount of inaccuracy and stability would be insignificant over the distances the light year is used to measure.)