light-year

A light-year is the distance that light travels in a year. The count of light-years to some object under observation also represents how far into the past this view of the object represents, i.e., if a star is ten light-years away, then when we see it, our view is of that star's appearance ten years ago. In the science of astronomy/astrophysics, the unit light-year is often used for communications to the public, but the unit parsec (pc) is generally used in scientific calculations and communication. Approximate equivalences:

• 5.88 × 10¹² miles.
• 9.46 × 10¹² km.
• 63241 AU.
• 0.3066 pc.

The International Astronomical Union (IAU) has standardized a precise definition of the light-year: the speed of light in a vacuum (c) times a Julian year, 365.25 days of standard (SI) seconds. This distance is an exact number of meters because the standard meter is defined in relation to c and the standard second: 9,460,730,472,580,800 meters.