A light year, abbreviated ly, is the distance light travels in one year: roughly 9.46 × 1012 kilometres (9.46 petametres, or about 5.88 × 1012 miles). More specifically, a light year is defined as the distance that a photon would travel, in free space and infinitely far away from any gravitational or magnetic fields, in one Julian year (365.25 days of 86400 seconds each). Since the speed of light in a vacuum is exactly 299,792,458 m/s, one light year is exactly equal to 9,460,730,472,580,800 m.
The light year is often used to measure distances to stars: A light year is not a unit of time. In astronomy, the preferred unit of measurement for such distances is the parsec which is defined as the distance at which an object will generate one arcsecond of parallax when the observing object moved one astronomical unit. This is equal to approximately 3.26 light years. The parsec is preferred because it can be more easily derived from, and inter-compared with, observational data. However, outside scientific circles, the term light year is more widely used by the general public.
A light year is also equal to 63,241 astronomical units (AU). For a list of lengths on the order of one light year, see the article 1 E15 m.
Units related to the light year are the light minute and light second, the distance light travels in a vacuum in one minute and one second, respectively. A light minute is equal to 17,987,547,480 m. Since light travels 299,792,458 m in one second, a light second is 299,792,458 m in length.
- It takes 8.3 minutes for light to travel from the Sun to the Earth (thus, we are about 8.3 light minutes from the Sun).
- The most distant space probe, Voyager 1, was 12.5 light hours away from Earth in January 2004.
- The nearest known star, Proxima Centauri is 4.22 light years away.
- Our galaxy, the Milky Way, is about 100,000 light years across.
- The observable universe has a radius of about 13,700,000,000 light years (130 yottameters). The reason for this is that the most accurate estimate yet for the Big Bang has it at about 13.7 × 109 years ago, thus it is impossible to see any farther back in time. This radius is expanding in all directions at a rate of one light-second per second.
Caveat: Since our galaxy is 100,000 light years across, a hypothetical spaceship travelling close to the speed of light would need somewhat longer than 100,000 years to cross it. However, this is only true for an observer at rest with respect to the galaxy; the spaceship's crew can experience the trip across the galaxy in a matter of minutes. This is because of the time dilation of moving clocks explained by special relativity. On the other hand, the crew will experience a length contraction of the galaxy: from their viewpoint, the galaxy will appear to become strongly flattened.