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 (nearly six trillion) 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 by the definition of metre, 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 perpendicular to the line of sight to the observer. 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.
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 (a distance of Template:Sn light years).
- The most distant space probe, Voyager 1, was 13 light hours (only Template:Sn light years) away from Earth in September 2004. It took Voyager 27 years to cover that distance.
- The nearest known star, Proxima Centauri is 4.22 light years away.
- The center of our galaxy, the Milky Way, is about 28,000 light years away. The Galaxy is about 100,000 light years across.
- The nearest large galaxy cluster, the Virgo Cluster, is about 60 million light years away.
- The observable part of the universe has a radius of about 46 billion light years, but light from the edge of the observable universe was emitted only 13.7 billion years ago (the age of the universe). The figures differ because distant objects have continued to recede from us due to cosmological expansion.
- One gigaparsec is equal to approximately 3.2 billion light years.
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