Light year
A light-year, abbreviated ly, is the distance light travels in one year: roughly 9.46 trillion kilometers (or about 5.88 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 vacuum is 299,792,458 m/s, one light-year is approximately equal to 9.46 × 1015 m = 9.46 petameter.
The light-year is often used to measure distances to stars. A light-year is not a unit of time. In astronomy, the perferred unit of measure for such distances is the parsec which 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 about 3.26 light years. The parsec is preferred because it can more easily be derived from observations without including conversion terms, whose value is imprecisely known.
A light-year is also equal to 63,240 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 meters. Since light travels 299,792,458 meters in one second, a light-second is 299,792,458 meters in length.
Miscellaneous facts:
- It takes 8 minutes for light to travel from the Sun to the Earth (thus, we are about 8 light-minutes in distance away 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 100,000 light years across.
- The observable universe has a radius of about 15,000,000,000 light-years. The reason for this is that the big bang occurred about 15 billion years ago, and you can't see anything further 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 space ship's crew will 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.
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Referenced By
Andromeda Galaxy | List of astronomical topics | List of astronomical topics (N-Z) | NGC 224
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