Light-year

A light-year or lightyear, symbol ly, is the distance light travels in vacuum in one terrestrial year.

Numerical Value

A light-year is equal to approximately

9,460,528,404,879 km (about 9.461 Pm)
5,878,482,164,161 statute miles^*
63,239.7263 AU

The actual, exact length of the light-year depends on the length of the reference year used in the calculation, and there is no wide consensus on the reference to be used. The figures above are based on a reference year of 31,556,925.9747 seconds, but other reference years are often used, such that the light-year is not an appropriate unit to use when extremely high precision is required.

The light-year is often used to measure distances to stars. 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.

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. Since the speed of light is defined as 299,792,458 metres per second, a light-second is exactly 299,792,458 m in length and a light-minute is exactly 17,987,547,480 m. In contrast to the light-year, the lengths of the light-minute and light-second are fixed with 100% precision.

Miscellaneous facts

Reflected sunlight from the Moon's surface takes 1.3 seconds to travel the light years to Earth.
It takes 8.3 minutes for light to travel from the Sun to the Earth (a distance of light-years).
The most distant space probe, Voyager 1, was 13 light hours (only light years) away from Earth in September 2004. It took Voyager 27 years to cover that distance.
The nearest known star (other than the Sun), Proxima Centauri is 4.22 light years away.
The center of our galaxy, the Milky Way, is about 26,000 light years away. The Galaxy is about 100,000 light years across.
The Triangulum Galaxy (M33), at a bit under 2.6 million light years away, is the most distant object visible to the naked eye.
The nearest large galaxy cluster, the Virgo Cluster, is about 60 million light years away.
The particle horizon (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 (see Hubble's law).

External links

Conversion Calculator for Units of LENGTH

Light | Astronomical units of length

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Numerical Value

Miscellaneous facts

See also

External links