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A light-year or lightyear (symbol: ly) is a unit of measurement of length, specifically the distance light travels in a vacuum in one year.

While there is no authoritative decision on which year is used, the International Astronomical Union (IAU) recommends the Julian year.

A light-year is equal to 9,460,730,472,580.8 km (about 9.461 Pm; 5,878,625,373,183.61 statute miles; about 63,240 astronomical units or about 0.3066 parsecs.

The 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 exactly 365.25 days (each of exactly 86,400 SI seconds).

A few examples of distances for light to travel are: Reflected sunlight from the Moon's surface takes 1.3 seconds to travel the 4.04 × 10-8 light years to Earth.

It takes 8.3 minutes for light to travel from the Sun to the Earth (a distance of 1.58 × 10-5 light-years).

The most distant space probe, Voyager 1, was 13 light hours (only 1.5 × 10-3 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.

Note:   The above text is excerpted from the Wikipedia article "Light-year", which has been released under the GNU Free Documentation License.
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