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A light-year, also written light year, is both a unit of length and time that light travels in one standard Earth year (365.25 days). Its symbol is ly, an abbreviation of "light-year". The vast distances are sometimes measured by astronomers the light year in parsecs (Parallex second). A light year is a distance of exactly 9,460,730,472,580.8  kilometres.  This is exactly 5,878,623,554,478 miles, 22 feet, 6.978048 inches  (about 5.9 trillion  miles).

Light travels at a rate of exactly 299,792,458 meters per second. This is just short of 300 kilometres per second and a little over 184,000 miles per second. The calculations above are based on the standard day of 24 hours, or 1/365.25 of a year. This is 31,557,600 seconds travelled in the standard year by light in a vacuum.

History

The first successful measurement of a light year was in 1838 by German astronomer and mathematician, Friedrich Bessel. After a transit of Venus in 1759 had made an accurate measurement of the distance between the sun and earth, a unit known as the "astronomical unit" had been used to measure space. Using this distance (around 93 million miles), had simplified measurements to stars based on parallax calculations.

A parallax is the apparent distance a stationery object moves when viewed from opposite ends of a measured distance. This is easily visualized by holding one's thumb up to cover an object at a distance and switching between closed eyes to view said object.  In like manner, distances to stars within about 1,600 light years are now (since 1989) able to be ascertained.  Further developments are hoped to increase the accurate measurement ten-fold.

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