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Not a gu 10 light bulb, a light year is in fact a unit of length which is the same as just under ten trillion kilometres. A light year, as defined by the International Astronomical Union, is the distance that light ravels within a vacuum in a Julian year. Often used as a means through which to measure distances on a galactic scale, the light-year is often employed in popular science fiction and in non-specialist publications. In astrometry, the preferred unit of distance is the parsec as it is more easily compared with, and derived from, observational data. A parsec is basically a distance from which an object will appear to move to the observer. The object will appear to move by one arcsecond of parallax if the observer themselves moves at one astronomical unit. The distance of a parsec is approximately 3.26 light-years.

Friedrich Bessel was the first to successfully measure the distance to a stat other than measuring the distance to the sun. Bessel successfully measured the distance to 61 Cygni in 1838 by using a superlative 6.2 inch heliometers that was designed by Joseph von Fraunhofer. In 1729, James Bradley had asserted that light travelled at a speed of 10,210 times faster than that of Earth. Venus’ transit was utilized in 1769 to reveal the distance of Earth from the Sun. This discovery, coupled with that of James Bradley, permitted the calculation of light’s speed. Under Bessel, it was worked out at what distance light would journey in a year, and the concept of a ‘light-year’ was finally articulated.