History
The first successful measurement of the distance to a star other than our Sun was made by Friedrich Bessel in 1838. The star was 61 Cygni, and he used a superlative 6.2-inch (160 mm) heliometer designed by Joseph von Fraunhofer. The largest unit for measuring distances across space at that time was the astronomical unit (AU), equal to the radius of the Earth's orbit (1.50×108 km; 9.30×107 mi). The use of this unit in trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, gave the distance to the star as 660 thousand astronomical units (9.9×1013 km; 6.1×1013 mi). Bessel realised that a much larger unit of measurement was needed to make the vast interstellar distances comprehensible.
James Bradley stated in 1729 that light travelled 10210 times faster than the Earth in its orbit. In 1769, a transit of Venus revealed the distance of the Earth from the Sun, and this, together with Bradley's figure, allowed the speed of light to be calculated as 3.01×108 m/s, very close to the modern value.
Bessel used this speed to work out how far light would travel in a year, and announced that the distance to 61 Cygni was 10.3 light-years. This was the first appearance of the light-year as a measurement of distance, and, although modern astronomers prefer the parsec, it is popularly used to gauge the expanses of interstellar and intergalactic space.
Read more about this topic: MLY
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