At the beginning of the nineteenth century, astronomers knew how far away some objects in the solar system are, but not how far away any of the stars are. During the nineteenth century they measured accurate positions for thousands of stars and...

The astronomical unit (AU or au or a.u. or sometimes ua) is a unit of length approximately equal to the distance from the Earth to the Sun. The currently accepted value of the AU is 149 597 870 691 ± 30 metres (nearly 150 million...