Astronomical unit was changed in 2012
In astronomy work with gigantic distances. And some of them have been forced to conclude, as the distances may vary – when the celestial bodies moving slightly relative to each other. So it is with an astronomical unit. It is defined as the distance between Earth and the sun.
But because the Earth’s orbit is slightly elliptical, the distance is not constant. Therefore, an astronomical unit is the average distance between the two, but it was only in 2012 that the International Astronomical Union decided that an astronomical unit equals 149 597 870 700 meters.
Siriometer as astronomical unit
In the early 1800s experimented German-born British astronomer William Hershel (1738-1822) with an even larger unit – the distance between the Earth and the brightest star, Sirius. He tried to calculate the distance to Sirius by comparing its brightness with the other stars, and he defined distance as a siriometer.
But since the differences in how strongly the stellar light also goes together with their different size, his calculation is not very accurate. We now know that the distance from the Earth to Sirius is 8.6 light years.
But his siriometer survived anyway as the unit of measurement. Various astronomers worked on various measurements and today has a siriometer come to correspond to one million astronomical units, which is equivalent to 15.813 light years.