The astronomical unit (AU) measures the relatively short distances within our solar system.
One AU is the mean distance from Earth to the Sun, about 93 million miles.
We can say that Pluto’s average distance from the sun is 40 AUs rather than saying it is 3.72 billion miles.
The units of light-years and parsecs take care of the even greater distances beyond the border of our solar system.
In the 2nd century CE, Ptolemy estimated the mean distance of the sun as 1,210 times the Earth radius.
Ptolemy determined this value by measuring the Moon’s parallax.
