Measuring the absolute value of the AU

[Alvin]

How can they be sure that the value for the AU is 149,597,870 km? What methods are used to measure it?

 

[neutrino]

An Astronomical Unit is approximately the mean distance between the Earth and the Sun. It is a derived constant and used to indicate distances within the solar system. Its formal definition is the radius of an unperturbed circular orbit a massless body would revolve about the sun in 2*(pi)/k days (i.e., 365.2568983... days), where k is defined as the Gaussian constant exactly equal to 0.01720209895. Since an AU is based on radius of a circular orbit, one AU is actually slightly less than the average distance between the Earth and the Sun (approximately 150 million km or 93 million miles).

http://neo.jpl.nasa.gov/glossary/au.html