Members of the International Astronomical Union have voted to approve a change to the definition of the famous "astronomical unit" aka, AU, from one based on variable data, to a definite number, an exact 149,597,870,700 meters, which is the average mean distance between the Earth and sun when viewed from the Earth. This change is key because the old calculation violated Einstein's Theory of Relativity in that using it should have given different answers depending on where the measurement was taken, i.e. from different objects in the solar system.
The famous astronomer Giovanni Cassini first noted the position of Mars while standing in Paris and compared it to its angle in the sky when viewed from a site in South America at the same moment in time. Using parallax he was able to come up with a very close estimate of just how far the Earth was from Mars, and then the Earth from the sun, which he said should be about 140 million kilometers, which came to be used as the standard for describing distances in the visible solar system.
To make the metric more accurate, the AU calculation was changed in 1976 to include a tie to the Sun's mass, which didn't take into account the fact that the sun is gradually shrinking. Subsequent new developments in science and technology have led to much more precise ways to measure the distance between solar objects, which served to make using the old calculation even more obsolete.
Source: The Daily Galaxy via IAU and http://phys.org/news