Measuring the absolute value of the AU

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The Astronomical Unit (AU) is defined as approximately 149,597,870 km, representing the mean distance from the Earth to the Sun. This value is derived from the radius of a theoretical circular orbit that a massless body would follow around the Sun in 365.2568983 days, using the Gaussian constant. Measurement methods include radar ranging and observations of planetary transits, which help refine the AU's accuracy. While the AU is a standard reference for distances in the solar system, it is slightly less than the average distance due to orbital eccentricities. Understanding the AU is crucial for astronomical calculations and navigation within our solar system.
Alvin
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How can they be sure that the value for the AU is 149,597,870 km? What methods are used to measure it?
 
Astronomy news on Phys.org
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
 

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