How is the astronomical unit defined and measured today?

Today, the Astronomical Unit (AU) is defined with extreme precision as exactly 149,597,870.7 kilometers (about 93 million miles). This fixed number was officially adopted by the International Astronomical Union (IAU) in 2012 to standardize distance measurements in space.

In earlier times, the AU was measured indirectly using Kepler’s laws and observations of planetary motion. Scientists compared the relative distances of planets and used parallax methods (observing a planet from two positions on Earth at the same time) to estimate the scale of the solar system.

With the advent of radar and radio astronomy, more accurate measurements became possible. Scientists bounced radar waves off Venus and other planets, measured the time taken for the signals to return, and calculated distances using the speed of light. These measurements allowed them to fix the AU as a precise number rather than a variable value.

Now, spacecraft tracking and laser ranging confirm these distances with millimeter precision.