What Is the Distance of One Astronomical Unit?

An astronomical unit (AU) is the distance between the Earth and its sun, but until 2012, the actual number wasn't fixed. Instead, astronomers relied on a complex formula based on several measurements for determining the distance between the two celestial bodies. In August 2012, the International Astronomical Union set the distance at 149,597,870.7 kilometers (about 93 million miles).

More about astronomical measurements:

  • The reasons for the difficulty in coming up with a fixed number included general relativity, the decreasing mass of the Sun and the reluctance of some astronomers to initiate changes.

  • The first recorded measurement of the Earth-Sun distance was established in 1672. Researchers at that time calculated the distance to be 87 million miles (140 million km), which was actually pretty accurate.

  • Some people have noted that the establishment of a fixed astronomical unit creates the need for updates in computer programs that use an earlier AU measurement.

More Info: www.iau.org

Discussion Comments


@anon312449 - yes, essentially center to center. average and mean mean the same thing - but I don't think that's how the astronomer's would characterize how they came to this specific number. I think it's more complex than that. They say it's the best estimate of the original expression.


Is this distance from the center of the earth to the center of the sun? Is this an average or mean of all the different distances throughout the year?

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