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"Astronomical Unit," or Earth-Sun Distance, Gets an Overhaul
Without fanfare, astronomers have redefined one of the most important distances in the Solar System. The astronomical unit (au) — the rough distance from the Earth to the Sun — has been transformed from a confusing calculation into a single number. The new standard, adopted in August by unanimous vote at the International Astronomical Union's meeting in Beijing, China, is now 149,597,870,700 meters — no more, no less.
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Most recently, the au was defined as (take a deep breath): “the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.01720209895 radians per day (known as the Gaussian constant)”.
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The revised definition wipes away the problems of the old au. A fixed distance has nothing to do with the Sun’s mass, and the meter is defined as the distance traveled by light in a vacuum in 1 / 299,792,458 of a second. Because the speed of light is constant in all reference frames, the au will no longer change depending on an observer’s location in the Solar System.
I had always considered an AU to be the mean distance between the Earth and the Sun. I had no idea that it was so complex. The old definition was even tied to the mass of the sun through the Gaussian constant. Which meant that as the sun loses mass, the AU distance changes. The old definition also predates Einsteins general theory of relativity.
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