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Astronomers Fix the Astronomical Unit
gbrumfiel writes "The Astronomical Unit (AU) is known to most as the distance between the Earth and the Sun. In fact, the official definition was a much more complex mathematical calculation involving angular measurements, hypothetical bodies, and the Sun's mass. That old definition created problems: due to general relativity, the length of the AU changed depending on an observer's position in the solar system. And the mass of the Sun changes over time, so the AU was changing as well. At the International Astronomical Union's latest meeting, astronomers unanimously voted on a new simplified definition: exactly 149,597,870,700 meters. Nobody need panic, the earth's distance from the sun remains just as it was, regardless of whether it's in AUs, meters, or smoots."
Since the Earth's orbit around the Sun is eliptical it's _never_ the same, is it?
This is correct. originally the AU was defined as the average distance between the Earth and the Sun. The problem then was to convert this distance to meters. The way to do this conversion in the end involves the product of the mass of the Sun and the Gravitational constant G. Both quantities are not well known (e.g., G is known to 4 or 5 digits only). But their product can be determined from modeling the motions in the Solar system to much higher precision. So by that time the AU was then redefined by defining the product GM (often called k^2, where k is called the "Gaussian gravitational constant"). It is my understanding that this has now been simplified. The difference between both is only a few meters.
You're missing my point. 150GM wasn't "between" the old measurements. It was just kinda-somewhere-near them.
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