If you guessed the astronomical unit, you are correct. The au — the rough distance between the Earth and the Sun — was previously determined by a confusing calculation ("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"). But thanks to a unanimous vote at the International Astronomical Union's meeting in Beijing, China, the astronomical unit is now one single, tidy number.
Writing in Scientific American, Geoff Brumfiel explains why the change was made:
As its name implies, general relativity makes space-time relative, depending on where an observer is located. The au, as formerly defined, changed as well. It shifted by a thousand meters or more between Earth's reference frame and that of Jupiter's, according to [Sergei] Klioner. That shift did not affect spacecraft, which measure distance directly, but it has been a pain for planetary scientists working on Solar System models.
The Sun posed another problem. The Gaussian constant is based on Solar mass, so the au was inextricably tied to the mass of the Sun. But the Sun is losing mass as it radiates energy, and this was causing the au to change slowly as well.
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.
Redefining the au has been possible for decades - modern astronomers can use spacecraft, radars and lasers to make direct measurements of distance. But "some of them thought it was a little bit dangerous to change something," says Nicole Capitaine, an astronomer at the Paris Observatory in France. Some feared the change might disrupt their computer programs, others held a sentimental attachment to the old standard. But after years of lobbying by Capitaine, Klioner and others, the revised unit has finally been adopted.
Top image via NASA.