Nature: A vote at a meeting of the International Astronomical Union has redefined the astronomical unit, the au, as exactly 149 597 870 700 meters. From Giovanni Cassini’s measurement in 1672 until late in the 20th century, the au was defined as the length of the semimajor axis of Earth’s elliptical orbit around the Sun; its value was determined by parallax calculations. The most recent, more precise definition was “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).” That calculation has several flaws but had remained unchanged for many years because of concerns over how the change would affect software and other applications. The new, noncalculated value makes the unit much easier to explain to students, and no longer varies because of general relativity or the decreasing mass of the Sun.