Since 2012 the Earth-Sun yardstick has been a fixed number of metres
The astronomical unit was once the average distance between Earth and the Sun, a figure astronomers kept refining. In 2012 the International Astronomical Union simply froze it at exactly 149,597,870,700 metres. Light covers that stretch in roughly 499 seconds, and Earth's actual distance swings a few percent either side.
Ancient estimates missed badly because the Sun is about 12,000 Earth diameters away, so tiny errors in Earth's size balloon. Around 280 BCE Aristarchus measured the angle between Moon and Sun at first quarter as 87 degrees; the truth is almost 89.9, so he put the Sun only 18 to 20 times farther than the Moon instead of nearly 390. Hipparchus offered 490 Earth radii, and China's Zhoubi Suanjing computed the distance from noon shadows at sites 1,000 li apart while assuming a flat Earth.
Precision mattered because the unit underpins star distances measured by parallax, the apparent shift of nearby stars as Earth circles the Sun. In 1976 the IAU defined the unit through celestial mechanics, tying it to a gravitational constant with the value 0.01720209895. That route suited planetary tables, since astronomers know the Sun's mass multiplied by the gravitational constant very accurately even though neither factor can be measured precisely alone.
Space probes then bounced radar and radio signals off planets, turning light travel times into distances, and in 2009 the IAU reported light-time for one unit to about a dozen decimal places. Relativity complicated matters: clocks on Earth tick at slightly different rates across the year as our distance from the Sun varies between about 0.983 and 1.017 units, so the 1976 definition lacked a frame of reference. After vigorous debate, the IAU adopted the fixed metre value in August 2012, based on the 2009 estimate.
Symbols wandered too, from A in 1976 to the ua the French-inspired bureau of weights and measures recommended in 2006, before the IAU settled on au. The unit suits distances inside planetary systems, like an asteroid's distance from the Sun, and keeps computer simulations of the Solar System free of rounding troubles. For interstellar space professionals use the parsec, itself defined from the astronomical unit, while the popular light-year is not an approved unit.
Source: Astronomical unit
