Convert link (Gunter's link) to astronomical unit

The astronomical unit is the mean Earth–Sun distance, fixed at exactly 149,597,870.7 km: Mars orbits at 1.5 au, Neptune at 30. The light-year is the distance light travels in a Julian year, 9.46 trillion km, and the light-second 299,792 km, about three quarters of the way to the Moon. The parsec, 3.26 light-years, is the distance at which the Earth's orbit spans one second of arc, and is what astronomers actually use: the nearest star is 1.3 pc (4.2 ly) away, the centre of the Milky Way about 8 kpc.

Gunter's chain of 1620 tied land measure to the acre: a chain is 66 ft or 100 links, 10 chains make a furlong (the length of a furrow), 8 furlongs a mile, and an acre is one chain by one furlong. Rods (16½ ft, a quarter chain) and chains still appear in US and Canadian deeds, and furlongs in horse racing. The US survey foot, 1200/3937 m, differs from the international foot by two parts per million, which adds up over a state; it was retired for new work on 1 January 2023, though survey data and legal descriptions in survey feet remain.