3 ms·

The Earth?

• tialaramex 8y ago
  Yes. In two ways. First, the assumption was that the Earth's rotational period was 86400 seconds. Mechanical clocks had given us no reason to doubt this was so. By the mid-20th century quartz timers were able to discern that the Earth has longer and shorter days, as numerous factors cause the spin to speed up or slow down. Instead of a single day the second was taken to be one 86400th part of an average day over the whole year. A few decades later the availability of atomic clocks made this seem silly and we uncoupled the second from the variably spinning Earth. The completely arbitrary seeming atomic clock definition of a second was basically chosen to be indistinguishable from our last guess at the "averaged over a year" second.