Al-Biruni, an 11th-century Persian scholar, was the first to use seconds to measure time, dividing a day into hours, minutes and seconds, but also thirds (one-60th of a second) and fourths (one-60th of a third). As such a second was first defined as one-86,400th of an average solar day. With the advent of ultra-accurate atomic clocks, scientists Louis Essen and William Markowitz converted the second into atomic time, using the frequency of radiation emitted by the caesium-133 atom as a yardstick. This established the standard measure of a second, officially adopted in 1967 and now used worldwide.
Answered by Alex Cheung.