The first documentation of human reaction time as a scientific variable comes from astronomy. In 1820, German astronomer Friedrich Bessel applied himself to the problem of accuracy in recording stellar transits, which was typically done by using the ticking of a metronome to estimate the time at which a star passed the hairline of a telescope. Bessel noticed timing discrepancies under this method between records of multiple astronomers, and sought to improve accuracy by taking these individual differences in timing into account. This led to various astronomers to seek out ways to minimize these differences between individuals, which came to be known as the "personal equation" of astronomical timing.