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personal equation

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A person's characteristic reaction time, or a correction for it, first investigated after an incident in which Neville Maskelyne (1732–1811), the eighth Astronomer Royal at Greenwich, reported in 1799 that he had found discrepancies averaging 0.8 second between his own observations of the transit times of stars across a hair-line, measured by counting the ticks of a pendulum clock, and those of his assistant, whom he had therefore fired, but in 1820 the German astronomer and mathematician Friedrich Wilhelm Bessel (1784–1846) followed up this report and discovered that even skilled astronomers vary consistently in the transit times that they report, and on the basis of this finding he introduced the personal equation for calibrating individuals to reflect these differences in what later came to be called reaction time. See also prior entry law.

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