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 chronography (recording the passage of time)

drum chronograph

Airy Transit Circle Chronograph, 1854

An astronomer making a star map takes two measurements: the angle at which the telescope is positioned to see the star cross the meridian and the time at which this happens. Traditionally the second measurement was estimated by listening to a clock facing the astronomer. With Airy's chronograph, the time was recorded electrically by tapping a key on the eyepiece of the transit circle.
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