A moment (momentum) was a medieval unit of time. The movement of a shadow on a sundial covered 40 moments in asolar hour. An hour in this case means one twelfth of the period between sunrise and sunset. The length of a solar hour depended on the length of the day, which in turn varied with the season, so the length of a moment in modern seconds was not fixed, but on average, a moment corresponds to 90 seconds: A day was divided into 24 hours (of unequal lengths, twelve hours of the day and the night each), and an hour was divided into four puncta (quarter-hours), ten minuta and 40 momenta.
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Source: Ultra Facts