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Currently, the international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms (see below).
Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human life spans.
They could be used to measure the hours even at night, but required manual upkeep to replenish the flow of water.
The Ancient Greeks and the people from Chaldea (southeastern Mesopotamia) regularly maintained timekeeping records as an essential part of their astronomical observations.
The French Republican Calendar's days consisted of ten hours of a hundred minutes of a hundred seconds, which marked a deviation from the 12-based duodecimal system used in many other devices by many cultures. A large variety of devices have been invented to measure time. An Egyptian device that dates to c.1500 BC, similar in shape to a bent T-square, measured the passage of time from the shadow cast by its crossbar on a nonlinear rule. At noon, the device was turned around so that it could cast its shadow in the evening direction.
Two contrasting viewpoints on time divide prominent philosophers.The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar.This Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year.Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.
Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy.Lunar calendars were among the first to appear, either 12 or 13 lunar months (either 354 or 384 days).