<P> Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy . Periodic events and periodic motion have long served as standards for units of time . Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart . 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 . </P> <P> Generally speaking, methods of temporal measurement, or chronometry, take two distinct forms: the calendar, a mathematical tool for organising intervals of time, and the clock, a physical mechanism that counts the passage of time . In day - to - day life, the clock is consulted for periods less than a day whereas the calendar is consulted for periods longer than a day . Increasingly, personal electronic devices display both calendars and clocks simultaneously . The number (as on a clock dial or calendar) that marks the occurrence of a specified event as to hour or date is obtained by counting from a fiducial epoch--a central reference point . </P> <P> Artifacts from the Paleolithic suggest that the moon was used to reckon time as early as 6,000 years ago . Lunar calendars were among the first to appear, either 12 or 13 lunar months (either 354 or 384 days). Without intercalation to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months . Lunisolar calendars have a thirteenth month added to some years to make up for the difference between a full year (now known to be about 365.24 days) and a year of just twelve lunar months . The numbers twelve and thirteen came to feature prominently in many cultures, at least partly due to this relationship of months to years . Other early forms of calendars originated in Mesoamerica, particularly in ancient Mayan civilization . These calendars were religiously and astronomically based, with 18 months in a year and 20 days in a month, plus five epagomenal days at the end of the year . </P> <P> 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 . Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of centuries, but it is now the most commonly used calendar around the world, by far . </P>

Who came up with the concept of time
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