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Lunar calendars were among the first to appear, either 12 or 13 lunar months (either 354 or 384 days).

The position of the shadow marks the hour in local time.

The idea to separate the day into smaller parts is credited to Egyptians because of their sundials, which operated on a duodecimal system.

Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities.

Time is used to define other quantities—such as velocity—so defining time in terms of such quantities would result in circularity of definition.

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.The opposing view is that time does not refer to any kind of "container" that events and objects "move through", nor to any entity that "flows", but that it is instead part of a fundamental intellectual structure (together with space and number) within which humans sequence and compare events.This second view, in the tradition of Gottfried Leibniz See Units of Time.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.The importance of the number 12 is due the number of lunar cycles in a year and the number of stars used to count the passage of night.