We measure things by comparing them to other, more familiar things. We measure distance by seeing how many short things (e.g., inches, centimeters, etc.) can fit into a longer thing, and liquids by seeing how many ounces or cups the larger volume can fill.
Measuring time was a major challenge, both because of the usefulness of it, and because of the complex relationships of the available measurement units. The most obvious repeating events were days and lunar cycles. Unfortunately, neither of these fit nicely into a year. Trying to force them by defining a year as 12 lunar cycles, for example, just meant that the year had no relation to the actual position of the earth or, more importantly, to the seasons. Some pretty whacked out schemes were devised, like the Babylonian calendar of 19 year cycles with years of different lengths. Try keeping that in your DayTimer or Filofax!
In The Discoverers: A History of Man's Search to Know His World and Himself, Daniel Boorstin credits the ancient Egyptians with finally breaking the mind set of using lunar cycles, and noticing that the time between Nile floods could be divided up into 12 months of 30 days, plus 5 extra intercalary days. That was successful enough that the ancient Romans adopted it, with some modifications by Caesar, resulting in the Julian calendar. Further refinements were made under Pope Gregory XIII. This, the most widely used calendar today, at least for day-to-day purposes, is known as the Gregorian calendar. Other calendars are still used for things like finding the dates of Easter, Passover, Ramadan, etc., which is why these holidays occur on different dates every year.
In some ways, the field of astronomy owes its existence to the need for time measurement. And because astronomy depended on accurate measurements and records, it was one of the first sciences.
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