Excellent overview of how our calendar developed, from the moon-watching of hunter cavepeople to the atomic clocks of today. Well written, humorous, and readable.
Along with agriculture, astronomy was the first science. Astronomers, were who often priests, charted the position of the sun, moon, and stars to determine the right time to plant and harvest crops. The earliest calendars, such as those by the Sumerians and Babylonians, were lunar and not very accurate.
The first solar calendar was developed by the Egyptians in 4236 BC, who observed that exactly once every 365 and a quarter days the star Sirius rose with the sun, which also coincided with the annual flooding of the Nile on which they depended for irrigation.
Julius Caesar learned of the Egyptian calendar through his contacts with Cleopatra and the astronomer Sosigenes, and instituted the solar calendar for Rome in 45 BC, renaming some months after Roman gods and one, July, for himself. This became the Julian calendar which was used for over 17 centuries.
But eventually it was clear the calendar was out of sync with the seasons, discombobulating important dates such as the spring equinox and Easter. This was due to an error of 11 minutes in the leap year which over the centuries had accumulated to 10 days . In 1582 Pope Gregory XIII convened a conference of astronomers to fix the error. Ten days were eliminated from that year, and to prevent future time lags the leap day was omitted in century years (those ending in 00) that were not divisible by 400. This is the Gregorian calendar we still use today, though Britain and the American colonies did not adopt it until 1752.
Other topics covered include Stonehenge, the 4,000 year old British edifice that aligns with the sun every solstice to this day; the Mayan calendar, one of the most complex and accurate calendars in history; the monk Dionysius Exiguus who invented the "BC" and "AD" dating system for Christians; the contributions of the Greeks, Hindus, Chinese, and Arabs to math, astronomy and timekeeping: Copernicus' assertion of a sun-centered solar system; Einstein's theory of relativity; and the atomic clocks used to keep world time today based on the metal cesium, which vibrates over 9 billion times per second.
ELPL also has excellent books for children on calendars, including The Story of Clocks and Calendars: Marking A Millenium by Betsy Maestro;
On Time: From Seasons to Split Seconds by Gloria Skurzynski; Tools of Timekeeping by Linda Formichelli and W. Eric Martin (with experiments for kids on timekeeping such as ancient peoples practiced).