Even though time has existed since the beginning of, well, time, it was still necessary to invent it.

© Jorge Láscar, CC BY 2.0. LEFT: Jakub Hałun CC BY-SA 4.0The Samrat Yantra is the largest purpose-built sundial in the world, with its gnomon — or tower — standing 73 feet (27 m) high. Built in the early 18th century, the massive instrument is part of the Jantar Mantar observatory in Jaipur, India. The gnomon (below) casts a shadow onto the flanking quadrant arcs, which can indicate the time to an accuracy of two seconds.
Time and astronomy are inseparable. Humans have been using the motions of the stars, Sun, and Moon for thousands of years to regulate their hunting, crops, religion, and lives in every way. And as astronomy developed, so did the need for more precise timekeeping.
There are many ways to ask, "What is the time?" Astronomers can use solar standard time, mean solar time, sidereal time, Universal Time, or Julian Date and its many modified forms. Astronomers describe three different types of twilight, the equation of time, 24 time zones, and an astronomical day. Understanding these different "times" gives us a better idea of our relationship with the sky above, and the spinning Earth on which we live.
The beginning of timeEarly civilizations developed two types of calendars.
The oldest is lunar in nature. It might seem more logical for the Sun to have been the first timekeeper, but archaeologists have found bones of mammoths and other animals dating over 20,000 years old that appear to have carvings recording phases of the Moon. During that period of human history, hunters tracking game needed to know how long they had been gone from their camp, making the Moon the obvious choice to track the passage of time.
It would be millennia before the Sun replaced the Moon in our modern calendar. This is because Earth and the Moon are involved in a cosmic mashup that is difficult to untangle. Most ancient cultures heralded the beginning of the month when the thin crescent or "New Moon" could be seen after sunset. There are 354 and a fraction days in a lunar year with 12 lunar months. Earth, however, revolves around the Sun every 365.242 days. While this was not a problem in a purely ceremonial or religious calendar, trying to mesh these two calendars was impossible.
The solution to this issue was proposed by Sosigenes of Alexandria, Cleopatra's court astronomer and arguably the most influential astronomer in all of history.
Julius Caesar employed Sosigenes to fix the old Roman lunar calendar. By Caesar's time, the lunar calendar had become so out of sync with the seasons that it required a decree from the emperor to remedy the situation. At Sosigenes' suggestion, the old lunar calendar was replaced with one which used only the Sun to delineate the year. The Moon was left to drift through the 12 months of Caesar's new calendar.This Julian calendar also implemented leap years, adding one extra day every four years. But this was not quite a perfect fix, as the last fraction of a day in a year is slightly less than one-quarter of a day. By the 16th century, the Julian calendar was also out of step with the seasons. This led Pope Gregory XIII to implement updates in 1582 that dictated leap years be skipped on years divisible by 100 except when divisible by 400. So while 1900 was not a leap year, 2000 was. Two thousand years later, the whole world still uses the modified calendar of Sosigenes.
Comment: China's mysterious Sanxingdui culture, thought to be of the same era as the Ce mentioned above, has provided of the most bizarre and exquisite finds yet.
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