How the Moon Made the Calendar

The moon has always been a prominent feature in the religious and cultural history of every culture in the world. Naturally, this is because it is one of the biggest, brightest things visible to the human eye. Considering the ease with which humans can observe a lunar cycle, it was practical centuries ago to establish a timekeeping method based on the moon.

A calendar was needed in the first place to track seasons. When humans started becoming settlers after the practice of agriculture was developed, there was a need to follow the changing seasons. Crops were harvested in one season, rains came in another, everything died of cold in yet another season, and more. While every single culture independently established its own calendar, the first to do so were the Sumerians.

Ancient Sumerian star chart

The Sumerian calendar was divided into months of 29 or 30 days, depending on whenever a full moon was sighted, which was the first day of the month. The progression of seasons ensured that there were 12 months in a year. Since the lunar cycle is shorter than 30 days, an extra month was added once every few years to keep calendars in sync with the seasons. This system was established as far back as the Bronze Age (3000 BC). The Egyptians borrowed this practice because of a very pressing need: they had to know when the Nile would flood.

The people, especially the educated representatives of religion, on whom the task of maintaining the calendar, had soon figured out that the position of the Sun changed with respect to seasons and decided to incorporate the Sun as a frame of reference in their calendar. They compared it with the lunar calendars and found that the Sun made a much more reliable anchor. So they adjusted their lunar calendars, making the Sun their primary focus around which dates were calculated. These calendars came to be known as the Lunisolar calendars. Most reforms to the then existing calendars were the inclusion of a few days to keep it in sync with the Sun, and then dividing the number of days by 12 to give the length of a month.

The Romans, by this point, through Romulus, had adapted the Greek calendar for their own use. Then Julius Caesar came along, and in 46 BC, reformed the Roman Calendar to keep it in sync with the movements of the Sun. Rome had a flourishing sea trade route with Egypt during Caesar’s time. This ensured that as the king of a highly successful commercial kingdom, Caesar had access to ancient libraries and the intellectuals of Egypt. He worked out that a solar year should be 365.25 days. He introduced the concept of a leap year, and gave the months we have today their number of days. It is important to know that at this point that the Greeks had known the length of a solar year almost a century before Julius Caesar came along, but chose to keep with the lunar calendar.

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Now comes the interesting part. By Julius Caesar’s calendar, each year was 365.25 days. That’s 365 days and 6 hours. However, the actual length of a solar year is 365 days 5 hours 49 minutes 12 seconds. That’s hardly ten minutes a year, but those ten minutes added up and in 128 years, there was one full day’s difference in the year. This was of particular concern to the Church in the early 16th Century. The Church realized that since the date of Easter was determined by the Spring Equinox, Easter was moving backwards in the calendar every year! The Julian Calendar had to be reformed.

It wasn’t until 1582 that an 80 year old pope, Gregory XIII, reformed the calendar to what we have today — the Gregorian Calendar. It’s still not perfect, though. Our calendar is 26.8 seconds longer than a solar year. It would need to be reformed again in 49th century.

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