Calendars: history and heliocentricism

The Calendar is a wonderful piece of human innovation. Not only is it so accurate that it calculates, to seconds, the revolution of Earth around the Sun, it is also a testament to some of the greatest minds in history. However, it is a bit of a paradox in itself. The calendar, almost entirely as we use it today, has been in existence for over 2000 years. A whopping two thousand years! But the calendar describes a solar year, the time it takes for a full revolution of the Earth around the Sun. Heliocentricism — the theory that the Earth revolves around the Sun and not vice versa — did not come into existence till 500 years ago. How then did people before Copernicus, who swore their lives to the supremely flawed geocentric theory, come up with it, if they believed the Earth was the center of the universe?

It’s a fascinating story.

To understand it, we need to look at the purpose of the calendar. Why did people need calendars? Keeping astronomy aside, what would they have used one for? Most obviously for seasons and festivals; but primarily for seasons. Historic calendars were devised to keep track of when the first snow came, when the Nile flooded, when the rains started, and when it got too hot to plant certain kinds of crops.

Later, when religion came into picture, people needed a certain day to celebrate a certain occasion. This was especially true for Christians, whose “year” was calculated forwards and backwards from Easter.

To make such a proto-calendar, there needed to be a frame of reference; something that had constant, visible cycles. And everyone’s eyes went to our nearest, largest, constant companion: the Moon. These people of centuries ago observed that the Moon completed one full cycle every 29.5 days, and seasons repeated themselves after approximately 12 lunar cycles. Naturally, the Moon became the reference point. The first calendars (that each evolved independently) were all lunar calendars with either 29 or 30 days each month (348 days and 360 days a year respectively). In fact, even today, most religious calendars are lunar calendars that have been passed down through centuries.

Today’s calendar, called the Gregorian Calendar, owes its origin to two important calendars of yore — the Greek and the Egyptian. The Hellenic (Greek) calendar had a year of 354 days, with alternating months of 29 and 30 days. The Egyptian calendar had months of 30 days each, with 360 days in a year. However, seasons would frequently become out of sync in Greece, and the Nile flooded later and later every year. So the Greeks added an extra “month” (called intercalary) of 10 days and Egyptians added five extra days of holiday to their calendars, bringing both calendars to a total of 364 and 365 days respectively. In fact, the Egyptians didn’t care much about the seasons. They simply accepted that the seasons get shifted by a few days every year. Their five days were added to the calendar to keep it in sync with the annual heliacal rising of their favorite star, Sirius, after a period of absence. We’re talking about some 3000 BC here.

The people, especially the educated representatives of religion, on whom the task of maintaining the calendar and adding intercalaries fell, 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. Since the addition of intercalaries in Rome was up to religious priests who presumably did so at their whim, the year 46 BC needed to be amended. So, Julius Caesar added 3 intercalary months, and from 355 days, made the calendar 365 days to keep it in sync with the solar year. This was our first Solar calendar.

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, our man Joo-C had access to ancient libraries and the intellectuals of Egypt. Upon conversing with Egyptian astronomers, Julius Caesar found out that the Egyptian calendar fell out of whack every 40 years. 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.

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.

Talks of reformation went on for almost a century. Incidentally or coincidentally, Nicolaus Copernicus, who had been studying the movements of Mars and Saturn, was approached and then submitted his proposal for the calendar reform. This was between 1512 and 1516. It was also around the same time that he secretly revealed his work on heliocentricism.

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.

So herein lies the unexpected legacy of the Calendar: all the accurate science it is a representation of was due to religion.


2 comments on “Calendars: history and heliocentricism

  1. vaidya

    Heliocentrism was there with the Greeks also, even back in the BCs (Aristarchus). It is only with the rise of the Church that the model changed. Indians “almost” got it right (planets circling the Sun, but the Sun circling the Earth, but that did not prevent them from making accurate calculations). Copernicus usually gets the name mainly because of having to stand up to the Church (not a small thing either, things were really going backwards).

  2. babak

    why didn’t jesus know the world was round? is it because he didn’t exist ? and what happened to his prepus?

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