The History of Calendars
Have you ever wondered how we ended up with the calendar we use today? It’s something I’ve Googled more than once because it is a rabbit hole of discovery!! In this week’s blog, I’m looking at the different types of calendars and the history of how we ended up with today's Gregorian calendar.
Before we dive in, what is a calendar? Simply put, it is a system that divides time over longer periods, such as days, weeks, months and years. The calendar we are most familiar with consists of 24 hours in a day, 7 days in a week, 30 or 31 days in a month (excluding February), and 12 months in a year. The year consists of 365 or 366 days depending on if February has 28 or 29 days. But is this entirely accurate?
A Lunar calendar is based on synodic months, which is the time it takes the moon to go through all eight phases, starting and ending with a New Moon. As a synodic month is roughly 29 and a half days, it doesn’t quite fit in with the solar year, or the year of the seasons.
A solar year is measured as the time between the Sun passing the celestial equator twice (the equinoxes) and because of the slow wobble of the Earth’s rotation, this is actually 365 days, 5 hours and 48 minutes. This is shorter than a sidereal year, which measures the time it takes the Sun to return to the same position in the sky (from our vantage point on Earth), and is 365 days, 6 hours and 9 minutes.
Then we have the anomalistic year which measures the time it takes the Earth to make two passages through the perihelion (the point at which the Earth’s orbit is nearest the Sun) and takes 365 days, 6 hours and 13 minutes.
And just for fun, it takes 225 million years for the solar system to revolve once around the centre of the Milky Way to give us a cosmic year.
Still with me?
Clearly, all these years and months are incompatible so a cycle of regular days with comparatively simple rules is the calendar we adopted that gives us a year with an error of less than half a minute. How did we get there though?
Let’s start with the Lunar calendar as it’s my personal favourite and the oldest.
The Sumerians were probably the first civilisation to develop a calendar that was based on the lunar phases and started on the first day of visibility of the New Moon. The problem is that there are 12 and a bit synodic months in a solar year and so extra days (intercalation) are needed to keep the lunar calendar in sync with the seasons.
It was around 380 BCE that royal Babylonian astrologers realised that the calendar was way out of step with the seasons and so they established rules for intercalations. The Metonic cycle gave 7 out of every 19 years an intercalary month, and it was the Greek astronomer Meton of Athens who followed the Babylonia precedent to give us this cycle.
The ancient Egyptians also used a lunar calendar. The first morning, after the Balsamic (waning crescent) moon became invisible, was the New Moon and the start of a lunar month, with the month itself named after the major festival celebrated then. A 13th month called Thoth was intercalated to keep the lunar year in line with the agricultural seasons and festivals. New Year's Day occurred when the star Sothis (Sirius) could be observed before dawn on the eastern horizon in midsummer. The timing of this would determine if the intercalated month would be used that year.
The Egyptians also kept a civil calendar established several thousand years before the common era and is the first known calendar to use a year of 365 days. This was divided into 12 months of 30 days and then an extra five days was grouped at the end of the year. As there was no leap year day the civil calendar rotated through the seasons and lined up again after 1460 years, known as a Sothic cycle. The Egyptian civil calendar used the same month names as the lunar calendar.
It was Julius Caesar in around 46 BCE who altered the Egyptian civil calendar by adding an additional day every four years. The Alexandrian astronomer Sosigenes advised Caesar that the solar year was 365 and a quarter days. The Julian calendar split the year into the 12 months we are familiar with today but because Sosigenes had overestimated the length of a year by 11 minutes, it meant that by the mid-1500’s the seasons were off by about 10 days.
The Gregorian calendar proclaimed in 1582 by Pope Gregory XIII, reformed the Julian calendar and meant that there would not be a leap year unless the year was divisible by 400. This system was adopted by different countries at different times, starting with Italy, France, Spain and Portugal in 1582. It was 1752 before Great Britain adopted the Gregorian calendar. There is currently a 13-day discrepancy between the Julian and Gregorian calendars (it will be 14 in 2100) and some churches still use the Julian calendar for fixing dates of worship, especially Easter.
The Gregorian calendar was universally adopted because it brought together the system of dating religious festivals based on the phases of the Moon with the seasonal activities determined by the Sun. As the Sun’s motion and the Moon’s phases are incompatible, the Gregorian calendar is a satisfactory compromise.
Lunar calendars are still used today. The Islamic, or Hijri calendar is a lunar calendar that makes no effort to stay in line with the seasonal years. Whereas, the Jewish and Chinese calendars are lunisolar. 12 months alternate between 29 and 30 days, with a leap year added using the Metonic cycle.
Solar calendars are great for tracking the seasons and agriculture cycles. Meanwhile, Lunar calendars are traditionally used to set religious dates. Personally, I prefer a lunar calendar for setting and tracking goals as it allows me to work with the moon’s energy and pace myself better.
I hope you found this week’s blog interesting. If you would like to find out more about working with a lunar calendar, then come and join the Moon Magic Masterclass - Lunar Planning on Friday 14th June.
