So today is one of the two equinoxes happening each year. On the equinox the sun is directly over the equator. The Sun will appear to rise and set directly in the East and West and the hours of day and night are the same.
The equinox has been a fixture of many cultures for a very, very long time. The one aspect that I thought I knew about was the calculation of Easter Sunday but then I started down an internet rabbit hole and now I know nothing. I can say that the equinox has had a huge influence on the modern calendar in the west. From 45 BCE to 1582 CE we used the Julian calender (named after Julius Caesar) which defines a year as 365 days and 6 hours. The real length is 365 days, 5 hours, and 49 minutes. While this seems like a small difference, just 11 minutes, it adds up to one extra day every 128 years.
So in 45 BCE the equinox was on March 25 (as decreed by Julius Caesar) by 1580 the equinox was on March 10. The problem comes in with the rather complicated calculation of Easter Sunday based on lunar and solar calendars but also must fit between certain events and that was drifting almost a day every century.
Pope Gregory XIII issued a papal bull called Inter Gravissimas which decreed a new calendar to start in 1582. The day after October 4 would be October 15 which meant that the equinox would be at (or near) March 21 which is the same date as when the First Council of Nicaea met in 325 CE. The First Council of Nicaea was a meeting of various Christian/Catholic church sects about several major disagreements, one of which was how to compute the date of Pasha/Easter.
The new, adjusted calendar was authored by doctor, astronomer, philosopher, and chronologist Aloysius Lilius and presented to the calendar reform commission is 1575. After Lilius died in 1576 Christopher Clavius, a Jesuit German mathematician and astronomer tweaked and championed the Lilius calendar. The protestant nations did not immediately accept the new Gregorian calendar so for almost 200 years there were two dates for Easter — one Catholic and one Protestant. Gradually the Protestant countries adopted the Gregorian calendar with the last ones being Great Britain and the colonies in 1752 and Sweden in 1753. Of course there were protests and demonstrations against the changes.
We now have a reasonably universal calendar system that accurately accounts for the real time that the Earth takes to orbit the sun. For many groups the equinox is one of the most important days used to set the calendar for the rest of the year.
P.S.
- There is a Lilian calendar developed at IBM which starts on October 15, 1582 — the first day of the Gregorian calendar. I think the problem this calendar avoids is the missing 10 dates from October 5 to October 14. The computer has a problem finding the number of days between any two dates if the calendar change is somewhere in that range. The Lilian calendar is only for integer days and does not account for hours, minutes, etc like the Julian day system (no relation to the Julian calendar).
- This is purely speculation but with two calendar systems in the west there is a problem of identifying which calendar is being used in a document. Neither side would even acknowledge that the other system existed so there must be some sort of code. I think the phrase “In the year of our Lord XXXX” was a code phrase to mean the Gregorian calendar. A different phrase was used for the Julian calendar.
- It must be a hazard for researchers to keep the calendar dates straight when reading ye olde documents.
- Keeping track of dates and times seems very simple until one digs into the math, astronomy, and culture details. Then the whole thing gets way complicated. Maybe we should adopt the Star Trek Stardate system. George Lucas never even attempted to figure out the calendar systems (must have been hundreds) in the Star Wars universe.
- Another rabbit hole is the various calendar systems. I kinda like the perpetual system where the first of January is always a Monday by adding one or two extra days to the last week of December. Make those extra days a national holiday.
As always please comment below if I got anything wrong, or tell us what is your favorite calendar system.