Yes, it’s time for the standard question of the amnesic or accidental time traveler. A year, like a day, tends to have an astronomical definition, the length of time it takes the earth to orbit the sun. Roughly 365.25 days, though not exactly, which is why leap years in the Gregorian calendar happen every four years, except years divisible by 100, unless they’re also divisible by 400. 1900 wasn’t a leap year, 2000 was, 2100 won’t be.
How to number those years? That’s a little more arbitrary. Except that it’s not. Calendar makers tend to pick a point in time as significant enough to be Year 1 and continue from there. This shows up in both fantasy and science fiction, as a common trope of future civilizations is they do not continue to use the Gregorian calendar. The advantage here is apparent when we read books like 2001 or 2010. A ship clearly did not travel to Jupiter and/or Saturn eleven years ago, and I certainly don’t remember Jupiter igniting into the new sun Lucifer two years ago. Resetting the calendar avoids the problem of reality catching up to the dates set aside in our science fiction.
So…that was easy. The current year is the number of times the earth has gone around the sun since a significant event in a culture’s history, called Epoch dates. Unless we’re talking about a purely lunar calendar, like the Islamic calendar discussed last time. The question becomes: what is a significant enough date?
Many calendars, including the predominant Gregorian calendar, pick a religious date. Jewish tradition counts the years since the creation of the world, as does the Byzantine calendar, though the years are different in each. Christian tradition counts the years since the birth of Christ, plus four years for incorrect math. Islamic tradition counts the lunar years after the Hijra to Medina. Some calendars, such as the pre-Julian Roman calendar, track the years since the founding of the culture. Some cultures, such as ancient Rome or modern England, maintain a calendar that starts over with the rise of the new king, queen, or emperor. There are even calendars that don’t traditionally number years, such as the Chinese calendar. Any attempt to determine an Epoch year for the Chinese calendar is largely from an external force.
So we’ve got three major questions when it comes to determining epochs and current years.
- Does your culture even have an epoch? Are years numbered at all?
- What significant event determined the epoch?
- Does the culture recognize negative years.
That third one is important. In a calendar such as the Hebrew or Byzantine calendar, intended to count the years since the beginning of time, negative years are meaningless. In a calendar such as the Christian/Gregorian or Islamic calendar, intended to count the years since a historic event, each has a notation for years before that event. Regency calendars would restart with each king or emperor, so wouldn’t have negatives but would have multiple year ones.
Oh, and an interesting fourth question that can be as important as you want it to be: Is there a Year 0? Which can be tied to the next question: Is there a number 0? Which is far too important of a question to go into right now.
This all works fantastically on earth, where we have three astronomical points of reference that are vastly different from each other: the length of a day, the cycles of a single large moon, and the time it takes the planet to orbit the sun. In our own solar system we see planets that would destroy any of these. To make things clear, I’ll use “day” to mean rotation and “year” an orbit of the sun. On Venus a year is just under two days long. On Mercury a day is twice as long as a year. Calendars developed on these planets would be vastly different to ones developed on earth.
Alright, I’ve focused this series almost entirely on time keeping. Which is an interesting subject, but now quite exhausted. Next up, directions, looking at the way most people overlay two-dimensional directions with three-dimensional directions.