Welcome to the future – what’s the date?

Or to 2012, at least. Changing the numbers on the calendar often prompts me to think about calendars, and I’m not the only one. This year even more so than usual, what with all the Mayan calendar hype, and a proposed calendar reform in the news.

What’s wrong with what we’ve got, and why are calendars so complicated anyway?

Humans need a way to keep track of time, from when to plant in the spring to when to celebrate birthdays and holidays, to managing our business and government affairs. We’ve tried different ways to do so for millennia. There are still at least a dozen calendars in use worldwide, although the Gregorian calendar is by far the most common.

Even the Gregorian calendar was established in 1582. Human culture has changed drastically since then: is it time for a new calendar?

Maybe, but based on what? We like to break time into manageable chunks: hours, days, weeks, months, years. But we also like to have those chunks correspond to the world around us. Spring happens during certain months, winter others, and so on. A calendar must be tied to the solar or lunar year, and to the day.

Like most if not all natural phenomena, astronomical days and months and years follow their own patterns determined by orbital mechanics, not the neat patterns that we’d like them to.

Days are the easiest to observe. Sunrise to sunrise, right? Except day length changes with season (unless you live at the equator), so that doesn’t work very well. Medieval timekeeping used 12-hour days and 12-hour nights, and let the hour length change with the season. We’ve standardized our hour length and our day length. A day still has 24 hours but they’re of equal length, and don’t match up with sunrise and sunset. An hour is 60 minutes, a minute is 60 seconds, and a second has been defined very precisely by relating it to physical constants.

The average length of a solar day is 24 hours, but individual days vary. Using a star instead removes the seasonal variation: star-rise to star-rise of a particular star should give a good measurement of day length. A stellar day is about four minutes less than 24 hours though, giving one more stellar day than solar day per year. So let’s just keep the average day length of 24 hours and not worry about it.

Years must be easier, right? A solar year is from the vernal equinox to the next, when the Sun returns to exactly the same place relative to the Earth. A solar year is about 365.25 days. (Wouldn’t it be nice if it were an integer?) If we just used a 365-day calendar all the time, we’d get farther and farther away from the calendar year matching the seasons: that’s why a leap day is added every 4 years. Almost: it isn’t exactly a quarter-day off, so once in a while we skip the leap day to keep the calendar from going off in the other direction.

Or we could measure years by comparing the position of the Sun to a star, same as we did for days. The sidereal year is about 20 minutes longer than the solar year. Never mind.

We’re making progress: we’ve settled on 24-hour days to match the annual mean, and on solar years of 365.25-ish days, with some kind of system to deal with that quarter-day to keep from getting too far off.

How to break up the year? Months could be based on the changing phases of the moon, from full moon to full moon. That’s called a synodic month, and there are lots of many other ways to define months. A synodic month changes length during the year, but the average value is 29.53 days. That doesn’t fit well with either our hours or our years.

It also doesn’t have anything much to do with the months of the Gregorian calendar, which range from 28 to 31 days in an irregular pattern due largely to Roman politics.

The seven-day week is historically-based too, and doesn’t match with either months (lunar or calendrical) or years, continuing in sequence across all both.

The Gregorian calendar, then, is rather a mish-mash of historical and astronomical quirks. Days of the week are the oddest, perhaps: consecutive years start on different days, so date and day are unrelated. Months are of irregular lengths, so quarters are also different lengths (a business accounting problem, among others).

And then there’s the epoch: where does the calendar start counting? A political date? Religious date (as in the Gregorian calendar)? Astronomical date? Confusing!

Can we fix it? Should we fix it?

What would it look like if we did?

There’s the Stardate system, which uses days and decimal days, counting from an unknown epoch. This makes a fair bit of sense in a universe where humans still want to work and sleep on a 24-hour schedule, but are completely divorced from the seasonal cycle on a particular planet.

The Hanke-Henry permanent calendar keeps the seven-day week and a 364-day year (not 365), but standardized the length of months and the relationship between day of week, day of month, and day of year. Under this system, not only is January 1, 2012 a Sunday, every New Year’s Day is a Sunday. If your birthday is always a Tuesday, well, too bad. This calendar manages the regular quarters and other business-friendly aspects, but isn’t so friendly to the social aspects that we’ve become accustomed to. It also requires adding a leap week after December every five or six years, which doesn’t seem very efficient to me. On the other hand, if you made that leap week a global holiday? I could be persuaded.

There are lots of other proposed calendar systems, both factual and science fictional. The calendar we use now has been changed many times. Do you think we should change it again? What kind of calendar would you like to live under?

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