It’s 1996, which means it’s time to take down your old Star Trek calendar and put up your new one.

Okay, so maybe you don’t have a Star Trek calendar. Maybe you have a World’s Fastest Cars calendar, or even (gag) a Friends calendar. The point is, for us, a calendar is a much an aesthetic and/or advertising medium as it is a way to see what day of the week it is. But in reality, every calendar is the amazing product of thousands of years of history.

A calendar is a system of marking off days, weeks, months and years. Its most important function is that it allows us to clearly designate a day for something to happen. Without the calendar, modern society would be utterly chaotic. (Even more so, I mean.)

The three basic elements of the calendar are the day, month and year. The day is the time it takes the Earth to rotate once: 24 hours. The month approximates the time it takes the moon to change from new moon to full moon to new moon again: 29.53 days. The year is the time it takes the Earth to complete one orbit around the sun: 365.2422 days.

Then there’s the week, a seven-day period that doesn’t correspond to any cycle in nature but reflects the Judeo-Christian story of the creation of the world by God in seven days.

The very first calendars were purely lunar: a new month was said to have begun whenever religious authorities saw a crescent moon. Month would flow smoothly to month, but 12 lunar months is only 354.36 days, almost 11 days shorter than a year. Dates in a lunar calendar, therefore, rapidly move backward through the seasons (as they still do in the Islamic calendar), so that a date in spring ends up in winter in a very few years. This made it difficult to use the calendar to know when to plant crops or to designate seasonal religious celebrations.

Early societies corrected their lunar calendars by throwing in an extra month or a few extra days whenever observations of the crops made it seem necessary. There were literally hundreds of variations of these “lunar-solar” calendars; the Jewish calendar, still in use, is one of these.

The Roman Empire originally used a lunar-solar calendar, but it ended up in a hopeless mess because the people charged with keeping the calendar started fiddling with it to lengthen their terms of office or hasten or delay elections. (Calendars change; politicians do not.)

Julius Caesar, in consultation with the Greek astronomer Sosigenes, put an end to the confusion with the creation of the “Julian” calendar. It ignores the phases of the moon and instead divides the solar year into 12 months of slightly varying lengths, to ensure that the year has 365 days. The year 46 BC was given 445 days to get the new calendar off on the right foot.

Every fourth year, an extra day was added at the end of February to compensate for the fact that the year is actually just a little longer than 365 days. But every 385 years, the Julian calendar created three leap years too many. Eventually it drifted so badly out of phase that religious festivals were again starting to slip into the wrong seasons.

This concerned Pope Gregory XIII. Like Caesar, he went to an astronomer, Christopher Clavius, to come up with a solution. The “Gregorian” calendar fixes the Julian calendar by making years ending in 00 leap years only when they are divisible by 400: 1900 was not a leap year, but 2000 will be. The new calendar was installed in 1582 in Catholic countries, and the 11-day error that had accumulated in the Julian calendar was eliminated by decreeing that Wednesday, October 4, 1582 (Julian) was followed by Thursday, October 15 (Gregorian).

It took a while, but the Gregorian calendar eventually became the standard throughout the West and in parts of Asia. There have been efforts to reform it, but all proposals run up against the obstinate fact that the year is not an exact number of days long. There must always be adjustments: leap days, leap years, leap months. So don’t look for anything new in the way of calendars soon.

Except, perhaps, in the pictures that come with them. Isn’t that new Star Trek calendar great?

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