# The Arbitrariness of Units of Time

**The Arbitrariness of Units of Time**

In the Gregorian calendar, today marks the new year. It is not the new year in the dozens of other calendars. This is not insignificant. Although the “new year” is celebrated with fanfare and increased alcohol consumption, aside from hangovers absolutely nothing changes between 31 December and 1 January. Nature does not know that it is a new year, nor does it care. Even if we humans care about delineating a new year there is nothing that indicates that 1 January should be the first day of a new year. Why not winter solstice or spring equinox? Non-Gregorian calendars, the Jewish and Islamic for example (which are lunar, not solar, calendars), consider the new year to begin on other dates.

The measurement of time is mostly arbitrary. Two units of time are connected to tangible events. We define the year as 365 days (except when it isn’t: leap year). We define a day as 24 hours long. But these are the least arbitrary of our time measurements because they are based on astronomical events relative to Earth. What tells us that a week is seven days long? That September has 30 not 31 days? That the day is to be divided into 24 hours? An hour into 60 minutes? A minute into 60 seconds? Then suddenly we switch to base-10 for counting time periods less than a second.

The point is that other than year and day, our units of time are artificial. What makes a second a second? Well, the Bureau International des Poids et Mesures (BIPM) defines a second as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium 133 atom.” Be sure and recalibrate your clocks to that. But that is still arbitrary despite the scientific precision of measuring activities of atoms. Humans chose one particular atomic isotope over the others available. That specific atomic time standard was adopted to synchronize with the long accepted definition of a second as 1/86,400 of a mean solar day.

The idea of a second is arbitrary. We don’t need it. We could instead divide the day into ten hours and an hour into one hundred minutes or something similar. Or we could divide the day into base-10 metric units. Then our units of time would sync up with how we divide mass and distance. Why don’t we say “I have been waiting for a centiday?” It is the same amount of time as 14.4 minutes. A milliday would be the same duration as 1.44 seconds. We arbitrarily use one set of units of time over others.

Then there are months and weeks. There is nothing that requires, or even suggests, that a year is twelve months and a week is seven days. What system we adopt does have to fit into the fact that there are 365 days to a year. Beyond that, it is all arbitrary. The Aztec *xiuhpohualli* calendar dealt with the year as having 18 months of 20 days each. Each month was divided into four five-day “weeks.” These otherwise symmetrical divisions created one “week” of five leftover days. These were considered empty days, days adrift without activities that people waited out until the order of the months returned.

The year could just as easily be divided into 20 months of 18 days each, 36 months of 10 days each, and so on. One still needs to figure out what to do about the leftover days. 365 is not a prime number but is only divisible by 5 and 73 and 73 is too long for a unit of days. The Gregorian calendar hides the leftover days by making months have three different (arbitrary) lengths. There is nothing aside from cultural habit based loosely upon world events that specifies we should choose one set of units of measurement over another.

The point of this exercise in the arbitrariness of units of time is this. Consider that much of what we take for granted is in fact based on arbitrary decisions that have strength only in their long-term acceptance. Things can be quite different than they are.

*Note: Since some may ask why I do not talk about the lunar year as much as the solar year, here is why. The lunar cycle is a mean of 29.53 days. Twelve lunations is about 354.37 days and therefore does not match well with the seasons that follow the solar cycle. An example is the purely lunar Islamic calendar which causes Ramadan to fall in different seasons.*