## Monday, February 29, 2016

### Leap Years and How Scientists Tell Time

I couldn't let Feb. 29th go by without commenting on leap years, especially since I have no expectation that this blog will still be around four years from now. Leap years originate in astronomy -- the length of the year (the time it takes the Earth to go around the sun), is not an even number of days (the time it takes the earth to rotate on its axis). The length of the year is roughly 365.2422 days. Lucky for us, the fractional part of the year (0.2422) is almost equal to 0.25 (or 1/4), so we can fix things by adding 1/4 of a day every year, or one day every four years. This "fix" was introduced by the ancient Romans, and it works pretty well, but it's still not perfect. The Gregorian calendar fixes the fix by taking away leap days every 100 years (so, for instance 1800 and 1900 were not leap years) and then adding them back in every 400 years, so 2000 was a leap year. Our leap year in 2000 is something we won't see again until 2400! This gives us a length of the year equal to 365 + 1/4 - 1/100 + 1/400 = 365.2425 days, which means the calendar will be off by one day every 3333 years -- not too shabby!

Calendar reform had a symbiotic relationship with the infant science of astronomy. It's really just a weird coincidence that a simple leap year works so well, but not well enough that it didn't need to be fixed later on. Would science have developed any differently if the year were more exactly a simple fraction of the number of days, so that the Gregorian reform had not been necessary? And what if the opposite had been true? What if the year weren't anywhere close to a simple fraction of the number of days? Would that have led to much greater calendrical confusion over the centuries?

Given all of this confusion, what unit of time do scientists use in their own work?  Days? Years? Hours? Minutes? The answer is none of the above.

The universal unit of time in science is the second. No other units need apply - we've thrown years, days, hours and minutes out the window. And the measurement of time itself has come unmoored from its origins in the solar system. Scientists now define the second using atomic clocks, which are much more accurate than the motion of the earth! Of everything in nature, time is now the the one quantity we can measure most accurately.

Vernor Vinge takes this to its logical conclusion in A Deepness in the Sky. His spacefarers measure time exclusively in seconds, so the novel constantly makes reference to kiloseconds, megaseconds, etc. Very clever, but also a bit annoying as I had to keep converting units in my head. While the "year" might be discarded as we move out into space, I'm not so sure we'll ever eliminate the "day" from our lexicon, since our biology ties us so closely to it.

#### 1 comment:

Kathy said...

I mislike fiction which uses unconventional units or references for time. Silverberg's "Roma Eterna," as well as Turtledove's "Agent of Byzantium" are good, solid alternate history reads, but they reference all dates either by the Founding of Rome or by the Orthodox notion of the Creation. So when Basil in the latter series comes across gunpowder and the printing press, I can't tell whether it's too soon compared to actual history.