“The Earth is a terrible timekeeper,” says Geoff Chester, the spokesman for this country’s official clock-master, the US Naval Observatory in Washington, DC.
The first problem is the Earth, but the second problem is us. We cheat to make the movement of the Sun and Moon match up with the calendars on our office walls, and, at a more rarefied level, we cheat so that physicists and astronomers can synchronize their scientific watches.
The Earth doesn’t rotate exactly 365 times during a full revolution around the Sun. (It rotates 365.2422 times, on average, if you must know.) To make up for that, since the time of Julius Caesar (45 BC), we have added a LEAP DAY to the calendar every four years. In 1582, Pope Gregory XIII made a slight modification, saying a leap day would not be added in years that are evenly divisible by 100, unless the year was also evenly divisible by 400 (which is why 2000 was a LEAP YEAR; 1900 was not, and 2100 won’t be, either).
That’s still not good enough, though, so we need more cheating.
The Earth doesn’t cooperate with physicists’ super-specific definition of “one second” (derivation of which is so complicated we’ll just mention the outermost electron in a cesium-133 atom and skip the parts about microwave radiation, vacuums, and magnetic fields). That definitely doesn’t change, but the Earth’s rotation is generally slowing (because of resistance from ocean tides and the movement of molten rock at the planet’s center). From time to time, to keep things matching, we need to add a LEAP SECOND. The last one happened on December 31, 2005; the next may happen on December 31, 2009, but maybe not, Chester says, depending on how much the Earth’s spin actually slows between now and then.
We could avoid leap seconds altogether if we were like the physicists who want time to run based solely on the atomic clocks. But that would force some other adjustment at some point. Some propeller-heads have proposed passing the buck to future generations via a scheme requiring someone to add an entire LEAPHOUR to some year a little more than 1000 years from now. Not likely. So we’re probably stuck with leap seconds. Adjust your watches accordingly.
Could be worse: some people are burdened with much, much more. Accountants and other people who need a fixed number of exactly-seven-day weeks per year use an occasional LEAP WEEK, giving some years 52 weeks and some 53. That makes them happy, but is too confusing to the rest of us and generally of very little import.
And then there are the people who use a LEAP MONTH. “The Moon does not go around the Earth exactly 12 times in a solar year,” says Chester. It’s about 11 days behind, which is why events on a lunar schedule (such as Easter in the Christian calendar, and nearly every holiday in the Jewish calendar) are “movable feasts” — i.e. their dates move around a bit from one year to the next. The Hebrew calendar uses a leap month every two or three years to keep itself relatively close to the solar calendar.Perhaps you’re ready to give up. (I know I am.) If so, try out the Islamic calendar, which is a purely lunar calendar, and ignores the solar calendar completely, not using leap days, weeks, months, or years. Happy February 29!