Thirty days hath September...

     Happy Leap Day, everyone! For those of you born on or fool enough to get married on this day (hi Mom, hi Dad), happy actual birthday/anniversary! Yes, this is one of those wonderful
years when we get an extra day of February. But why is that? Why, in our modern age, do we have a month that changes length every four years?

     As any elementary school student can tell you, we have Leap Year every four years. The reason it comes around every four years is that the solar year and the calendar year are off by a factor of about ¼ of a day. Hence, every four years, we’ve racked up an extra day. Julius Caesar,  of all people, came up with the idea of having an extra day every four years. Or, rather, Julius Caesar commissioned a bunch of scientists to come up with a solution. His frustration with the calendar then in existence is understandable - before we had a leap day, we had a leap month. And that leap month was not consistently applied. Caesar decided on an every four years automatic change to prevent the problems of his time - winter festivals were being celebrated in spring, as the mismatch between the solar year and the calendar year became more and more noticeable. The new, corrected, oh so modestly named Julian Calendar remained in effect until the sixteenth century. At that point, it was becoming evident that the solar year isn’t off by exactly a quarter of the day. It’s off by ¼ of a day minus 11 minutes. This seems trivial, but it adds up over the centuries. The new Gregorian calendar knocked off three leap years per century, and remains in effect today. Mostly. A fascinating combination of social and political differences throughout the world meant that it was well into the 20th century before every country was using the Gregorian calendar. The Julian calendar remains in effect for certain applications within astronomy, as astronomical time isn’t overly concerned with what Earth season matches up to what month.
    
      Calendars are one of those things that, the more you think about, the weirder they are. Theoretically, we mark the length of a year by how long it takes the Earth to complete an orbit of the sun. But how, exactly, do we do that? One way is to measure the time from equinox to equinox, or solstice to solstice. Pick a fixed point on the Earth (preferably not on the Equator), measure the length of a day, and measure how long it takes until you get another day of that exact same length. You should wind up with a measurement of 365.24 days. Alternately, you can measure the time it takes the Earth to move from perihelion (the closest point in it’s orbit to the Sun) back to perihelion. Thus, this measures one full circuit of the sun. But that gives you a measurement about 25 minutes longer than the equinox-equinox method. Wait, what?

    The equinox-based method measures the circuit of seasons on Earth, a cycle controlled partly by the tilt of the Earth’s axis. The perihelion method measures the actual orbit of the Earth around the Sun. The two cycles don’t quite line up, hence, the difference in a year length as calculated by either method. For most purposes on Earth, the seasonal equinox cycle is perfectly fine, and since most people and cultures strongly associate the calendar year with seasons, works better for day to day use. Time, after all, is relative.

    This isn’t even getting into the weirdnesses of the lunar year, or why we define day length the way we do, or, God help us all, daylight savings time (we’ll wait until that actually starts to get into it). Calendars seem immutable, but are really an array of fixes meant to more or less make time measured as we perceive it match up with time as it actually progresses. Imagine how much fun this is going to be when we colonize space