THE SOUTHWORTH PLANETARIUM 207-780-4249 www.usm.maine.edu/planet 70 Falmouth Street Portland, Maine 04103 43.6667° N, 70.2667° W Founded January 1970 "Campaign-free news source!" THE DAILY ASTRONOMER Monday, February 29, 2016 Leap Day Astronomy Ask a planetarium audience what aspect of astronomy is most compelling and every person will invariably shout, "calendar reckoning!" Unlike the tedious, boring topics such as 'black holes,' 'exploding stars,' 'extraterrestrial intelligence,' or 'parallel universes,' calendar reckoning ignites the imagination beyond constraint and has inspired generations of children to select lucrative, groupie-accumulating astronomy careers. (And we've never regretted it.) Now, in a tidy Universe, calendar reckoning would be a complete non-event. Our planet would describe a perfect circle and its constant rotation period would be a pleasant integer multiple of the orbital period. For instance, an ideal Earth would spin on its axis exactly 300 times during each orbit, so every year would be precisely 300 days long, down to the picosecond. Once established, the calendar would retain its integrity forever. Well, in our untidy, troublesome Universe, the situation is a complete mess. Earth's orbit is not a circle, but an ellipse (more on this issue later); and the amount of time our planet requires to complete a circuit about the Sun is hardly a well-behaved integer, but approximately equals 365.24219265 days.* (365 days, 5 hours, 48 minutes, 45.5 seconds, or thereabouts.) Let's pretend we go back in time to a few thousand years ago, and we don't have the convenience of knowing the value we just introduced. However, we've been hired to construct a calendar that everybody can use for sowing, harvesting, and festivals. We have to keep in mind the four seasonal points: the first day of spring, summer, fall and winter. Fortunately, we already conned some other unsuspecting time travelers into ascertaining these points.** Our calendar must maintain these seasonal points: so, to take a wild example out of our hats, the Vernal (spring) Equinox must be on March 21, or very close. Well, we're going to be completely lazy and decide that each year has 365 days. Everybody will abide by it and all will be well. At least it will be for us, because our work is done and we can now spend the rest of our lives engaged in ancient world amusements, such as playing the Wii Sports Resort. Initially, all seems well. The calendar and the seasons are in lovely sync. A few years elapse and we notice a slight problem. The seasonal points are shifting, albeit slightly. We're reaching the Vernal Equinox and the other points later than we should. "Hmmm," we think, "why is our calendar off?" We reluctantly tear ourselves away from our amusements and investigate. We discover that Earth's year, which we regard as the time the Sun requires to move around us, is a bit longer than we thought. Calculations performed on our watches (remember, we're time travelers) indicate that this discrepancy equals about one day every four years. Mind you, such a discovery would require a damn long time, but we're compressing the scenario. Our calendar is wrong. Fortunately, we don't have to make any changes because we fled back to the modern world before anybody really noticed. Many years later after our cowardly departure, a Roman dictator named Julius decides to take matters into his own divine hands. Exactly 708 years after Rome's mythical founding by Romulus and Remus (46 BCE or 46 BC to us), Julius imposes a new calendar system to compensate for the older one's deficiencies. His year still consists of 365 days. However, every four years, a leap day is inserted. This extra day accounts for the extra time the Sun needs to complete its orbit around Earth. As Julius was the only vain leader in Roman history, he names this system the Julian calendar. A little while later, Julius dies because he chose the wrong friends, but the calendar survives. Centuries elapse! And, now, in the beautiful, mythological, age of castles, dragons and hobbits, people find that the calendar is showing other errors. The Vernal Equinox is now happening EARLIER than March 21st. Much earlier. In fact, spring is beginning around March 11th. The calendar had shown this "creep effect" ever since the Roman era, but only after hundreds of years has this error accumulated enough to be troublesome. The Vernal Equinox now precedes the actual Vernal Equinox date by more than a week and a half. Now, we know where this is leading: if left uncorrected, the Vernal Equinox (and other seasonal points) will migrate farther away from their established dates. The equinox will move toward the solstice date and the solstice dates toward the equinoxes. Eventually, the Vernal Equinox will occur around December 21st, the date once reserved for the solstice; and we'll reach the summer solstice in March and, well, what a mess! The real trouble, ecclesiastically, is the Easter date, the calculation of which involves the Vernal Equinox.*** We reach 1582, and Pope Gregory XII (Ugo Boncompagni to his friends) takes it on himself to set things right. He issues new calendar rules designed to account for the smaller, but still significant, errors inherent in the Julian calendar. Along with Christopher Clavius, the astronomer who does the real heavy lifting, Pope Gregory XII introduces a dramatic calendar reform. First, he removes 11 days from the calendar so set the Vernal Equinox back in place. He essentially gives the calendar a swift kick.. He then modifies the leap year rules that Julius developed: Every four years will have a leap day; except for those years equally divisible by 100, unless those years are also divisible by 400. Let's take a few examples: 1900 was not a leap year; but 1896 and 1904 were 2000 WAS a leap year, but 2100 will not be. 1600 WAS a leap year, but 1700 was not. By imposing these extra rules, the calendar will match the seasonal year much more closely. Now, in a neat world, everybody would embrace these changes at once. Instead, various countries take their sweet time adopting them. (Religious strife, communication issues and sometimes stubbornness impedes the calendar's promulgation and adoption.) England waits until the 18th century; and Greece waits until 1923, to cite a few examples. The Gregorian calendar is now widely accepted because it is so beautifully accurate! Mind you, the Vernal Equinox is not always on March 21st because our orbit not circular and date variations are inherent in our orbit. However, with this new reform, our calendar remains in sync with the seasons and will for centuries to come. These leap days do come in handy. *We could really tangle the knots with information about the different types of years. The value cited in the article's main body is the "tropical year," defined as the time related to the seasonal changes. We will cheerfully neglect the other definitions. **How would one make these determinations? An easy method would involve sighting the sunrise-sunset points along the horizon. Provided one maintains the same observation point, one could insert posts to mark the various rise/set points. The Sun rises due east and sets due west on the equinoxes, John, and these points proceed southward throughout the autumn, until reaching the southeastern/southwestern most points on the winter solstice. Conversely, they proceed progressively northward through the spring, until reaching their northeastern/northwestern points on the summer solstice. One could designate posts specific to the seasonal points. (Granted, such determinations require more than one year. ***The First Council of Nicaea (325 AD) established the Easter date as being the first Sunday after the Full Moon following the Vernal Equinox.