“Dear Librarian…” My continuing struggle against one of the oldest enemies in Los Angeles…IGNORANCE!
Elise T. of Burbank asks: “A DJ on the radio just said this year it takes 366 days to revolve around the sun. Please explain leap year to these people.”
Well, Elise, anyone who is getting their directions in life from a DJ really deserves everything they get, eh? I may grant exceptions to David Bowie or Indeep…or anyone at KXLU. But if they endorse John and Ken on the radio, maybe Facebook isn’t the worst master we could ask for. Bring back Art Bell!
When the Flying Spaghetti Monster was creating the Universe, one of the many numbers it couldn’t quite jibe was the rotation of the Earth upon its axis along with the orbit of the Earth around the Sun. Neither of them is a round number, nor do they have any mathematical relationship (like the orbits of some planets are now synchronized after millions of years.) Because of this inconsistency we are constantly adjusting the length of the day and the year.
The Earth orbits the Sun in 365.256363 days. From this irregular number our entire system of degrees was probably created, with a perfect circle divided into 360 degrees. No doubt many theological arguments raged in ancient times about the discrepancy between the malleable 360 (which is divisible by 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 24, 30, 36, 40, 45, 60, 72, 90, 120 and 180, making it an extremely useful way of measuring the world) and the longer length of an actual year, but hopefully we can accept that the Universe of our beloved FSM is slightly imperfect and move on. The number 360 can also be derived from the number 12, the number a person can count single-handed by using their thumb to count 8 knuckles and 4 fingertips; the number of fingers and thumb (5) and all fingers and toes (20), and the 3 sides of an equilateral triangle (with 60 degrees in each corner), as it was venerated in ancient Mesopotamia.
If our calendar was held steady at 365 days, we would lose a month in just over a century. By adding one day every four years, the special leap year that we are celebrating today, we reduce that error significantly…but the calendar would still change over a period of centuries, because the year is not exactly 365.25 days either.
To complicate things further, the length of an actual seasonal year is not 365.256363 days, the sidereal year or the time when the sun appears in the same point in the sky, but a “real” year is currently 365 days, 5 hours, 48 minutes and 45.19 seconds (or about 365.2421897 days, as of 1 January 2000). Known officially as the tropical year, this difference of just over 20 minutes a year is caused by the precession of the equinoxes, a subject I explored when I was previously asked about the significance of 21 December 2012 in the Mayan Calendar… The tilted axis of the Earth wobbles like a top in a cycle of 25772 years (yet another imperfect number), so that the shortest day in the Northern Hemisphere (21 December) would have moved to 21 June over 12886 years, losing about one day every 72 years. To accommodate precession, we don’t add a leap day at the beginning of each century (as in 1800 or 1900) unless the year can be divided by 400 (as in 1600 or 2000); this adds about a week to every millennium. With this formula we will only lose one day every 8000 years, an error less than other changes in the length of the year.
Because yes, all of these periods are not fixed, but change very slowly; the precession of the equinoxes, the length of the day and the orbit of the Earth are all influenced by the gravity of other planets and especially the Moon. The Moon has a huge pull on us, able to lift the oceans from thousands of kilometers away; this gravitational drag from our neighbor would increase every day by 2.3 milliseconds in a century, except for the fact that the Earth is still rounding after the melting of billions of tons of polar ice in the most recent Ice Age (not including our present climate change). This change of shape currently speeds up our planet 0.6 milliseconds a day every century, so currently every day is slowing by 1.7 milliseconds a century. To correct this (and resolve other political issues about the relationship between atomic clocks and the solar day) a leap second is added on an irregular schedule; the next one will be added on 30 June of this year, so enjoy it!
Incidentally, February was honored both as the shortest month and the month to receive the “leap day” because, until about 450 BCE, it was the last month of the year in the Roman calendar, the time of Februa (Purification). And the reason why we split the day into 24 hours is lost in prehistory, but most likely is because 12 seemed like a good division of time during the day and night, matching the number of lunar cycles (and later months) in a year, and number of constellations in the Zodiac. The ancient Egyptians also counted time at night by noting the rising of certain “decan” stars in 12 groups, and designed their sundials with 12 divisions to match.
I hope that answers your question, and that you celebrated the birth of your favorite leap year babies, such as Balthus, Tempest Storm, Richard Ramirez, William Wellman, Ja Rule, Dinah Shore, Tony Robbins and Jimmy Dorsey.