One Giant Leap
The Calendar >
Is a leap year really necessary? Isn't it just an ancient formality we still hopelessly cling to? Well, unless you want to celebrate Christmas in shorts and sandals, or risk a snowstorm during a Fourth of July fireworks extravaganza, you'd better accept it as one of life's "good things."
The Earth, moon, sun, and stars follow the laws of nature, not the laws of man. Our planet spins around on its axis at just the right rate, it is tilted at just the right angle, and it circuits the sun in exactly the right amount of time to make life very, very comfortable on this planet. That all these don't allow a nice, neat human-made calendar is not of great concern at the cosmic scale.
We all know the typical calendar year is 365 days long --- 365 spins of the globe. That fact has been known for millennia by many people groups. The sun, from our point of view, takes that long to travel one complete circuit through the stars. Or we moderns might say that it takes that long for Earth to orbit the Sun once. But it isn't exactly 365 days; it's just a wee bit longer than that.
Actually it takes 365 days, 5 hours, 48 minutes, and 46 seconds to complete one exact trip around the sun. The immediate effect those extra hours have is unnoticeable. After one year, an uncorrected man-made calendar and the natural seasons will be off by just a quarter day. After a decade they will disagree by just over two days.
But if you don't correct for it, you'll notice after generations pass that the holidays your great-great-grandparents celebrated in the spring are now taking place in what feels more like winter, and the special days your people would traditionally celebrate as summer rituals are now taking place when the leaves are budding on the trees.
The
Roman calendar, the chief influence in our modern calendar, was one unholy
mess by the time Julius Caesar came to power. By the first century B.C.
their uncorrected calendar was off by 3 months compared to the seasons!
Caesar, with the help of an Alexandrian astronomer named Sosigenes, was
determined to clean it up.
So, in 46 B.C. he boldly declared that 67 days were to be added to the end of November so that the calendar would match again with the seasonal celebrations. And each month --- from January to December --- which before his decree had a seemingly random number of days assigned to it, was given the amount of days we have now on our calendar (e.g. 31 for January, 28 for February, etc.)
But wisely he went one step further. Sosigenes knew that the year was really about 365 1/4 days. So to avoid calendrical confusion down the line, Julius decreed a leap day into February every four years to make up for the lost day and assure that the seasons and calendars would remain close. Hence the 2000-year-old tradition of leap day was born.
But the year, remember, is not exactly 365 1/4 days long and after another 16 centuries passed, even the leap days weren't keeping the calendar and seasons together. So, in 1582, the Gregorian calendar --- after Pope Gregory XIII --- was introduced which again would yank days right out of the calendar to correct it, but then add a new rule.
Every century year (e.g 1600, 1700, etc.) would not get the leap day unless it was divisible by 400. That is why the century year of 2000 got a leap day, it being divisible by 400. The year 2100 will not.
Got all that? There's more. To bring the calendar even closer to the seasons for as long as humans walk the earth, it has been recently agreed upon that the years 4000 and 8000 will also get their leap year status revoked. Make note of that if you are planning anything then.
Keeping our calendars in harmony with the seasons may be a pain, but it's a small price to pay for a planet with the perfect spin, the ideal tilt, and the just right year.
Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.
Posted by Administrator at 2004.02.21 12:45 PM | Comments (0)
