The history of time keeping is interesting.
We based time using lunar cycles first, as the moon is more visible. The length of a lunar month, from one new moon to the next, is 29.5 days. So twelve lunar months are 354 days, approximately 11 days short of a solar year. In a lunar year each of the twelve months slips steadily back through the seasons. It was hard to align the months with the seasons, paving the way of a more season-accurate method of time keeping – the solar calendar.
Scientists used the position of the stars to measure one solar year = 365 days. Soon enough they noticed that the brightest star, Sirus, appears a day later every 4 years. That’s because a full solar year is 365 days and 6 hours. The Romans made a calendar adjustment by adding the leap year, the Julian Calendar.
Only much later, does a flaw yet again appear. The reason is that the solar year is not 365 days and 6 hours but 365 days, 5 hours, 48 minutes and 46 seconds. There was an extra day every 130 years. Rounded off, the error amounts to 3 days every 400 years.
Pope Gregory XIII employs a German Jesuit and astronomer, Christopher Clavius, to find a solution. Clavius suggests an ingenious adjustment: century years (or those ending in ’00’) should only be leap years if divisible by 400. This eliminates three leap years in every four centuries and neatly solves the problem. The result, in the centuries since the reform, is that 1600 and 2000 are normal leap years, but the intervening 1700, 1800 and 1900 do not include February 29. And this is the Gregorian Calendar that we use until today.
As the Gregorian calendar is based on rounding off values, 3/400 instead of 1/130, the discrepancy will add one day in every 3,323 years. But we may not live that long to make another calendar change.
Anyhow, happy Chinese New Year!