Leap Years Explained (????)

The Gregorian Calendar (partly Adapted from Formilab's Calendar Converter & aboutastro.com)

In the year 1572, Ugo Boncompagni became Pope Gregory XIII and there was a crisis of the calendar. One of Christianity's most important dates was falling behind with respect to the seasons. Easter, which is based on the date of the vernal equinox was being celebrated too early in March. The cause of this confusion was the over 1,600 year-old Julian calendar, established by Julius Caesar in the year 46 BCE.

Julius Caesar took control over the chaotic Roman calendar, which was being exploited by politicians and others with the haphazard addition of days or months. It was a calendar out-of-synch with the seasons of the earth, which are the result of the rotation of the earth around the sun. Caesar's new calendar of 364.25 days, closely approximating the length of time it takes the earth to go around the sun from the beginning of spring until the next beginning of spring.

Caesar's calendar year was normally 365 days long but included an extra day (a leap day) every four years to account for the extra one-quarter of a day. The intercalary day was added prior to February 25 each year.

Unfortunately, while Caesar's calendar wasn't quite accurate enough as the year isn't 365.25 days, but approximately 365.242199 days. Hence, the calendar of Julius Caesar was 11 minutes and 14 seconds slow each year, which would add to be a full day in error every 128 years.

It took from 46 BCE to 8 CE to get Caesar's calendar functioning properly as at first leap years were observed every three years instead of every four years. By the time of Pope Gregory XIII one day in error every 128 years had added up to a full ten days. It was only by coincidence the Julian calendar happened to have leap years on years divisible by four as during Caesar's time the numbered years of today didn't exist.

A serious change needed to take place, and Pope Gregory XIII decided to repair the calendar. Gregory was aided by astronomers developing a calendar that would be more accurate. The solution developed was almost perfect.

The Gregorian calendar was proclaimed by Pope Gregory XIII and took effect in most Catholic states in 1582, in which October 4, 1582 of the Julian calendar was followed by October 15 in the new calendar, correcting for the accumulated error between the Julian calendar and the equinox as of that date.

When comparing historical dates, it's important to note that the Gregorian calendar, used universally today in Western countries and in international commerce, was adopted at different times by different countries. Britain and her colonies did not switch to the Gregorian calendar until 1752, when Wednesday 2nd September in the Julian calendar dawned as Thursday the 14th in the Gregorian.

Only a few countries were ready or willing to change to the new calendar in 1582. It was adopted that year in Italy, Luxembourg, Portugal, Spain, and France. The Pope issued a reminder on November 7 to nations that they should change their calendars, many did not comply. Had the calendar change been ordered a century earlier, more countries would have been under Catholic rule and would have done the Pope's bidding. By 1582, Protestantism had spread across the continent and politics and religion were in disarray; additionally, the Eastern Orthodox Christian countries would not change until years had passed.

Other countries joined the fray over the following centuries. Roman Catholic Germany, Belgium, and the Netherlands switched by 1584; Hungary in 1587; Denmark and Protestant Germany by 1704; Great Britain and its colonies changed in 1752; Sweden changed 1753; Japan changed 1873 as part of Meiji's Westernization; Egypt changed in 1875; Albania, Bulgaria, Estonia, Latvia, Lithuania, Romania, and Turkey all switched between 1912 and 1917; the Soviet Union changed in 1919; Greece switched to the Gregorian calendar in 1928; lastly, China switched after the 1949 revolution.

Change wasn't always easy. In Frankfurt and London, people rioted over the loss of days in their lives. With each change to the calendar around the world, laws established that people could not be taxed, paid, nor would interest accrue over the "missing" days.

The Gregorian calendar is a minor correction to the Julian. The Julian says every fourth year is a leap year in which February has 29, not 28 days, but in the Gregorian, years divisible by 100 are not leap years unless they are also divisible by 400. Therefore, the years 1700, 1800, 1900, and 2100 would not be leap years, but 1600 and 2000 would. This change was so accurate that today, scientists need only add leap seconds every few years to the clock in order to keep the calendar matching the actual year. As in the Julian calendar, days begin at midnight.

The average length of a year in the Gregorian calendar is 365.2425 days compared to the actual year from equinox to equinox of 365.24219878 days, so the calendar accumulates one day of error with respect to the solar year about every 3300 years. As a purely solar calendar, attempts aren't made to synchronise months to the Moon's phases

Speaking of "Gregorian dates" prior 1582 is incorrect, but, the calendar can be adjusted to prior dates. This uses the convention that the year prior to year 1 is year 0, which differs from the Julian in which there is no year 0, the Julian year before year 1 is year -1. Thus, December 30th, year 0 in the Gregorian calendar corresponds to January 1st, year 1 of the Julian calendar.

A slight modification of the Gregorian calendar would make it even more precise. Add the additional rule that years evenly divisible by 4000 aren't leap years. This results in an average solar year of 365.24225 days per year compared to the actual mean year of 365.24219878, an error of one day over a period of about 19,500 years; this accounts for errors due to tidal actions on the rotation of the Earth.

After the change dates were written with O.S. (Old Style) or N.S. (New Style) following the day so people examining records could understand whether a Julian date or Gregorian. So, George Washington was born on February 11, 1731 (O.S.), his birthday became February 22, 1732 (N.S.) under the Gregorian calendar. The change in the year of his birth was due to when the change of the new year was acknowledged. Before the Gregorian calendar, March 25 was the new year but under the new calendar it became January 1. Since Washington was born between January 1 and March 25, the year of his birth became one year later.

Today, we use the Gregorian calendar to keep us almost perfectly in line with the rotation of the earth around the sun. If such a change were mandated today, chaos would likely reign. If you have any technical (or ANY) questions as to how this all works, I'm definitely NOT the person to ask! Until next time. Take care.