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January 2008 About the leap year

This year was a leap year, and there was another day in February.It is said that the Olympic Games will be held in the leap year, but there are actually three times that the Olympic Games were not held in the leap year.In 1916 (Berlin), 1940 (Tokyo), and 1944 (London), all were canceled due to war.Then, on the contrary, has it been held in an average year?There is only once.The Paris Olympics are 1900 was a normal year."Oh, is 1900 normal?So how about 2000 in Sydney?You might well wonder.How was the leap year decided in the first place?

The web encyclopedia Wikipedia states:
1) Years that are divisible by 4 are leap years
2) However, the year in which the year is divisible by 100 is not a leap year
3) However, the year in which the year is divisible by 400 is a leap year

It's hard to understand, but it's almost similar in every book.I will rewrite it a little so that it can be programmed.
a) Year in which the year is divisible by 400.
b) Years in which the year is divisible by 4 but not 100.
It becomes clear that the year that meets one of these two conditions is a leap year, and the other is a normal year.This year is a leap year according to b), 2000 is a leap year according to a), and 1900 is a normal year because neither applies.

But why did it become such a confusing rule?The current calendar, called the Gregorian calendar, has been in effect since 1582, but in the Julian calendar used before in Europe, all years in which the Western calendar was simply divided by 4 were leap years.1400 and 1500 were leap years.The Gregorian calendar was established to reduce leap years.The average number of days in a year is leap year once every four years in the Julian calendar.
  365 + 1/4 = 365.25
In the Gregorian calendar, the leap year is set to 400 / 4-400 / 100 + 400/400 = 97 times in 400 years.
  365 + 97/400 = 365.2425
However, the exact number of days in a year is 365.24219 ··· day (return year), so in the Julian calendar, the difference is 0.0078 ≒ (1/128) days per year.Even this slight error produces an error of one day after 128 years.Because no one lives to be 128, daily activities do not need to be so precise, but for the purpose of holding events on specific days each year for a long period of time, this is insufficient.Since Easter, an important Christian event, is basically “the next Sunday after the first full moon after the Equinox Day” (with various theories), the Equinox Day must be accurately determined.Since the Julian calendar was used for more than 1200 years, the Equinox day kept moving forward and became March 11 in the 16th century.So, Pope Gregory XIII at that time convened scholars to create a committee to study the calendar, and in 1582 the Gregorian calendar was established.The day after October 4th (Thursday) was October 15th (Friday), so it was blank for 10 days, but the days of the week were continuous.

The Gregorian calendar is used immediately in Roman Catholic countries (Spain, Portugal, France, Italy, South German cities, countries, etc.) but was delayed by about 200 years in Protestant countries (England, Sweden, North Germany, etc.).The United States and Australia were British colonies, so they followed Britain.The latest was the Eastern Orthodox church countries (Russia, Greece and other Eastern Europe), which used the Julian calendar until around World War I.Asian countries switched from the lunar calendar to the solar calendar in the late 19th and early 20th centuries, and they did not use the Julian calendar.In Japan, the solar calendar has been in use since 1873. To be precise, December 3, 1872 the old calendar was replaced by January 1 of the Gregorian calendar in 1873.So in 1872, there were no more than 3 days in December.Letters and politician declarations were issued to change the calendar.However, in order to avoid confusion, they say that the date of the old calendar was also written down as a reference in the official calendar (the calendar officially recognized by the government) until the calendar of Meiji 42.

The Gregorian calendar does not completely solve the leap day problem, either, by 1 day in every 3320 years.In 1923, the Greek Orthodox Church, which had previously used the Julian calendar, established a new calendar called the revised Julian calendar.
1) Years that are divisible by 4 are leap years
2) However, the year in which the year is divisible by 100 is a normal year
3) However, the year in which the remainder of the year divided by 900 is 200 or 600 is a leap year
1) and 2) are the same as the Gregorian calendar, but 3) are different.According to this, there are 900/4 - 900/100 + 2 = 218 leap years in 900 years, so 1 year has an average of 365.242222 days, which is an order of magnitude accurate than the Gregorian calendar and there would be no error for more than 30,000 years.In the future, 2800 will be a leap year in the Gregorian calendar, but it will be a normal year in the revised Julian calendar, and the date will be off by one day.In 2900, the revision of the Julian calendar is the only leap year; the dates match again, but the same happens again in 3200 and 3300.Although it is accurate, it is rather complicated, so I don't know which country will use it.It seems that it was decided because of the difference between the Catholic Church and the Eastern Orthodox Church.

This new and accurate calendar is actually easy to create.We look for natural numbers i and j such that, if there are j leap years between i years, the average number of days in 1 year is 365 + j/i days, which is close to the 365.242194 ・・・ days mentioned above.When examining over 1000 years, the smallest error is 1 year = 365.2421959 days when the leap year is set 225 times in 929 years, but it is difficult to actually set the leap year based on this.A practical new calendar is 31 (= 128 / 4-1) leap years in 128 years, 1 year = 365.2421875, and the day will not shift for hundreds of thousands of years.The actual calculation is as follows:
1) Years that are divisible by 4 are leap years
2) However, the year in which the year is divisible by 128 is a normal year
Then you can use it immediately.In the current calendar, it is a leap year, but in this new calendar, when is a normal year or vice versa?

By the way, according to the civil law, whether it is a leap year or a normal year, “the age will change at the moment when the day before the birthday ends.That's right.Therefore, if you were born on February 29, your age will change at the moment that February 28 ends.Be careful about renewing your driver's license and various maturities.

Details are on my Web page so please take a look.

Kazuyuki Sakka