This day in history Feb. 29, 2020-leap years
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Started by metmike - Feb. 29, 2020, 8:59 p.m.

Read and learn about history. Pick out a good one for us!

https://en.wikipedia.org/wiki/February_29


1504Christopher Columbus uses his knowledge of a lunar eclipse that night to convince Jamaican natives to provide him with supplies.




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By metmike - Feb. 29, 2020, 9 p.m.
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Leap years

Main article: Leap year

Although most modern calendar years have 365 days, a complete revolution around the Sun (one solar year) takes approximately 365 days, 5 hours, 48 minutes, and 46 seconds (or, for simplicity's sake, approximately 365 days and 6 hours, or 365.25 days).[5] An extra 23 hours, 15 minutes, and 4 seconds thus accumulates every four years (again, for simplicity's sake, approximately an extra 24 hours, or 1 day, every four years), requiring that an extra calendar day be added to align the calendar with the Sun's apparent position. Without the added day, in future years the seasons would occur later in the calendar, eventually leading to confusion about when to undertake activities dependent on weather, ecology, or hours of daylight.

Solar years are actually slightly shorter than 365 days and 6 hours (365.25 days), which had been known since the 2nd century BC when Hipparchus stated that it lasted 365 + 1/4 − 1/300 days,[6] but this was ignored by Julius Caesar and his astronomical adviser Sosigenes. The Gregorian calendar corrected this by adopting the length of the tropical year stated in three medieval sources, the Alfonsine tables, De Revolutionibus, and the Prutenic Tables, truncated to two sexagesimal places, 365 14/60 33/3600 days or 365 + 1/4 − 3/400 days or 365.2425 days.[7] The length of the tropical year in 2000 was 365.24217 mean solar days,[8] Adding a calendar day every four years, therefore, results in an excess of around 44 minutes every four years, or about 3 days every 400 years. To compensate for this, three days are removed every 400 years. The Gregorian calendar reform implements this adjustment by making an exception to the general rule that there is a leap year every four years. Instead, a year divisible by 100 is not a leap year unless that year is also divisible by 400. This means that the years 1600, 2000, and 2400 are leap years, while the years 1700, 1800, 1900, 2100, 2200, 2300, and 2500 are not leap years.

By metmike - Feb. 29, 2020, 9:04 p.m.
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Leap Years


https://www.mathsisfun.com/leap-years.html


Because the Earth rotates about 365.242375 times a year ...

... but a normal year is 365 days, ... 

... so something has to be done to "catch up" the extra 0.242375 days a year. 

 

So every 4th year we add an extra day (the 29th of February), which makes 365.25 days a year. This is fairly close, but is wrong by about 1 day every 100 years. 

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So every 100 years we don't have a leap year, and that gets us 365.24 days per year (1 day less in 100 year = -0.01 days per year). Closer, but still not accurate enough!

So another rule says that every 400 years is a leap year again. This gets us 365.2425 days per year (1 day regained every 400 years = 0.0025 days per year), which is close enough to 365.242375 not to matter much.


How far away each year is from the average
leap year graph 

Example: look just before 2100, the worst year is 1.2 days ahead, but because 2100 is not a leap year they all get adjusted back by 1 day.

So this keeps us pretty close, and any other adjustments can be done way in the future (when the Earth may be rotating a little slower, anyway!)