Why we have leap year.

The Ask A Science Teacher book is published by Experiment Publishing, New York City. I am so very proud of this book, as it has been carefully copyedited and fact checked. The release date was December 17, 2013. It is available on Amazon.com and is in Barnes and Noble stores across the United States. Ask A Science Teacher will also be found in smaller and independent book stores. Below is a Q & A from the book.
QUESTION:
Why do we have leap year?
ANSWER:
We have a leap year every four years because the Earth does not revolve around the Sun in an even number of days. There is no reason why it should; it would be a freak accident of nature if the Earth rotated on its axis a whole number (no fractions) of times for every one full orbit around the Sun.
The Earth needs 365.25 days to go around the Sun. That extra quarter, or one-fourth, day, added up four times, means that we need to add a day to the yearly calendar every four years. That extra day, every four years, is February 29 in the Gregorian calendar that we all follow. We had leap years in 2008, and 2012, and the next ones will be 2016, 2020, and 2024.
However, it gets a bit more complicated. Technically, the Earth does not need 365.25 days, but rather 365.2422 days. (We could also say this as 365 days, 5 hours, 48 minutes, and 46 seconds). That extra day every four years overcompensates for the error.
Here’s the fix. Over a period of 400 years, the totaled errors amount to three extra days, so the calendar leaves out 3 leap days every 400 years. There are February 29ths in the 3 century years, (integer or whole number multiples of 100) that are not whole number multiples of 400.
The year 1600 was a leap year. The years 1700, 1800, 1900 were not leap years. However, the year 2000 was a leap year. The years 2100, 2200, 2300 will not be leap years.
The year 2400 will be a leap year. The year 2500 will not be a leap year, and so on.
There are a few simple steps to determine a leap year. First, the year must be evenly divisible by 4. If the year can also be evenly divided by 100, it is not a leap year, unless the year is also evenly divisible by 400, in which case it is a leap year.
If this seems complicated, and it is, just get a calendar from Barnes and Noble; they are sure to have it done properly. That’s what I plan to do in 2100!
The name ‘leap year’ is derived from the fact that a fixed date on the calendar advances one day of the week from one year to the next. However, in a leap year, the day of the week will advance two days, from March forward, (no pun intended) because of the extra day of February 29.
For example, Christmas Day in 2010 was on a Saturday, on a Sunday in 2011, on a Tuesday in 2012, and on a Wednesday in 2013. Christmas Day “leapt over” from Sunday to Tuesday in the leap year of 2012.
It is a tradition in Britain and Ireland that women may propose marriage on leap years. In Greece, marriage in a leap year is considered unlucky. In some countries, if a man refuses a marriage proposal from a woman on leap day, he is expected to pay a penalty, such as a gown or money. In other countries, if a man turns down a marriage proposal on leap day, he is expected to buy the woman 12 pairs of gloves, supposedly to hide the embarrassment of not wearing an engagement ring.

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