*and a quarter*times (six extra hours) on its axis by the time it has completed a full year's orbit around the sun, which means that every so often the calendar has to catch up. Since those six extra hours add up to 24 hours over the course of four years (4 × 6 = 24), our calendar includes a leap year every fourth year. (It's similar to receiving a free ice cream cone after getting your frequent buyers card stamped the fourth time.) That is the reason the month of February has 29 days instead of 28 for a total of 366 days in the year.

This year of 2020 is a leap year, but why is the word "leap" used? Believe it or not, it has to do with patterns. (All math is based on patterns!) Typically, a calendar date that is on, say, a Monday one year will fall on a Tuesday the next year; then Wednesday the year after that, and so on. However every fourth year, thanks to the extra day in February, we "leap" over Thursday and that same calendar date lands on a Friday instead. (For example, in 2016, Christmas was on a Sunday, but because 2020 is a leap year, this year, Christmas will be on Friday, not Thursday.)

**BUT**this is only a temporary fix! This will work for about 3,300 years, at which point we will be a day off -

**again**!

For example, 1600 and 2000 were leap years, but 1700, 1800, and 1900 were not. In the same way, 2100, 2200, 2300, 2500, 2600, 2700, 2900, and 3000 will

**be leap years, but 2400 and 2800 will be. Therefore, in a period of two thousand years, we will have 485 leap years. By this rule, the average number of days per year will be 365 + 1/4 − 1/100 + 1/400 = 365.2425, which is 365 days, 5 hours, 49 minutes, and 12 seconds.**

__not__So why does this formula have to be so difficult? Because, in reality, the exact number of days in a solar year is slightly less than 365.25 (365.242374, to be exact), so the algorithm is designed so that a leap year is omitted every so often to account for underestimating the length of the earth's orbit.

Unfortunately, there's an exception to the "divide by 4" rule. (You knew there would be). For some time, astronomers have been able to more precisely estimate the earth's orbit. In reality, that number is roughly 365.2422 days, or 365 days, 5 hours, 48 minutes and 46 seconds, just a smidgen under the 365.25 days previously discussed. By comparing the numbers, we see that the number above is off by 26 seconds. To make up for this, a rule states there can only be 97 leap years over the span of 400 years, not 100 as you may think. [Source: U.S. Navy Astronomical Center] One way to remember the rule is this:

**This is why 1900 wasn't a leap year but the year 2000 was.**

*Years that occur at the turn of centuries such as 1900 and 2000 must be evenly divisible by 400.*