How many years in a leap year?

Answer 1

Answer #1:

I think the terminology is unclear, but it is so common now that there is no changing it. What we experience about every 4 years on February 29 should be called a leap day, and not a leap year. When we periodically add a second to the clock (a change that most people don't even know occurs) it is called a leap second. A day is added to the calendar now and then, just to keep the calendar and seasons in line with the yearly cycles of our orbit. This is needed because the exact year is not evenly divisible into whole days. So 'how many years in a leap year' isn't really the question. It is just that now and then, typically every four years, we add a day to a year to keep the calendar in sync with earth's movement. Because the year is not precisely 365.25 days in length, we have to adjust the 4-year rule now and then, according to an established set of exceptions. When people casually ask whether or not a given year is a 'leap year', they are asking if February of that year will have a 29th day.

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Answer #2:

There is exactly one year in every year. It doesn't matter if it's a leap year or not because it takes exactly one year for Earth to completely orbit the sun and that's what we mean by a year. Of course the question should really be how many days are in a leap year, not how many years. But then the answer to that is that there are exactly the same number of days in every year, including leap years: 365.2424 days.

Of course our calendars wouldn't be much use if the last day of the year had 0.2424 days (almost 6 hours). That is, if the year started at midnight on 1st January, the following year would need to start at almost 6am on 1st January. After 4 more years the start of the year would land on 2nd January. If allowed to continue, the calendar would soon be completely out of sync with the seasons; the northern winter would happen in the northern summer months!

To compensate for this, a calendar year has to be exactly 365 days while every 4th year we add a "leap day" at the end of February (February 29th), thus creating a leap year with 366 days. This isn't quite enough to keep us in sync because 0.2424 times 4 is only 0.9696 days, but we've actually added a full day. However, we deal with this by skipping the leap day every 100 years but add it every 400 years. In this way our calendars are kept in sync with the actual year.

Why February 29th? This is simply because the year actually begins on March 1st, not January 1st, making February 28th the last day of a normal year, and February 29th the last day of a leap-year. This also explains why September, October, November and December are the 9th, 10th, 11th and 12th months: originally they were the 7th, 8th, 9th and 10th months reflecting their ordinal origins (sept, oct, nov and dec).