Quote (AiNedeSpelCzech @ May 19 2016 05:49pm)
It equals most years. No actual year has a quarter day. :P
why do you think some months have extra days
There are approximately 365.25 days, but the true value is something like 365.2422.
And actually, the earth's rotation is slowing down as well--by something like 0.00002 seconds/year--so the number of days per year is shrinking while the days themselves get longer.
365.2422 is indeed very close to exactly 365.25, but yes, it is just a coincidence. Had we reached this point in our astronomical knowledge a couple million years sooner or later in earth's history (an eye-blink in terms of the age of the earth), the value wouldn't be quite so close to a nicely small-denominator fraction.
Note, too, that the kind of year you're talking about is the "tropical year." There are other ways to measure years (and days, for that matter), which naturally come out to different numbers of days/year. The Wikipedia article on the year defines all the kinds of years you're ever likely to encounter, and then some.
Also note: that fractional part, 0.2422, is really close to 0.2425, which is 1/4 - 1/100 + 1/400, which is why the leap year rule is "every four years, except century years, unless the year is divisible by 400". The "every four years" gets you an extra 1/4 = 0.25 days per year. But that's just a bit too much. It's 0.0078 days too much, to be precise. So we take away one leap-day per century, which is equivalent to subtracting .01 days per year, leaving 0.24. But then that's not quite enough. Now we're short by 0.0022 days/year. So we add back in one leap year every four centuries, or another 1/400 = 0.0025 days. The net result is 0.25 - 0.01 + 0.0025 = 0.2425, which is only 0.0003 days too much, and we call that close enough. Fun with fractions!