That's how much longer a "tropical" year is (on average in this epoch) than 365 solar days. (A solar day is measured from the point the sun crosses the meridian from one day to the next, as opposed to a sidereal day, which is measured by the time a distant star would require to pass through the same complete cycle.)
The calendar in a non-leap year does not account for this. So, let's see, if we add a 24-hour day every four years, 6 hours per year... Hey! That's gonna be pretty close! But it is too much--the calendar will measure 11 minutes, 13 92/100 seconds too many every four years.
This is not a real big deal, unless you're worried about centuries, which you probably should worry about if you want your calendar stable. So, in 400 years, the calendar with a leap day every 4th year would be 1123 minutes, 12 seconds too long. Taking out three leap days every 400 years just about does the trick. This removal of a leap day is done at the end of every century, except those divisible by four. (We had a leap day in 2000.)
The calendar ends up losing only 48 minutes every 400 years. Not too shabby.
Posted by The Owl at 02:29. Filed under: Mathematics and science
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