Posted by: Bystander PJ23 FEB 2012
Since 2012 is a leap year, we gain a bonus day next week. Wouldn’t it be great if it was a real bonus — a national holiday?
We all know that a leap day is added every four years because the solar year is about a quarter of a day longer than the calendar year. But since the solar year is actually slightly less than 365.25 days, some fine tuning is need, and thus years evenly divisible by 100 are normally not leap years. Even finer tuning means that this rule does not apply to years also divisible by 400, so that 2000, unlike most century years, did acquire an extra day.
But even this does not precisely align the mean calendar year with the solar year. It has been suggested that the year 4000, although divisible by 400, should be a non-leap year. However, since it is impossible to forecast the precise length of future solar years, we cannot predict what calendrical microadjustments may be needed in future millennia.
But why are leap days tacked on to February? It is because that month was the last month of the early Roman calendar and thus the logical place for an extra day. When Julius Caesar revised the calendar in 45BC he moved the start of the year to January, close to the winter solstice, thus leaving leap days stranded in their awkward position ever since.
Leap days are also known as intercalary days or bissextile days. Based on Latin roots, the adjective “intercalary” was concocted early in the 17th century to mean a day inserted into the calendar. But “bissextile” is less easy to understand. Its origins lie in the way the Romans slotted in their extra day. Instead of adding it to the end of February, they placed it six days earlier. At this point in any month the Romans counted backwards from the start of the following month, so that the extra day was called “bis sexto”, meaning a repeat of the sixth day before the beginning of March.