A leap year, where an extra day is added to the end of February every four years, is down to the solar system’s disparity with the Gregorian calendar.
A complete orbit of the earth around the sun takes exactly 365.2422 days to complete, but the Gregorian calendar uses 365 days.
So leap seconds – and leap years – are added as means of keeping our clocks (and calendars) in sync with the Earth and its seasons.
Julius Caesar vs Pope Gregory
The Roman calendar used to have 355 days with an extra 22-day month every two years until Julius Caesar became emperor in the 1st Century and ordered his Alexandrian astronomer Sosigenes to devise something better.
Sosigenes decided on a 365-day year with an extra day every four years to incorportate the extra hours, and so February 29th was born.
In 46 BC, Julius Caesar, featured here in a self-decreed minted coin, created a calendar system that added one leap day every four years.
The system was tweaked, however, about 500 years later.
As an earth year is not exactly 365.25 days long Pope Gregory XIII’s astronomers decided to lose three days every 400 years when they introduced the Gregorian calendar in 1582.
The maths has worked ever since but the system will need to be rethought in about 10,000 years’ time. Perhaps mankind’s robot overlords will think of something.
Why does the extra day fall in February?
All the other months in the Julian calendar have 30 or 31 days, but February lost out to the ego of Roman Emperor Caesar Augustus.
Under his predecessor Julius Caesar, Feb had 30 days and the month named after him – July – had 31. August had only 29 days.
When Caesar Augustus became Emperor he added two days to ‘his’ month to make August the same as July.
So February lost out to August in the battle of the extra days.
Source – telegraph