One day on Earth is the time it takes for our planet to complete a full spin on its axis, while one year on Earth is the time it takes for the planet to complete a full orbit of our star, the Sun. However, the two are not linked, and Earth’s orbit around the Sun doesn’t exactly take 365 days. In fact, it takes roughly 6 hours longer, meaning that every four years, calendars would be one day out of sync.
This is important as the longer this is left, the calendar would become more out of sync with the seasons, which we associate with certain months. Within a century, our calendar would be off by 25 days and in 700 years, the seasons would be flipped – meaning the Northern Hemisphere would celebrate Christmas in the summertime.
Although a 365 day calendar is too short, 366 days is too long, so an extra day is added to the calendar every four years, which restores the balance.
Well, sort of, because an extra day every four days is too much. To correct this, the leap year rule is ignored every century; meaning that 1896 and 1904 were leap years, but 1900 wasn’t. However, this also causes problems as this puts the calender behind by one day every 400 years. As a result, if a year can be divided by 400, a leap day is added; this means that, whilst 1800 and 1900 weren’t leap years, 2000 was.
This prevents the calendar we use from becoming out of sync with the seasons and orbit of the planet. That said, the current rules regarding leap years mean we will need an extra day once every 8,000 years, but we will cross that bridge when we come to it…