A normal year has 365 days, but every fourth year sees an extra day fall into place. However, the time it actually takes for the Earth to orbit the Sun – to be exact – is closer to 365.25 days.
Therefore, in order to ensure the calendar remain synchronised with the solar seasons, an additional day is marked on the calendar every four years.
Planetary scientist James O’Donoghue, who works at the Japanese space agency (JAXA) has created a simple animation which shows Earth’s true orbit and why we need a leap year.
Mr O’Donoghue told Business Insider: “The way we do leap years is fairly messy looking, but I can’t think of a better way to handle them.
“We do it so our seasons don’t migrate over time.”
According to Mr O’Donoghue’s animation, if we were not to have a leap year, after 400 years our calendar months would drift into new seasons.
Another way to make up for the time anomaly which occurs on Earth’s orbit would be to add six hours to the end of each year.
However, Mr O’Donoghue said this would wreak havoc.
He continued: “One cannot simply add six hours to the end of our year to fix this, because then the sun would rise six hours earlier the next day.
“We could do it, but only if we don’t care that the 24-hour clock will cease relating to sunrise and sunset.
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“I actually really admire the leap year system we have now: It presents a great opportunity for people to discuss the celestial mechanics of our planet.”
According to Daniel Brown, associate professor in the School of Science and Technology at Nottingham Trent University, if we didn’t observe Leap Years, then in around 750 years’ time, the coldest time of the year would fall in June.
Professor Brown said: “That’s how much this shift would really make an effect on the calendar itself.”
Although Leap Years are thought to take place every four years, there are exceptions to the rule.
James Evans, a physics professor at the University of Puget Sound, said: “As it turns out, if you stick in one Leap Day every four years, that’s a few too many.”
Therefore, Leap Years fall on every year that is divisible by four, except for years that are exactly divisible by 100.
However, years divisible by 100 are still Leap Years if they are also exactly divisible by 400.
Consequently the years 1600 and 2000 were still Leap Years, while the years 1700 and 1900 were not.