Creative accounting

Why do we have leap years? Illuminating video exposes truth about Earth's wonky orbit

If we did not have leap years, it would "ruin our lives."

NASA/Joshua Stevens

In 2020, we are blessed with an extra day, February 29. Every four years, we experience a leap year — a 366-day-long year that exposes a discrepancy between our calendar year and how long it actually takes for the Earth to orbit the Sun.

But a new visualization reveals that far from being just a conveniency of the calendar, leap years are actually critical if we want everyday life on Earth to run the way we need it to.

The visualization, created by James O’Donoghue, planetary scientist at Japan Aerospace Exploration Agency, explains the astronomical reasoning behind leap years

Earth takes a little more than 365 days to orbit around the Sun. In fact, the Earth takes exactly 365.2422 days, which is about six hours longer than what we account for in a calendar year.

As a result, each year the Earth falls a little short of being in the exact same spot as last year in its season.

“It’s funny because we measure our year in the number of rotations of Earth, that is already the problem right there,” O’Donoghue tells Inverse.

“We don't have a set number of rotations per orbit so there’s always going to be a little bit of a change in the end, and in this case it’s .24 days.”

Leap years account for that extra time. Over four years, the six hours add up to 24 hours — penciled in to the calendar as February 29.

Why we need leap years

An alternative solution would be to add an additional six hours at the end of every year, O’Donoghue says.

“This is all fine, if you don’t mind the Sun rising six hours earlier on January 1,” he says.

“We do it every four years so we can pretty much make one day as a shift, otherwise we would ruin our lives.”

Earth takes a little more than 365 days to orbit around its host star.NASA's Goddard Spaceflight Center

So what if we didn't have leap days at all, and instead just pretended the extra few hours did not matter?

Without the extra days, celestial events like equinoxes and solstices would shift a little bit each year, and in turn shift the seasons' dates around.

In fact, if we stopped accounting for those extra six hours for just one century, "summer" would shift to mid-July, according to NASA.

Leap years only take place in years that are divisible by 400. But even this is not a perfect accounting for the extra time it takes for Earth to make its way round the Sun. In reality, 0.24 days does not equate to six hours, but five hours, 48 minutes and 45 seconds.

To make up for our rounding up, we have to do a little extra accounting work every century. Each 100 years, we skip a leap year to make up for the accumulated 0.03 days. As a result, the year 2400, which is supposed to be a leap year, will be treated as if it was nothing of the sort, with just 365 days counted.

Celebrating science explained

O’Donoghue started creating his astronomical visualizations in late 2018, when he was working at NASA’s Goddard Space Flight Center and a government shutdown kept him out of the office for 35 days.

During his time off, he dabbled with Adobe After Effects, which he had used before to illustrate a video for a paper he co-authored on Saturn’s rings in December, 2018.

“I substituted my hobby for playing video games and made about 40 animations during the year 2019,” he says.

O’Donoghue shares his visualizations through his YouTube channel and on Twitter, where they receive millions of views.

“That’s not bad for a hobby,” he says.

O'Donoghue's videos explain simple concepts related to astronomy, like the seasons, and the scale of the Solar System — things we take for granted, but, without an astronomer's training, aren't actually well understood.

“Given that there’s only about 10,000 astronomers on the planet, there is a very narrow overlap between us astronomers and people that would make an animation,” O’Donoghue says.

All of O’Donoghue’s videos are available to watch for free online. The planetary scientist would like to see more people have access to this kind of science, regardless of their level of education or background.

“I feel like there’s definitely not enough science out there, I don’t think it’s celebrated enough,” he says.

“I think it needs to have a larger role in society like sports, music and entertainment," he says. "It would be nice to see the public more scientifically literate.”

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