This animation shows why leap years are necessary. Without them, December would eventually drift into summer.

• Leap Day is February 29. A simple animation shows the purpose of this bonus day.
• Leap years realign our calendars with the equinoxes and solstices to keep up with changing seasons.
• Without leap years, December would drift into summer in 400 years.

Leap year happens once every four years for a good reason: Without it, December would eventually drift into summer.

We get a bonus day on Thursday — Leap Day, February 29 — to keep the seasons in check.

An animation from James O'Donoghue, a planetary scientist who has worked at NASA and Japan's space agency (JAXA), illustrates how leap years do that and why years like 2024 are 366 days long.

"The way we do leap years is fairly messy looking, but I can't think of a better way to handle them," he told Business Insider in an email. "We do it so our seasons don't migrate over time."

As his video (below) makes clear, leap years resolve a problem with the way we track time: Even though our calendars measure a year as 365 days, Earth's orbit around the sun actually takes 365.242 days.

O'Donoghue's animation reveals what would happen if we didn't have leap years — after 400 years, our calendar months would drift into new seasons.

Over the years, O'Donoghue has spent lots of his free time making animations of astronomy concepts like the moon's retreat from Earth and the vastness of our solar system.

Why leap years are necessary

Astronomically, seasons are marked by equinoxes and solstices. The equinox happens twice a year, in March and September, when Earth's equator lines up with the center of the sun.

On those days, day and night are nearly equal lengths everywhere on the planet. After that, the seasons change. Days grow longer and nights become shorter on one side of the planet, and vice versa on the other.

The solstice is the opposite: On those days, the sun appears at its furthest point from the equator, and the lengths of day and night are the most unequal.

Because it takes Earth 365.242 days to travel from one March equinox to the next, our calendars fall 0.242 days (about six hours) short of the actual seasons every year. That extra quarter of a day, of course, adds up to one full day of drift every four years.

So leap days re-align our calendars with the astronomical seasons.

"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," O'Donoghue said. "We could do it, but only if we don't care that the 24-hour clock will cease relating to sunrise and sunset."

Leap years are the least confusing solution, he added.

"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," O'Donoghue said.