Spring is already halfway through here in the northern hemisphere. But the season is just beginning in the northern hemisphere of Mars. Today is the vernal equinox on the Red Planet. The Sun crosses the planet’s equator from south to north, ushering in spring.

Mars has seasons for the same reason that Earth does: the planet is tilted on its axis. In fact, Mars is tilted at almost the same angle as Earth. So as the planet orbits the Sun, the northern and southern hemispheres take turns receiving more sunlight.

But the Martian seasons are a bit more complicated than those on Earth. That’s because Mars’s orbit is much more lopsided than Earth’s. The planet’s distance from the Sun varies by about 25 million miles, compared to only about three million miles for Earth. So when Mars is closest to the Sun, it receives a lot more energy than when it’s farthest from the Sun.

Right now, Mars is near the middle of that range in distance. As the northern spring progresses, though, Mars will move farther from the Sun. It’ll then start moving closer, and get closest to the Sun near the end of autumn. As a result, northern winters and summers are much more temperate than the southern seasons.

As the distance to the Sun changes, so does Mars’s orbital speed. That means there’s a good difference in the length of the seasons. Northern spring is the longest season. It lasts more than six months, so it’ll continue until the middle of fall here on Earth.

Script by Damond Benningfield