The planets of the solar system are all different. But most of the planets with atmospheres have something in common: seasons. Today, in fact, marks the start of autumn in the northern hemisphere of Mars.

The seasons are caused by a planet’s tilt on its axis. Mars is tilted by about 23 degrees — almost exactly the same as Earth. Because of that, the Martian poles dip toward or away from the Sun at different points in the planet’s orbit. When a pole dips toward the Sun, its hemisphere sees more sunlight, so it gets hotter.

Right now, at the equinox, the poles receive equal amounts of sunlight. But the south pole will tip toward the Sun over the coming months, heading toward southern summer.

One planet with an atmosphere that doesn’t have seasons is Venus. It’s tilted by only three degrees, so its poles don’t really dip toward the Sun — they barely nod, in fact. So the northern and southern hemispheres don’t see much change in either the amount of daylight or the temperature.

And Mars’s orbit is lopsided, so the planet’s distance to the Sun changes by a good bit. That amplifies the seasonal changes. But Venus’s orbit is almost circular. So conditions stay just about the same all across Venus, all year long.

Look for these worlds in the southeast at first light. Venus is the “morning star,” with Mars close to its lower right. The Moon slides past them the next couple of mornings, forming a bright trio in the dawn sky.
 

Script by Damond Benningfield