In the dark about physics? Perhaps the shadow knows

Dr. Wendy Sue Universe

Dr. Universe,

Why do we have seasons?

— Bella, 9, North Carolina

It turns out seasons can be quite different depending on where you live. But no matter where you live, the reason for the seasons has to do with the way the Earth rotates.

To find out exactly why we have seasons, I talked to my friend Vivienne Baldassare, a physics and astronomy professor at Washington State University.

She said the Earth makes a complete path, or orbit, around the Sun every 365 days or so. While the planet orbits the Sun, the Earth itself is also spinning around like a top.

“The Earth doesn’t spin straight up and down; it’s actually a little bit tilted,” Baldassare said.

It’s the Earth’s tilt that gives us our seasons. The tilt also means sometimes the North Pole will be pointing closer to the Sun, while the South Pole will be pointing farther away from the Sun.

When the Earth is in this position, it will be spring or summer in the Northern Hemisphere — the top half of the planet. But in the Southern Hemisphere — the bottom half of the planet — it will be fall or winter.

The opposite situation also happens. Maybe you can figure out what seasons we experience when the South Pole is tilted toward the Sun and the North Pole is tilted away from the Sun.

If you guessed it would be spring or summer in the Southern Hemisphere and winter or fall in the Northern Hemisphere, you are correct.

When it’s summer in places like Brazil or South Africa, it’s winter in places like Washington state or North Carolina.

Each season brings new changes for life on Earth, but the Earth isn’t the only planet with seasons.

“All you need in order to have seasons is the tilted axis,” Baldassare said.

Mars has a bit more tilt than the Earth. The red planet moves more slowly in its orbit than the Earth does, so a year on Mars is about two Earth years. Mars also has four seasons nearly twice as long as those we experience on Earth.

Meanwhile, Venus and Jupiter don’t have much of a tilt compared to the Earth. There isn’t much difference between seasons on these two planets. The seasons change slowly, and each season is about three years.

Here on Earth, you may experience four different seasons: spring, summer, fall and winter. But there are actually some places on our planet with only two seasons.

Believe it or not, both the Arctic at the South Pole and Antarctica at the North Pole experience only summer and winter. Antarctica experiences constant daylight during its summer and six months of darkness in the winter. That’s all because of the Earth’s tilt.

A lot of people have a favorite season. Baldassare and I both really enjoy fall in the Northern Hemisphere. It’s a great time to see the leaves change colors and to explore the outdoors. Do you have a favorite season? What do you enjoy about it, and what changes do you observe during that time of year?

Dr. Wendy Sue Universe, WSU’s resident science cat and writer would love to hear your thoughts. She can be reached by email at dr.universe@wsu.edu.

Recommended for you