Soon the leaves will turn red, the shorts will go back in the closet, and the turkeys will flee for their dear lives – another day, another year, and another season. Sitting at the end of summer, it’s perhaps an opportune time to ask why we experience seasons to begin with.


Image of an old globe
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Earth orbits around the sun while rotating around its own axis. However, the rotation axis of the Earth is not perpendicular to its solar orbit. Rather, the Earth exhibits an axial tilt of 23.44°. If you were to imagine a bead suspended on string that you held vertically with one hand at each end, earth’s rotation axis would look the same as if you moved both of your hands slightly away from your body to produce a diagonal line from your point of view. This represents Earth’s axial tilt. The axial tilt does not change its orientation as Earth orbits around the sun.

Spread the sunshine

Image of sunshine streaming through a field of flowers
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Because of the axial tilt, the Northern and Southern hemispheres receive different amounts of sunlight depending on Earth’s orbit around the sun. When the northern hemisphere is closest to the sun, it experiences the hot season. When the northern hemisphere is farthest away, it experiences the cold season. The southern hemisphere experiences the opposite amount of sunlight due to the tilt, which is why countries such as Australia experience summer from December to February and winter from June to August.

The days are getting longer

Aerial image of trees with leaves changing color
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Earth’s axial tilt also affects the diurnal cycle. The portion of the Earth that is closest to the sun at any given point not only receives more heat from its proximity but also experiences a longer duration of sunlight throughout each complete rotation (one day). The opposite is true during the winter. These effects increase with latitudes diverging from the equator and are most profound at the poles. The North Pole goes through the “polar night,” a period of six months without sunlight, and the south pole goes through the “midnight sun,” during which time there is no night. The mid-latitudes are the only regions where a four-season year is experienced.

Switching things up

Photo of a globe in front of a map
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You may be wondering whether or not the axial tilt changes over time, and the answer is yes. The Earth exhibits obliquity, i.e. change in axial tilt. As the tilt of the Earth increases away from the axis perpendicular to its orbit around the sun, the effect of seasonal temperature shifts become more extreme, but don’t panic. Obliquity occurs in a cycle every 40,000 years with the axial tilt oscillating between 22.1-24.5°.

But whence?

Photo of Earth from the surface of the moon
Credit: NASA

The question arises as to the source of Earth’s axial tilt. The current theory traces back to the Earth’s first breaths. As a solar system forms, particulate matter begins to swirl around the gravity pool of a new star. This particulate matter clumps into aggregations called planetesimals and protoplanets that further aggregate to form planets. Early during Earth’s formation, the beginnings of our protoplanet were struck by another protoplanet, which was absorbed into what became Earth. The collision threw Earth’s rotation axis off tilt. In addition to the axial tilt, this collision also released debris, which accumulated some distance away from Earth to form our moon.

Simple stuff

Image of dry, cracked desert dirt
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So there you have it. We are riding a gargantuan mass of aggregated star dust spinning askew as a result of a momentous collision from over 4 billion years ago, flying in circles around a gigantic flaming ball of gas in space.

And that’s why you wear jackets in the winter and shorts in the summer.