Though we often experience seasons as a particular type of weather (snow in winter, rain in spring), or a set of holidays (Memorial Day, the Fourth of July, Labor Day, all to mark the phases of summer) — or even a specific taste (like “pumpkin spice” in fall), what really defines a season is the position of the planet (and your location on it) relative to the sun.
Though the Earth rotates around the sun in an ellipsis, it’s not the planet’s changing distance but rather its changing orientation that determines seasonal shift.
In December, the south pole points toward the sun; in June, it’s the north pole that’s angled towards the star. Whatever pole is facing sun gets the most direct light; the pole that’s tilted away is lit only indirectly. The way we mark the official passage of seasons—equinoxes and solstices—springs from this solar relationship. In any given hemisphere, solstices mark the highest or lowest angle of the sun.
(Though it falls on the same day in December, it’s a summer solstice below the equator and a winter one above it.) In all hemispheres, the equinox marks the midpoint: when both poles receive equal light.
It wasn't always this way. Around 4.5 billion years ago, scientists believe the Earth did not experience seasons: The surface of the planet received the same amount of light day in and day out year round. What changed was the moon.
According to the prevalent theory for the moon’s formation — the “giant-impact hypothesis” — the Earth’s axis (the invisible line that runs through the planet and connects its two magnetic poles, around which the planet spins each day) used to run perpendicular to its orbit.
It got knocked off center by a planet-sized object scientists called “Theia,” which directly collided with Earth.
Scientists who analyzed the geological content of both Earth and the moon hypothesized, in a 2015 paper published in Nature, that this explosive impact sent debris from Theia and Earth into space, where it likely coalesced to become the moon: part Earth, part Theia.
The Earth, likewise, absorbed the remainder of Theia: a remade, and now seasonal, planet.
But just because there are seasons, that doesn't mean you have to get used to fluctuating temperatures. If you favor one season — or even one specific temperature — over another, take a look the interactive tool Perpetual Temperature, which will tell you where on Earth to find your ideal weather no matter the time of year. And then start planning that epic around-the-world journey for your never-ending summer (or fall, or spring, or winter).