The REAL First Day of Spring

Panorama of COSI on a sunny day

Spring is on the way. Really! Despite the chilly weather and occasional snow dump, the second half of March invariably brings on scientific-sounding descriptions of something called the vernal equinox, the day when daytime and nighttime are finally equal. It marks the time when daylight starts lasting longer, nights get shorter, and (eventually) warm weather returns to our fair land.

What is rarely discussed, though, is why. Why should the Earth's axis tilt in such a way to create varying seasons? Why are seasons so predictable, year after year after year? Do other planets have similar cycles? Will our pattern ever change?

To answer these questions, we have to go back to the REAL first day of spring, or at least the first spring we would recognize as such. We have to go back around four and a half billion years to our planet's early history, a time when we orbited our Sun without a companion, lonely and alone.

The Earth was a different world then, glowing hot and battered by collisions with asteroids, comets, and all manner of rocky and icy junk flying about the early solar system. A huge asteroid would careen into our planet and a molten pool of rock would form all around the wound, then slowly cool until the next collision occurred. It was a hot, hellish, unrecognizable world. Yet it was the world that would become our home.

One day on this hot and unfamiliar planet, something deeply profound and nearly catastrophic happened. Our world was hit, not by a bit of space junk the size of a city, but rather by another world.

This interloping planet, named Theia, was perhaps the size of Mars, perhaps even larger. Its impact nearly ripped the Earth apart. But by chance this monster collision was at just the right angle to produce a shower of debris in the Earth's vicinity. That debris, in time, coalesced to form our Moon.

The collision also did something else. It knocked the Earth over on its side, resulting in a spinning planet with a tilt of just about 23 degrees from vertical. Why does this matter? It is just this tilt that produces our seasons. As the Earth orbits the Sun, first the northern and then the southern half receive more direct sunlight, resulting in more daylight and warmer temperatures. We call this time summer. When our hemisphere is tilted away from our star, we experience our darker, colder winters. At the two halfway points of the Earth's yearly trip round the Sun, we celebrate the first day of spring (the vernal equinox, if you want to get technical) and the first day of fall (the autumnal equinox).

But the connection goes much deeper even than that. Mars, with no large moon in orbit, has an axis tilt almost the same as our Earth's. But this is just an accident of history; Mars's tilt changes over time as the planet wobbles through space and is tugged by other passing worlds. Earlier and later in history, Mars' tilt was and will be be very different from Earth's, wreaking havoc with seasons on the Red Planet.

Earth, on the other hand, has a large Moon that orbits relatively nearby. That big, nearby Moon has kept the Earth's axis tilt stable for billions of years, giving us (you guessed it) regular seasons, year after year after year.

So as you celebrate this year's vernal equinox, take a moment to thank the Moon, whose violent birth and everyday gentle tug makes springtime possible.

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