Summer solstice: Did a rampaging planet create Earth's seasons?

The summer solstice brings attention to Theia, a Mars-sized planet-wannabe some scientists say helped create the moon and altered Earth's axis.

Mike Segar/Reuters
Summer solstice: People practice yoga on Monday morning, in celebration of the first day of summer in New York's Times Square. Some scientists say a planetlike object called Theia crashed into the earth, altering its axis and creating the moon.

Northern hemisphere? Happy summer solstice!

Southern hemisphere? Eat your heart out (for now).

Monday marks the official start of summer for those living between the equator and the North Pole. The celestial hand-off from spring to summer came at 11:28 Universal Coordinated Time this morning.

IN PICTURES: Summer Solstice at Stonehenge

In short, the northern solstice represents the moment when the northern end of an imaginary, oversize No. 2 pencil – poked through the Earth along its axis of rotation – is leaning closest to the sun as the planet moves along its orbit.

Which means that for all the general talk of "highest point above the horizon at noon," which depends on where you live, the summer solstice arrived at 3:58 p.m. in Kabul, during the morning commute in Boston, and at 1:28 a.m. in Honolulu.

Summer's arrival is celebrated with everything from casts of thousands at Stonehenge to festivals of all sorts, a kind of collective open-armed welcome to warm weather and (hopefully) bountiful crops.

But amid the revelry, perhaps we also should pay homage to what may be the ultimate source of Earth's seasonal swings: Theia – a Mars-sized planet-wannabe that some astronomers have invoked to explain the formation of the moon.

As the story goes, Theia grew out of a collection of smaller building blocks from the disk of dust and gas surrounding the young sun. That collection gathered in a gravitational sweet spot roughly 60 degrees from Earth along the planet's orbital path, a bit like leaves in a whirlpool.

Theia grew too large, however, to remain gravitationally bound in that sweet spot, so it broke free and headed in a wobbly way toward its collision with Earth.

That smack is thought to have knocked the Earth's axis off kilter by some 23.5 degrees, relative to the plane of Earth's orbit around the sun, as well as dislodged material that formed the moon.

Without that tilt, we might've ended up with more seasons than we could deal with over the course of Earth's history, suggests Neil Comins, an astronomer at the University of Maine. He's written substantially on what alternate Earths might be like and how hospitable they would be for the emergence of life.

The collision thought to have generated the tilt also created the moon, which is responsible for stabilizing Earth's spin axis. Without a moon, "the Earth spinning on its axis is an unstable system in which the tilt goes from straight up and down to far closer to the plain of it orbit than the axis is today," he says.

At its most extreme, this could leave Earth orbiting the sun and spinning on its axis like a chicken roasting on a spit with head and tail alternately aimed at the fire, rather than cooking the bird broadside. Earthlings from one pole to the equator would be in darkness during the solstice, while the other half would be in toasty sunlight. The poles would swap lighting conditions at the next solstice.

Moreover, the moon's presence and its effect on the oceans act as a brake on Earth's rotation rate. By some estimates one turn of the Earth on its axis some 4.5 billion years ago would have taken 6.5 hours, versus 24 today. No moon means a far more speedy cosmic rotisserie.

"Differences in ocean temperature, currents, fill in the blank, would be quite significant," Dr. Comins says.

Indeed, a collision-free, tilt-crazy planet might not have been hospitable to life at all, he and others suggest. Ocean tides, for instance, at first enormous because the moon was far closer to Earth than it is today, would have stirred up nutrients in the ocean for uptake by organisms that could take root and thrive in vast intertidal zones.

In the absence of the moon, tides would be wimpy, even by today's standards.

So when you break out the sun screen and switch on the Nook for that splash of summer reading, give Theia a thought or two. But no more. Her existence, after all, represents a hypothesis that still contains some inconsistencies. And there's too much summer to enjoy.

IN PICTURES: Summer Solstice at Stonehenge


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