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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.

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.

Mike Segar/Reuters

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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.


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