December 11, 2011 - First Recorded Sighting of the Aurora Borealis in America

– 1719

The Aurora Borealis is another name for the Northern Lights—the dancing “curtains” of light that sometimes appear in the night sky in the far north. (The same thing in Antarctic regions is called the Aurora Australis.)

Of course we know that this natural phenomenon had been seen many, many times before 1719—by, for example, the many native peoples of Alaska and Canada—but for the English settlers in New England, it was a new and unexplained sight. Many witnesses in New England, on this day in 1719, saw a mysterious face looking back at them from the sky. Some responded with fright, thinking that this was a religious experience foretelling the “last judgement.”

Now we know the true explanation: the sun not only emits light, but also charged particles such as free electrons and positive ions. These energetic particles pour down on the surfaces of all the planets and moons, but because Earth has a magnetic field, the particles are redirected and then funneled downward to the north and south poles. When the particles collide with atoms in the upper atmosphere, those atoms emit light.

Auroras tend to get brighter whenever there are sunspots on the sun, because these dark spots indicate solar magnetic storms that cause the sun to emit more charged particles than usual.

By the way, Earth acts like a giant magnet because of the rotation of its molten core. Venus, which is the closest planet to the Earth and also its twin in size, does not have magnetic fields. Scientist think that Venus must have a iron core similar to Earth's, but the planet's very slow rotation—one Venus day is 243 Earth days!—is just too slow to produce the dynamo effect that causes Earth's magnetic fields.

Can you find out which other planets do or do not have magnetic fields? (Hint: most of them do.)

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