June 16 – See Spot Run

Today’s Factismal: The black dot of a sunspot is actually brighter than moonlight; it only appears dim because the rest of the Sun is so bright!

The Egyptians saw the Sun as a giant ball of poop being rolled across the sky by a dung beetle. The Greeks thought it was more like a young man riding a chariot from East to West (and racing back under the oceans each night). For the Norse Viking, it was a compass that lead him to land even after it had set. And to Aristotelian philosophers like St. Augustine, it was a perfect unchanging and unblemished sphere. Boy, were they wrong!

Galileo's drawing of sunspots (Image courtesy The Galileo Project)

Galileo’s drawing of sunspots
(Image courtesy The Galileo Project)

When Galileo turned his newly invented telescope to the skies, one of the first things that he looked at was the Sun. (NOTE: DON’T try this at home; it is a good way to go blind.) And one of the odd things that he noticed about the Sun was that it wasn’t unchanging and it wasn’t unblemished. As a matter of fact, it turned out to have lots and lots of what he named “sunspots”. He quickly proved that the spots were actually on the Sun (thereby proving that the Sun wasn’t perfect) and that they changed in position (thereby proving that the Sun rotated) and shape (thereby proving that the Sun wasn’t immutable). And there the state of the art stood for nearly 200 years.

In 1843, Heinrich Schwabe discovered that sunspots came in cycles; this was actually a byproduct of his search for a planet orbiting closer to the Sun than Mercury. (Just another example of how pure research can give interesting and unusual results.) At roughly the same time, Joseph Henry was able to measure the temperature of the Sun and sunspots. You can imagine his shock when he discovered that the Sun is roughly 10,000°F but sunspots are merely 7700°F. He soon realized that the sunspots were darker because they are cooler and that the sunspots would outshine the Moon if they weren’t surrounded by even brighter material.

A "butterfly diagram" of sunspots (Image courtesy NASA)

A “butterfly diagram” of sunspots
(Image courtesy NASA)

And one of the most impressive discoveries happened in 1874 when astronomers at the Royal Observatory in Greenwich made the first plot of sunspot size versus location. You can imagine their surprise when the plots came out looking like the wings of a giant, colorful insect – what we now call a “butterfly diagram”. The diagram clearly showed that sunspots start at high latitudes and slowly drift toward the Sun’s equator. The angle of the “wings” tells us how quickly the spots move and the size and color of the wings tells us how many spots were on the Sun at any given time.

And that last part is the most interesting. It turns out that the number of spots on the Sun changes. Sometimes there are very few or even no spots on the Sun (what astronomers call a solar minimum) and at other times, there are lots and lots of spots (what astronomers call a solar maximum). It takes about 22 years to get from one peak to another, though no-one is certain why. What we do know is that the amount of energy that the Sun gives off is closely related to the number of sunspots; more sunspots means less energy (for obvious reasons).

A sunspot compared to Earth (Image courtesy NASA)

A sunspot compared to Earth
(Image courtesy NASA)

But not all sunspots are created equal. Some are huge enough to swallow Jupiter where others could just barely choke down the Moon. Some are nearly perfect circles and others are irregular Rorshach tests of our physics understanding. Scientists hope that by classifying the various sunspots, we can learn more about how they influence the Sun and life on Earth. And that’s where you come in. If you’d like to page through image after glorious image of sunspots and help identify important features, then head over to Sunspotter.org. They’ll show you the images; all you have to do is help sort them out!

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