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Staring at the Sun, Just as Galileo Did

Astronomers at the Mount Wilson Observatory sketch sunspots every day, continuing a tradition started by Galileo.

By Dava Sobel
May 27, 2009 5:00 AMNov 12, 2019 6:40 AM


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Mount Wilson, Calif.—As a confessed Galileo groupie, I chose to hold my personal observance of the 2009 International Year of Astronomy on this summit in the San Gabriel Mountains east of Los Angeles. All around the world, astronomy enthusiasts are collaring the uninitiated to look skyward and see the things Galileo saw, from mountains on the moon to the star lanes of the Milky Way. But here at the Mount Wilson Observatory, a few astronomers still pursue an archaic artistic activity that Galileo pioneered: They make daily drawings of sunspots.

The existence of spots on the sun—announced more or less simultaneously four centuries ago by Galileo in Italy and several contemporaries in other countries—inflamed theologians and academics alike. The sun was presumed to be an immaculate, immutable body. How could its face be marred by dark blemishes that bloomed, grew, and moved about Galileo tried to answer those fundamental questions by watching the spots over time and rendering their individual characteristics. Aware that observing the magnified sun directly would blind him, he allowed the solar image to fall through his telescope tube onto a piece of paper, where he traced the elaborate chiaroscuro of the spots.

When Galileo recognized a particularly distinctive group of spots rolling back into view after it had drifted out of sight two weeks earlier, he realized the spots were not so much moving as being carried around. He had discovered the monthly rotation of the sun. Later, following the trails of sunspots over the course of the year, he saw them tilt upward, then downward. He interpreted the changing orientation as a perspective effect and evidence that Earth moves around the sun on an inclined orbit.

Scientists ever since have continued to read the daily sunspot pattern for clues to other major astronomical discoveries. Just keeping count of the number of spots, for example, led to recognition of the 11-year sunspot cycle that waxes from “solar minimum,” when very few spots are seen, to “solar maximum,” when great conglomerations of planet-size splotches pockmark the photosphere, or visible surface of the sun. At times of maximum solar activity, the magnetic ferment represented by sunspots frequently releases and leaps across space to Earth—to foment magnetic storms that disrupt communications networks and light the polar skies with auroral displays.

Fortunately or unfortunately, the timing of my visit to Mount Wilson (not to mention the entire International Year of Astronomy) coincides with a period of pronounced solar minimum. The current minimum, in fact, is the deepest and longest-lasting of the past half century. Some scientists speculate that the sun may be entering a prolonged inactive phase, similar to the one that lasted from 1645 to 1715 and coincided with the “little ice age” in Europe—although there is no evidence that the sun will rescue us from global warming.

For weeks before my arrival, I checked the daily sunspot drawing posted on the observatory’s Web site to find only a round blank slate, accompanied by the message “No Spots Seen Today.” But on the day I drove up the switchback road to meet the men who draw the sunspots, four small spots appeared.

The 150-foot solar tower on Mount Wilson stands in geometric contrast to the site’s several traditional domes sheltering telescopes aimed at more distant stars. Its skeletal frame resembles a construction crane or a bridge truss. Two flat mirrors and an objective lens atop the tower catch the sun’s image and toss it down a tube to the ground-level observing room, where the sunspot drawings are made. (Buried another 80 feet belowground lies the spectrograph for analyzing the sun’s light. A 60-foot solar tower nearby is dedicated to helioseismology—the study of the sun’s oscillations in order to determine its internal dynamics.)

The first thing I noticed on entering the observing room of the 150-foot tower was a bust of George Ellery Hale, founder of the observatory and an early expert on the sun’s magnetic behavior. In 1912 Hale initiated the daily practice of drawing sunspots here and established the archive that now holds some 26,000 such drawings. Unlike Galileo, however, Hale was no artist. He sketched the sunspot contours with scribble and crosshatch, intent mainly on mapping the locus (solar latitude and longitude) of each magnetic outburst and gauging the polarity and field strength (in gauss) of every sunspot. The expression on the bust suggests he is pleased to watch the program continue.

When on-duty solar observer Steve Padilla of UCLA adjusted the mirrors atop the tower, the sun’s reflection swamped the observing table. We could see the four small sunspots jumping about like a family of fleas. Mount Wilson boasts 300 clear days (and nights) per year, and this day was surely one of the clearest, but the “seeing” was disturbed by strong January gusts that jiggled the sunshine. The wind also shook the tower from time to time, causing a further blurring of the picture.

The huge solar image—17 inches in diameter—called for a correspondingly large sheet of white paper. Padilla keeps his drawing pencils on a ledge at the edge of the observing table in an old metal box labeled “Koh-i-noor.” The small assortment ranges in grade from 2-B to 9-H, which allows for ample gradations of shade on better days, when spots might mimic Rorschach inkblots or elaborate mold growth. This day’s minimal offerings could be recorded swiftly in monotone. The largest one was probably twice the size of Earth but measured only a quarter of an inch across the page. Padilla was almost apologetic. He offered, by way of compensation, a photograph from April 6, 1947, of the largest sunspot group ever observed from Mount Wilson. Here the grainy photosphere erupted in crowds of spots, each with an irregular black center, or umbra, marking a relatively cool area on the sun’s nearly 10,000-degree-Fahrenheit surface. The surrounding lighter area, or penumbra, of each spot seethed with texture.

The ideal time to create the sunspot drawing is the hour just after sunup, according to Larry Webster, another experienced observer who lives on the mountain as the site manager of Georgia State University’s Chara (Center for High-Resolution Astronomy) Array.

“Within an hour of sunrise the atmospheric conditions are steadiest,” Webster explained, “because the sun hasn’t yet heated up the mountain. But if it’s cloudy in the morning, you wait for a break in the afternoon and try it then. The drawing can take anywhere from 10 minutes to three hours, depending on the solar activity. In November 1978, when I first started, we were at solar maximum, and one day I had 23 sunspot groups to draw. Some guys would just put a circle with a line through it for each sunspot, but I tried to make mine as nice as I could, shading the umbra and the penumbra. I’d even take a little artistic license and draw the fibrils radiating from the penumbra.”

It was time for Padilla to go up to the top of the tower to adjust the mirror for the afternoon magnetogram observations, so he put on a hard hat and buckled himself into the “elevator,” as the small open bucket is euphemistically called.

Site superintendent Dave Jurasevich stood outside with me to watch the ascent. He recalled how Ferdinand Ellerman, one of the first solar observers at Mount Wilson, used to halt the bucket halfway up the tower, take a cigar from his pocket, then pull out a magnifying glass. For the benefit of any spectators below, or perhaps just for his own amusement, he would light his cigar with a focused sunbeam before resuming his climb. Padilla, who doesn’t smoke, just waved to us without stopping.

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