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Eclipses through Time

by Louise Good

The solar corona as viewed through 5 filters

A composite of the solar corona as viewed through five different filters during the total solar eclipse of August 1, 2008. The image has been processed to reveal the fine structures of the corona. The green-bluish hue corresponds to solar gas at a temperature of about 2 million degrees C, while the reddish hue corresponds to gas at 1 million degrees C. The data were taken in the Gobi desert in China by a team led by S. Rifai Habbal (UH IfA), and from Mongolia by a team led by M. Druckmüller and P. Aniol. Composite produced by M. Druckmüller.

Shadia Rifai Habbal
Shadia Rifai Habbal

On November 18, IfA solar astronomer Shadia Habbal gave a Frontiers of Astronomy Community Lecture entitled "The Magic of a Disappearing Sun" at the University of Hawaii at Manoa's Art Building auditorium. Habbal has led a number of eclipse expeditions in recent years, including her most recent one to Enewetak in the Marshall Islands in July 2009, one of the longest eclipses in recent times. Thus, I expected to hear about what she and her colleagues had learned from data gathered during these expeditions. And, indeed, there was some of that included in the talk, but most of it was a very fascinating history of solar astronomy and eclipse expeditions.

Total eclipses generally occur every year or two, and they are the best opportunity astronomers have to study the Sun's corona, the extremely hot ionized gas (more than 3 million degrees F) that is the Sun's outer atmosphere. Though eclipses were known, often feared, and even predicted in ancient times, it was not until the 19th century that scientists began to use them to study the Sun. They were aided by the invention of photography in the 1820s and 1830s. The first photograph of a total solar corona was a daguerreotype made during the solar eclipse of July 28, 1851.

It took eclipse observers a while to figure out that the corona is indeed part of the Sun. Though he had never seen an eclipse, the German mathematician and astronomer Johannes Kepler opined in 1605 that the corona was light reflected from matter around the Sun. In 1724, Giacomo Filippo Maraldi, who had seen an eclipse, concluded that the corona was part of the Sun because the Moon traverses it during an eclipse. It was Jose Joaquin de Ferrer who gave the corona ("crown" in Spanish) its name. After viewing the eclipse of June 16, 1806, he declared that the corona was part of the Sun, not the Moon, because of its great size. But the most convincing evidence that the corona is part of the Sun was discovered by the French spectroscopist Jules César Janssen. He noticed that the shape of the corona changes with the sunspot cycle. It was rounder in 1871 at solar maximum than it was at solar minimum in 1878.

A similar question arose about solar prominences, streams of glowing gas that shoot out from the Sun's upper chromosphere and lower corona. Were these also part of the Sun? Eclipse observations from 1851 to 1860 established that they were.

Eclipse observers benefited greatly from the invention of the spectroscope in 1859. Spectra of prominences taken during the August 18, 1868, eclipse showed that prominences are composed primarily of hydrogen. During an eclipse in August 1869, two observers independently discovered what they thought was a new element not seen on Earth. They named it coronium. It was not until 1941 that it was correctly identified as an ion of iron with 13 electrons missing. In 1871, English solar physicist Norman Lockyer discovered helium in the corona. At that time, helium was not known to exist on Earth.

Observations made during solar eclipses have been used to confirm the prediction made by Einstein's general theory of relativity that gravity bends light. On May 29, 1919, British astronomer Arthur Eddington and his colleagues observed an eclipse from two sites, one near Africa and one in Brazil. They noticed that stars near the eclipsed Sun appeared to have moved from their actual positions, thereby confirming that the gravity of the Sun bent the light from these more-distant stars.

Another interesting fact is that over the years eclipse expeditions have become considerably smaller both in terms of the equipment required and the number of people needed to make the observations. Expeditions in the late 19th and early 20th century must have required large ships or trucks to transport their equipment. This year, members of the IfA eclipse expedition and all their equipment made it to Enewetak in one small airplane that was able to land on that island's short runway.