Since the Middle Ages, the observation of total solar eclipses, like the one we will witness next Monday, April 8 in southern Quebec, has enabled major scientific discoveries. It also revealed enigmatic physical phenomena that have still not been resolved to this day.
A total solar eclipse is a unique opportunity to see the solar corona. When the Moon completely obscures the solar disk, it then becomes possible to observe it. About a million times less luminous than the sphere of the Sun, the solar corona is invisible in broad daylight, in particular because our eyes are primarily dazzled by the brilliance of the Sun.
The first written mention of the existence of the solar corona appears in the annals of the Court of Constantinople, dated December 22, 968. It is described as “a thin brilliant ring encircling the disk of the Sun which has become black and without light which appears at the time of an eclipse. “The solar corona is actually the atmosphere surrounding the sphere of the Sun. It is a very large and distended atmosphere that stretches for a distance of up to twice the radius of the solar sphere. It is composed of plasma, that is to say very hot gases, mainly hydrogen, where the majority of atoms have lost their electrons,” explains professor in the Department of Physics at the University of Montreal Paul Charbonneau. , world expert in solar physics.
Writings dating from the eclipse of 1er May 1185 report “burning embers” which appeared around the solar disk obscured by the Moon when the eclipse became total. At that time, it was believed that these were clouds from the lunar atmosphere which were illuminated by the Sun, recalls Mr. Charbonneau. But during the eclipse of May 2, 1733, using a telescope equipped with a filter, the astronomer Birger Wassenius described with more precision these structures present in the solar atmosphere which we today call protrusions.
“The protrusions which appear during eclipses in the form of very bright structures within the corona are in fact accumulations of plasma which find themselves confined by the magnetic field of the Sun,” explains the specialist.
However, it was the first photograph taken of an eclipse in 1851 which demonstrated once and for all that the corona visible during the entire eclipse is indeed that of the Sun, and not that of an extended atmosphere of the Moon. And that the prominences are located in the solar atmosphere and not in that of the Moon.
This confirmation gives impetus to the science of the Sun and eclipses. During the eclipse of August 18, 1868, Norman Lockyer and Jules Janssen used spectroscopy, a technique recently developed at the time, to observe the spectrum of a prominence present in the solar corona. They detect a spectral line which does not correspond to any chemical element previously known on Earth. They attribute this line to a new element that they call helium, in honor of Helios, the Greek god of the Sun. The helium discovered in the solar corona was finally isolated and identified in the laboratory in 1895, specifies Mr. Charbonneau.
One million degrees Celsius
In 1869, Charles Young and William Harkness in turn spotted a line in the spectrum of the solar corona which, once again, was not linked to any recognized chemical element. They associate this spectral line with a new element which they call coronium. But in 1941, two specialists in very high temperature and very high pressure spectroscopy, Walter Grotrian and Bengt Edlén, discovered that the spectrum previously associated with coronium actually coincided with the lines of iron and nickel found in a highly ionized state. By this observation, they prove that coronium is not a new element. “For iron and nickel to end up in such an ionization state where they have lost several electrons, the temperature must be at least one million degrees Celsius. The discovery of Grotrian and Edlén therefore indicated that temperatures within the solar corona reach one to two million degrees Celsius,” notes Mr. Charbonneau.
“Since the surface temperature of the Sun is only about 6000 degrees Celsius, the energy emitted by the Sun is therefore not sufficient to heat plasma to a million degrees by thermodynamic processes. It is still unclear what causes such overheating. This is one of the big questions in solar physics that remains unresolved. Two hypotheses have been proposed, but neither is really convincing. The question remains a very active subject of research,” he adds.
The gases in the corona which are heated to temperatures ranging from one to two million degrees cannot remain in place around the Sun, unlike those in the Earth’s atmosphere which are cold and therefore are maintained around the planet by its gravity, underlines Mr. Charbonneau. The astrophysicist Eugene Parker demonstrated in 1958, through theoretical calculations, that the gases of the solar corona, due to their very high temperature, disseminate in interplanetary space, thus forming a “solar wind” which accelerates and reaches supersonic speed as it approaches Earth. The existence of this solar wind as well as its speed were subsequently confirmed by probes Lunik in 1960, Explore 10 in 1961 and Marinate 2 in 1962.
The theory of general relativity
But perhaps the most significant scientific contribution arising from the study of total solar eclipses is the confirmation of Einstein’s theory of general relativity. “This theory made some really bizarre predictions, including one stating that near a large mass, like the Sun for example, space-time is distorted and light rays that normally travel in a straight line are slightly deviated” , summarizes the physicist.
Einstein’s theory predicted that, during an eclipse, a ray of light from a star passing near the Sun would be deflected, causing the star to be seen at a certain position in the sky. which would be slightly different from where it really is. “The deviation caused by the mass of the Sun that was predicted by theory was 0.000472 degrees on a protractor, a tiny deviation that was measured during the 1919 eclipse at two different locations,” says M. .Charbonneau.
Total solar eclipses have probably not finished sending scientists running and dazzling ordinary observers.