Czech scientists have rejuvenated, improved and coloured one of the most famous images in the history of physics. The original of 1919 was the first practical proof of Einstein's general theory of relativity. For Easter Monday of 2021, NASA's Association of Universities for Space Research chose this photograph, originally taken less than 102 years ago, as the Geoscientific Image of the Day, which has now been revived by Czech scientists in colour and previously unseen details. Miloslav Druckmüller from the Faculty of Mechanical Engineering at BUT took part in the renovation of the image.

The image was taken during a trip of the Royal Greenwich Observatory, which had the main task: to take the best possible image of a solar eclipse. So good to see how much the gravity of our star bends the light of the stars around it. The task of the expedition was to verify that the gravity of the Sun bends light to the extent that the general theory of relativity, which is famous today but was only a few years old at the time, predicted.

Einstein, already a famous physicist at the time, worked his way up to the really correct form of his gravitational field equations. However, he had to convince the rest of the world that they were right. He succeeded in convincing some of his colleagues relatively quickly, but the mathematics behind his discovery was complex and not accessible to many physicists, much less the general public. A clearer proof would be to measure the practical implications of the new concept of gravity in practice. It was possible for the first time thanks to the planet Mercury. As astronomers have noticed without Einstein's push, its trajectory does not fully correspond to what the classical Newtonian theory predicts. This was the first proof of the correctness of the general theory of relativity, which helped Einstein personally a lot. But it is especially the now revived photograph of the eclipse from 1919 that has become known to the public.

The image captures the Sun when almost the entire disk is covered by the Moon. Thanks to this, the solar corona can be seen - a flared plasma envelope of the Sun shaped by its magnetic field. The corona is quite "bushy" because the eclipse took place at a time when the sun was relatively active. Numerous magnetic loops can be seen in the image, the solar magnetic poles can also be recognized (top left, bottom right). What is interesting, though not in terms of the theory of relativity, is the protuberance in the image in the upper right. Protuberance is a cloud of cooler plasma hovering over the sun's surface. This is not a rare phenomenon, but taking a picture of it during a total solar eclipse is a rarity. At the same time, it is the largest protuberance ever captured photographically during a solar eclipse.

The authors of the image at the time were mainly interested in the relatively poorly and faintly visible dots at the bottom right, which are two stars belonging to the constellation Taurus. Thanks to the eclipse, the stars are bright enough to be photographed, even if they are so close to the sun's disk. Astronomers have thus been able to compare the position of these stars in the sky during an eclipse with their position at a time when the Sun is not present near them (i.e. at night.) It turned out that the image of the stars is indeed slightly shifted.

In 1914, a German-American expedition to the Crimea tried to measure the deviation. At first, however, it was unlucky for historical circumstances: the eclipse was supposed to be on August 21, but on August 1, Russia declared war on Germany. The German part of the expedition could not take part in the observation. The remaining American astronomers were not lucky with the weather. Subsequently, on November 10, 1917, the British Standing Committee on Solar Eclipse decided to organize expeditions to the long solar eclipse belt on May 29, 1919. There were two expeditions, coordinated by Frank W. Dyson and Arthur S. Eddington. One of them, led by Andrew C. D. Crommelin (1865-1939), traveled to Sobral in Brazil. Eddington then led an expedition to Prince's Island in the Gulf of Guinea, off the west coast of Africa. Both expeditions, despite some difficulties with weather and technology, eventually celebrated the triumph. Their images showed a much greater shift in the apparent position of the stars than Newton's laws predicted.

The results were first published in the specialized press, but gradually travelled around the world and ensured recognition both to the members of the expedition and to Einstein himself. Images taken during the 1919 eclipse went down in 20th century history. But the original images did not have a glorious fate: the original records were lost. Fortunately, there were copies that were sent to observatories around the world so that all scientists could see for themselves the evidence supporting the theory of relativity. One such copy of the expedition, which photographed the eclipse from the island of Sobral off the coast of Brazil, also reached the Landessternwarte Heidelberg-Königstuhl observatory, which recently digitized it as part of the Heidelberg Digitized Astronomical Plates (HDAP) project.

Petr Horálek from the Institute of Physics in Opava, in cooperation with the European Southern Observatory, obtained a digitized copy of the original record in the highest possible quality. At first glance, it was clear to him that the quality of the image did not reach the current possibilities. The scan was scratched and full of dust particles. And according to Horálek, it was clear from it that at the time of the eclipse on Sobral, the clouds were hampered. The scientists decided to "clean up" the image using modern methods and adjust it so that the real structures of the solar corona stand out. They improved the result with the help of the NAFE program of Miloslav Druckmüller from BUT, who is a specialist in astronomical imaging and became famous, for example, for photographs of solar eclipses. The program can also highlight fine details in the structures of otherwise rather blurred images, thanks to which it was possible to obtain even greater details in the solar corona, but mainly in the protuberance, which then appeared during the eclipse. Finally, Petr Horálek was able to reconstruct the colour of the entire image from the known colour properties of some phenomena in the image.