Eclipses that changed science

For most of history, a total eclipse was the only chance to see what surrounds the Sun. The Sun’s bright disc drowns out everything near it, so astronomers planned expeditions years ahead, shipped telescopes across oceans and deserts, and then hoped for a clear sky on the day. Many came home with nothing but cloud.

The lucky ones brought back discoveries that changed physics. Eclipses showed that the red flames at the Sun’s edge belong to the Sun, added helium to the list of elements and proved that gravity bends light.

1. 1851

   The first photograph of totality

   On 28 July 1851, at the Royal Prussian Observatory in Königsberg, the daguerreotypist Johann Julius Friedrich Berkowski exposed a silver-coated plate for 84 seconds through a small refracting telescope. It was the first successful photograph of a total solar eclipse.

2. 1860

   Prominences belong to the Sun

   On 18 July 1860, Warren De la Rue photographed the eclipse from Rivabellosa in northern Spain, while the Jesuit astronomer Angelo Secchi photographed it from a site several hundred kilometres to the south. The red prominences matched in both sets of pictures, so they could not be an effect of the Earth’s atmosphere or the Moon: they were part of the Sun.

3. 1868

   Helium is discovered in the Sun

   During the eclipse of 18 August 1868, Jules Janssen, observing from Guntur in India, saw a bright yellow line at 587.49 nanometres in the light from the edge of the Sun. Two months later Norman Lockyer observed the same line and concluded that it came from an unknown element, which he and the chemist Edward Frankland named helium, after helios, the Greek word for the Sun. On 26 October 1868 the French Academy of Sciences honoured both Janssen and Lockyer with a medal. Helium was not found on Earth until 1895.

4. 1869

   The mystery of coronium

   At the eclipse of 7 August 1869, Charles Young and William Harkness independently recorded a green line in the spectrum of the corona that matched no known element. It was put down to a new element called coronium. In the 1930s Walter Grotrian and Bengt Edlén showed it came from highly ionised iron, which can only form at extreme temperatures. The corona turned out to be around a million degrees, far hotter than the Sun’s surface.

5. 1914

   An expedition arrested by war

   In August 1914 the German astronomer Erwin Freundlich led a team to Crimea, hoping to test Einstein’s idea that gravity bends light. The First World War broke out days before the eclipse, the team was arrested, and its equipment was seized. Einstein’s prediction had to wait five more years.

6. 1919

   Starlight bends around the Sun

   On 29 May 1919, Arthur Eddington observed from the island of Príncipe off West Africa, while Charles Davidson and Andrew Crommelin observed from Sobral in north-east Brazil. They measured the positions of stars near the eclipsed Sun and compared them with photographs taken months earlier. The stars appeared shifted, as Einstein’s general theory of relativity predicted, and the results were announced to the Royal Society and the Royal Astronomical Society on 6 November 1919. A day later The Times ran the headline “Revolution in Science”, and the New York Times followed with “Einstein Theory Triumphs”.

7. 1973

   74 minutes under the shadow

   On 30 June 1973, a prototype Concorde, 001, took off from Las Palmas in the Canary Islands with a team of scientists and their instruments on board. Flying at about Mach 2 and 17,000 metres, it kept pace with the Moon’s shadow across Africa and gave scientists 74 minutes of totality, far longer than any observer on the ground has ever had.

8. 2017

   Volunteers film the corona for 93 minutes

   For the eclipse of 21 August 2017, the Citizen CATE project placed identical telescopes run by volunteers at 68 sites across the United States. As the shadow crossed the country they captured 83 of a possible 93 minutes of continuous images of the inner corona, and saw plumes moving at about 320,000 km/h.

Old eclipses that measure the Earth

Eclipse records from Babylon, China, the Arab world and medieval Europe turned out to be useful for something their writers never imagined. If the Earth had always turned at today’s speed, many ancient eclipses would have been seen in the wrong places. By working out where they were actually observed, the astronomers Richard Stephenson, Leslie Morrison and Catherine Hohenkerk used records from 720 BCE to 1600 CE and showed that the day has been getting longer by about 1.8 milliseconds per century. That is less than the 2.3 milliseconds that the Moon’s tides alone would predict, a clue to other processes inside the Earth.

Why eclipses still matter

Instruments called coronagraphs can now block the Sun artificially, and spacecraft watch the corona constantly. But a total eclipse still gives the clearest view of the inner corona, close to the Sun’s edge, which is exactly where the questions of how the corona is heated and how the solar wind is launched are decided. That is why research teams still travel to every total eclipse, including the long one on 2 August 2027.