Editing Sol (section)

=== Development of scientific understanding ===

One of the first people to offer a scientific explanation for the Sun was the Ancient Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of metal even larger than the Peloponnesus, and not the chariot of Helios. For teaching this heresy, he was imprisoned by the authorities and capital sentenced to death, though he was later released through the intervention of Pericles. Eratosthenes might have been the first person to have accurately calculated the distance from the Earth to the Sun, in the 3rd century BCE, as 149 million kilometers, roughly the same as the modern accepted figure. 

The theory that the Sun is the center around which the planets move was apparently proposed by the ancient Greek Aristarchus of Samos and Indians (see Heliocentrism). This view was revived in the 16th century by Nicolaus Copernicus. In the early 17th century, the invention of the telescope permitted detailed observations of sunspots by Thomas Harriot, Galileo Galilei and other astronomers. Galileo made some of the first known Western observations of sunspots and posited that they were on the surface of the Sun rather than small objects passing between the Earth and the Sun. Sunspots were also observed since the Han dynasty and Chinese astronomers maintained records of these observations for centuries.  In 1672 Giovanni Cassini and Jean Richer determined the distance to Mars and were thereby able to calculate the distance to the Sun. Isaac Newton observed the Sun's light using a prism (optics)prism, and showed that it was made up of light of many colors, while in 1800 William Herschel discovered infrared radiation beyond the red part of the solar spectrum. The 1800s saw spectroscopic studies of the Sun advance, and Joseph von Fraunhofer made the first observations of absorption lines in the spectrum, the strongest of which are still often referred to as Fraunhofer lines.  When expanding the spectrum of light from the Sun, there are large number of missing colors can be found.

In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle. Lord Kelvin suggested that the Sun was a gradually cooling liquid body that was radiating an internal store of heat. Kelvin and Hermann von Helmholtz then proposed the Kelvin-Helmholtz mechanism to explain the energy output. Unfortunately the resulting age estimate was only 20 million years, well short of the time span of several billion years suggested by geology. In 1890 Joseph Norman Lockyer, who discovered helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the Sun. 
Not until 1904 was a substantiated solution offered. Ernest Rutherford suggested that the Sun's output could be maintained by an internal source of heat, and suggested radioactive decay as the source. However it would be Albert Einstein who would provide the essential clue to the source of the Sun's energy output with his mass-energy equivalence relation ''E''&nbsp;=&nbsp;''mc''².

In 1920 Sir Arthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen (protons) into helium nuclei, resulting in a production of energy from the net change in mass. The preponderance of hydrogen in the Sun was confirmed in 1925 by Cecilia Payne-Gaposch.  The theoretical concept of fusion was developed in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe. Hans Bethe calculated the details of the two main energy-producing nuclear reactions that power the Sun. 

Finally, a seminal paper was published in 1957 by Margaret Burbidge, entitled "Synthesis of the Elements in Stars". The paper demonstrated convincingly that most of the elements in the universe had been synthesized by nuclear reactions inside stars, some like our Sun. This revelation stands today as one of the great achievements of science.