On this day, in 1871, French astronomer Jules Janssen analysed the spectrum of the light around the rim of a solar eclipse seen from India. The observation confirmed his own theory that the corona, normally only visible during a

NASA’s Goddard Space Flight Center/Gopalswamy

solar eclipse, is a physical part of the Sun and is composed of both hot gases and cooler particles. We know today that the temperature of the solar corona reaches two million degrees Celsius , much more than the 5500 degrees Celsius of the surface, but it is made of very thin, though very energetic, gas.