In 1814, German physicist, Joseph Von Fraunhofer, noticed the presence of very dark lines in the Sun's continuous spectrum. He measured their relative positions using a spectroscope of his invention, and marked the main ones by letters of the alphabet (a terminology still in use today).:
Fraunhofer lines
In 1859, another German physicist, Gustav Kirchhoff showed that these dark lines were characteristic of chemical elements; the sodium double line 'D' being the most famous.
