Édouard Stephan

Who was Édouard Stephan?

Édouard Jean-Marie Stephan was a French astronomer. His surname is sometimes spelled Stéphan in some literature, but this is apparently erroneous.

He was born in St Pezenne and attended the Ecole Normale Superieur, and graduated at the top of his class in 1862.

He was the director of the Marseille Observatory from 1864 to 1907. In the early part of his career there, he had limited opportunities to do observations because he was preoccupied with improving the observatory. He discovered the asteroid 89 Julia in 1866. In 1867 he used the new telescope to observe a transit of Mercury.

Between 1870 and 1875, Stephan systematically studied nebulae, precisely recording their positions and discovering many new ones. His goal was to enable the exact measurement of stellar proper motions by creating a reference system of fixed objects.

In 1873, Stephan was the first person to attempt to measure the angular diameter of a star using interferometry, converting the 80 cm telescope at Marseille Observatory into an interferometer. He did this by obscuring the reflector with a mask containing two vertical slits. The star he chose to perform this experiment was Sirius.