Pierre Gassendi. Engraving of the French mathematician and philosopher Pierre Gassendi (1592-1655). Gassendi was ordained as a priest in 1616, the year before he became Professor of Philosophy at Aix, France. In 1645 he was made Professor of Mathematics at the College Royale in Paris, France. He observed the motion of the planet Mercury and gave the Aurora Borealis (northern lights) its name. Gassendi was a pioneer of atomic theory. He believed that the properties of atoms were due to differences in their size, shape and weight. While he did try to reconcile atomic theory with Christian doctrine he was a strong supporter of experimental science.