  Gassendi's interests in unusual celestial phenomena dates back as early as 1621, when he observed the colorful illumination of the sky and dubbed these lights ‘aurora borealis’. Based on his correspondence with observers as far away as the Levant, he located the source of the illumination at very high altitude, above the Northern Polar region. In 1629, he observed the rare phenomenon of parhelia, or false suns, which he explained in his Parhelia seu soles…, in terms of the reflection of sunlight by ice or snow crystals at high altitude. This account, shown to be accurate in the nineteenth century, relies on the views of Gassendi and Peiresc—based on their microscopical observations—that crystal formations of snow and ice are highly reflective. The great triumph of Gassendi's scanning of the skies was his observation of Mercury's transit before the Sun (1631), the first such recorded observation and a confirmation of Kepler's prediction of the planetary orbits in accordance with the Three Laws. This confirmation in turn enabled the subsequent calculations (Halley and Gallet, 1677) of the distance between the Earth, the Sun, and the other planets.