Johannes Hevelius (yōhän´əs hāvā´lēŏŏs), 1611–87, Polish astronomer, b. Danzig. From a finely equipped observatory in his house at Danzig, assisted by his wife Elizabeth, he made valuable observations of the moon's surface, discovered four comets, and collected data for his catalog of 1,564 stars. He recorded his pioneer study of lunar topography in Selenographia (1647), noted for excellent lunar maps. Many of the names given by him to the lunar mountains, craters, and other features are still used. He was one of the first to observe (1661) a transit of Mercury. He improved astronomical instruments but resisted the introduction of telescopic sights. His surname appears in various spellings, among them Hevel, Hewel, Hewelcke, and Höwelcke.