On Dec. 20, 1900, astronomer Michel Giacobini spotted a 10th-magnitude comet while working at Nice Observatory, in France. Calculations following his discovery suggested the comet had an orbital period of a bit less than seven years. When German astronomer Ernst Zinner rediscovered the comet on Oct. 23, 1913, happening across it by accident while observing Continue reading "Dec. 20, 1900: The discovery of 21P/Giacobini-Zinner" The post Dec. 20, 1900: The discovery of 21P/Giacobini-Zinner appeared first on Astronomy Magazine .