Complex sentence: Huygens knew the eminent Polish astronomer Johannes Hevelius and gave the manuscript copy he was offered to him, who appended it to his report on the Mercury transit, "Mercurius in sole visus Gedani", published in 1662.
Split into simpler sentences: Huygens knew the eminent Polish astronomer Johannes Hevelius and gave him the manuscript copy. Hevelius appended it to his report on the Mercury transit, "Mercurius in sole visus Gedani", published in 1662.