Horologium Oscillatorium: sive de motu pendulorum ad horologia aptato demonstrationes geometricae (Latin for "The Pendulum Clock: or geometrical demonstrations concerning the motion of pendula as applied to clocks")[1] is a book published by Christiaan Huygens in 1673; it is his major work on pendulums and horology.[2] This work is regarded as one of the three most important works done on mechanics in the 17th century, the other two being Galileo’s Discourses and Mathematical Demonstrations Relating to Two New Sciences (1638) and Isaac Newton’s Philosophiæ Naturalis Principia Mathematica (1687).[3]