Final answer:
Isaac Newton is best known for the significant event of discovering universal gravitation, as documented in his 1687 work, Philosophiæ Naturalis Principia Mathematica.
Step-by-step explanation:
The significant event that has happened in Isaac Newton's life is the discovery of universal gravitation. Sir Isaac Newton, a prominent figure during the Scientific Revolution, published his landmark work, the Philosophiæ Naturalis Principia Mathematica in 1687. In this work, he presented a series of scientific laws that we still use today to describe the motion of objects, including his Law of Gravity which provided a predictable framework for understanding the physical universe. Robert Fulton is better known for the invention of the steamship engine, not related to Newton's achievements.