Final answer:
Newton's observation of a falling apple led to his revolutionary idea of a universal law of gravitation that applies to both terrestrial objects and celestial bodies, such as the Moon and the Sun, providing a cohesive framework for understanding motion in the universe.
Step-by-step explanation:
When Sir Isaac Newton saw an apple fall to the ground, he pondered why it fell straight down rather than moving off to the side. This observation led him to theorize about a force that must extend from the Earth to the apple. His subsequent thought was that if this gravitational force could reach the apple, perhaps it could reach as far as the Moon, or even the Sun, creating a universal force that governed astronomical motions as well as objects on Earth. This insight was foundational to his formulation of the universal law of gravitation and his laws of motion, which together provided a unified explanation for both terrestrial and celestial phenomena. The recognition that the same force is responsible for both an apple falling and the Moon's orbit was revolutionary and provided strong evidence for the underlying simplicity and unity in nature explored by scientists. Newton's work greatly expanded upon earlier ideas from Galileo and was further explained and supported by his contemporaries, creating a leap forward in our understanding of gravity and motion.