Final answer:
Sir Isaac Newton was the first to define gravity mathematically and to explain how it governs both earthly and celestial motions, such as the orbits of planets around the Sun.
Step-by-step explanation:
The first person to suggest that gravity is what keeps the planets in their orbits around the Sun was Sir Isaac Newton. He was not the first to recognize the presence of this force; forerunners like Galileo Galilei had already posited that falling bodies and planetary motions might share the same cause. However, it was Newton who provided a precise mathematical form for the gravitational force and demonstrated how it could explain both falling objects on Earth and the elliptical orbits of planets and moons.
His work showed that all material bodies exert a gravitational attraction, positing that the universal force of gravitation extended from the Earth to the Moon and beyond, to other planets in their paths around the Sun. This was a remarkable leap in understanding the fundamental workings of the universe, drastically transforming science and laying the foundation for modern physics.
Newton's insights, along with the contemporary work of scientists such as Robert Hooke, Christopher Wren, and Edmund Halley, led to a comprehensive understanding of gravitational forces. Additionally, the translation and augmentation work of Émilie du Châtelet played a significant role in the widespread acceptance of Newtonian gravitation.