1687: Isaac Newton Unites Heaven and Earth

Portrait of Isaac Newton, aged 46, painted by Godfrey Kneller in 1689.

Although each of the scientists featured on this website made important contributions to cosmology, a special place in the history of science is reserved for Isaac Newton, a professor at Trinity College, Cambridge University, as the first person to realize that the physical laws that govern our lives on Earth are the same as those that govern the stars and planets.  In a very important sense, Newton’s insights and mathematical abilities opened people’s eyes to the recognition that we are part of a broader universe, accessible not only by what we can see with our eyes, but also by what we can envision with creative ideas checked by the rigor of mathematics.

In order to fully appreciate the enormous intellectual barriers that Newton had to overcome, it is necessary to take a brief excursion 2,000 years into the past, to view the cosmos as envisioned by Aristotle. The world of the heavens seemed so different from the world here on Earth that Aristotle envisioned the two realms to be governed by two entirely different sets of laws. Inside the orbit of the moon was the changeable world inhabited by people, where objects would fall downwards, toward the center of the universe—which he thought to be the center of the Earth. Beyond the orbit of the moon was the world of the heavens, where objects would travel in perfect circles. The two realms even consisted of different materials—earth, air, fire and water for the world we inhabit, and quintessence as the substances that make up the world beyond.

Aristotle’s ideas reigned for more than 2,000 years. Some progress was made during the middle ages, and important contributions were made by Galileo. But Aristotle’s idea that the laws that governed our world here on Earth were fundamentally different from the laws that governed the heavens were not fully extinguished until Newton proposed an alternative theory and demonstrated that the same laws which governed falling objects on Earth applied equally well to the Orbit of the Moon.

Newton’s contributions to cosmology were immense. His invention of the Newtonian telescope embodies the principle that underlies nearly all large optical telescopes today. His three laws of motion and the universal law of gravitation made it possible to understand the motions of the planets of the solar system, the formation and movements of stars and galaxies, and to some extent, the past and future of the universe itself