Sir Isaac Newton’s calculations changed the way people understood the universe. No one had been able to explain why the planets stayed in their orbits. What held them up? Less than 50 years before Isaac Newton was born it was thought that the planets were held in place by an invisible shield. Isaac proved that they were held in place by the sun’s gravity. He also showed that the force of gravity was affected by distance and by mass. He was not the first to understand that the orbit of a planet was not circular, but more elongated, like an oval. What he did was to explain how it worked.