Final answer:
Isaac Newton concluded that Earth's shape was an oblate spheroid, flattened at the poles due to centrifugal force from its rotation, leading to a bulge at the equator. His understanding of gravity as an attractive force between all masses explained why the Earth and other celestial bodies follow elliptical orbits rather than circular paths, as earlier models proposed.
Step-by-step explanation:
Isaac Newton reasoned that Earth was not perfectly spherical due to C) the flattening at the poles and bulging at the equator. This irregular shape of Earth, also known as an oblate spheroid, can be attributed to the centrifugal force that results from Earth's rotation. This force causes the planet to bulge outward at the equator and flatten at the poles, leading to a larger radius at the equator compared to the poles by about 30 kilometers.
Newton's insight into the nature of gravitational forces and their effect on celestial bodies led to the realization that the force of gravity is not limited to Earth alone. He proposed that the same gravitational force that causes objects to accelerate toward Earth could extend further into space, keeping the Moon in its orbit around Earth, and likewise, the Sun's gravitational pull could maintain the planets in their orbits. Understanding that Earth's mass is not evenly distributed due to the equatorial bulge, Newton could also infer that the strength of gravity would vary from the poles to the equator.
In the context of historical developments in astronomy, Newton's ideas marked a transition from the geocentric Ptolemaic model, which placed Earth at the center of the universe, to the heliocentric Copernican model, which rightly positioned the Sun at the center of the known universe. Ptolemy's model involved complex deferent circles and epicycles to explain the planets' motions, while Newtonian physics offered a simpler, more elegant solution based on universal gravitational forces.