The Mathematical Principles of Natural Philosophy

Three books in Latin, published in 1687, that took the scattered observations of astronomers and turned them into one system. Newton lays down the three laws of motion and the law of universal gravitation, then shows that the same rule pulling an apple to the ground holds the moon in its orbit and swings the planets around the sun. From there he works out the tides, the slow wobble of the equinoxes, the flattening of the earth at its poles, and the paths of comets. The mathematics is geometric and severe, closer to Euclid than to a modern physics text, which is part of why it proved so hard to argue with. Free in PDF and EPUB.