“Just as the eye was made to see colors, and the ear to hear sounds, so the human mind was made to understand, not whatever you please, but quantity,” wrote Kepler.
Kepler based his investigations on careful observations and complex mathematics, but always with a mystical bent, perhaps from his work as an astrologer. As Tycho Brahe’s assistant and successor, he used Tycho’s careful observations to discover the three mathematical laws that describe a planet’s orbit, the speed at which it travels, and the time it needs to complete one revolution around the Sun.
He proved that Earth and other planets travel in orbits that are ellipses, not circles as had been believed since ancient times. And he showed that the speed of a planet in its orbit is not uniform, but decreases as its distance from the Sun increases, thus overthrowing another long-held belief.
Kepler also was a founder of the modern science of optics. He was the first to correctly explain how people see –– he noted that the pupil of the eye functions as a diaphragm and that light rays are focused on the retina.
He also explained why eyeglasses help people see better. After Galileo Galilei sent him one of the first telescopes, he explained how the instrument works and improved its design.