Huygens, Christiaan
The Dutchman Christiaan Huygens (1629 - 1695), son of an influential family, was a mathematician, physicist, engineer, astronomer, and inventor.
He had a long-term interest in the study of light refraction and lenses, and worked out practical ways to minimize the effects of spherical and chromatic aberration.
In 1657, he invented the pendulum clock.
In 1655, he began grinding lenses to build refracting telescopes. He discovered Saturn's biggest moon, Titan, and was the first to explain Saturn's strange appearance as due to a thin, flat ring, nowhere touching, and inclined to the ecliptic. He developed what is now called the Huygenian eyepiece, a telescope with two lenses.
In optics, he is best known for his wave theory of light, which was initially rejected in favour of the corpuscular light theory of Isaac Newton.
He calculated the period of a pendulum made of an arbitrarily-shaped swinging rigid body. He was the first to derive the formula for the period of an ideal mathematical pendulum.
He designed a mechanical planetarium that could display all the planets and their moons then known circling around the sun, which he completed in 1680.