Galileo Galilei ( 15 February 1564 - 8 January 1642) was an Italian polymath: astronomer, physicist, engineer, philosopher, and mathematician. He has been called the "father of observational astronomy", the "father of modern physics", the "father of the scientific method", and the "father of science". Galileo studied speed and velocity, gravity and free fall, the principle of relativity, inertia, projectile motion and also worked in applied science and technology, describing the properties of pendulums and "hydrostatic balances", inventing the thermoscope and various military compasses, and using the telescope (a Dutch optical instrument) for scientific observations of celestial objects.