Newton’s theory of projectiles and orbits involved his laws of motion: the law of inertia, which describes
objects moving with uniform velocity; the second law, F=ma, which describes how objects accelerate
when subjected to a force; and the third law, which states that the gravitational attraction between two
bodies is equal and opposite. Newton worked out the quantitative details of his theory of gravity with the
help of Kepler’s discoveries about the motion of the planets, Galileo’s work on kinematics, and
Copernicus’s theory of a Sun-centered Solar System. Equally important was the concept of the universe as
a vast, nearly empty space extending without limit in all directions, as had been proposed by Thomas
Diggs in England and Giordano Bruno in Italy during the previous century. Newton’s detailed and
comprehensive theory of motion and of gravity was published in his major work, Philosophiae Naturalis
Principia Mathematica (1687), usually referred to as The Principia. Newton’s comprehensive theory
made it possible to determine the orbits of all the planets and known moons of the Solar System, to predict
when comets would return, to explain the tides, and even to start thinking about launching artificial
satellites as indicated by his drawing and description (Figure 2) of how a projectile may achieve a stable
orbit.