For an apocryphal legend, the tale of Newton and the apple is something of a snoozer -- especially when you consider how the man actually thought about the physics of gravity. In laying out his law of universal gravitation, Newton described a mountain so gigantic that its summit poked into space -- and that's where he placed the giant cannon.
No, Newton didn't plan to fire at alien invaders. His orbital cannon was a mere thought experiment explaining how one object might orbit another. Load too little or too much gunpowder into this theoretical super weapon, and the cannonball will either fall back to the Earth's surface or sail off into outer space.
Just the right amount of powder, however, and you'd give the cannonball sufficient velocity to fall toward the Earth at the same rate that the planet curves away from it. The cannonball, Newton writes, would continue in free fall all the way around the planet, in effect, orbiting it.
First published in 1687, Newton's law of universal gravitation theorized that all particles exert a gravitational force and that gravity -- affected by both mass and distance -- universally commands the movements of everything from terrestrial rain to planetary orbits. While Einstein would later update some of the details of the Newtonian view, the 16th- and 17th-century physicist laid a solid groundwork for our modern understanding of gravity.