Newton's Cannon refers to a thought experiment he published in his Principia in 1687, in which he considered what would happen if a cannon fired a series of cannonballs horizontally at ever increasing speeds. His hypothesis, that at a sufficient speed a projectile would orbit the Earth just like the Moon, was the key insight enabling him to reconcile celestial motion with ordinary terrestrial gravity. The now famous image made its way into NASA's 1977 Voyager Golden Record; a sort of interstella