Today, we know that Earth completes an orbit around the Sun every 365 days and moves at more than 100,000 km/hour. Copernicus, a sixteenth-century Polish astronomer, was the first to place the Sun at the center of the universe. His heliocentric theory contradicted geocentric theory, the long-held belief that Earth was fixed at the center of the universe and that all other bodies, including the Sun, revolved around it.

If Earth was indeed moving at great speed, why did it seem to all senses to be motionless? Galileo developed a thought experiment to answer this question. Thought experiments are hypothetical scenarios that generally cannot be tested due to practical considerations. In this thought experiment, Galileo began by thinking that a ball dropped by a rider atop a stationary horse falls straight down beside the horse. Even if the horse is galloping and the ball is dropped, it lands beside the horse -- just as when the horse is stationary. How can this be? Because, Galileo reasoned, the forward motion of the horse (which represents Earth) is communicated to the ball through the rider's hand. In other words, the ball moves in the same direction and at the same speed as the horse and will always fall beside the horse when dropped.

Galileo concluded that all objects on Earth and within its atmosphere share in its motion. As a result, they are unaffected by its motion, just as if they were stationary. Ironically, Galileo's thought experiment involving the horse and rider could easily have been tested, but he was so certain of the results that he never did it.