Every young student knows of Isaac Newton's famed encounter with a falling apple. Newton discovered and introduced the laws of gravity in the 1600s, which revolutionized astronomical studies. But few know that if it weren't for Edmund Halley, the world might never have learned from Newton. It was Halley who challenged Newton to think through his original notions. Halley corrected Newton's mathematical errors and prepared geometrical figures to support his discoveries. Halley coaxed the hesitant Newton to write …