Sir Isaac Newton’s answer comprised many many words, but contained no more than the following information: that one could do it by telling the time with an excellent sea-going chronometer, which no one knew how to make yet; or by watching the satellites of Jupiter through an excellent sea-going telescope, which no one knew how to make yet; or by looking at the position of the moon and comparing it against calculations derived from his, i.e., Sir Isaac Newton’s, lunar theory, which was not quite finished yet but would be coming out any minute now in a book. In the timeless and universal manner of authors conversing in public places, he did not fail to mention its title: Volume III of Principia Mathematica, entitled The System of the World, available shortly where books are sold.