For instance, men from earliest times observed the heavenly bodies and the regularity of their motions. Theories of the universe were invented which should harmonize with the known facts. As new facts were discovered, the theories had to be changed and extended. First it was believed that the earth was fixed in mid-space, and sun and stars were daily carried around it. Hipparchus improved this theory by placing the earth not exactly in the center of the sun's circle. Ptolemy, three hundred years later, considered that the sun and moon move in circles, yearly, around the earth, and the other planets in circles, whose centers again described circles round the earth. Copernicus simplified the whole system by teaching that the earth rotated around its axis, and around the sun. Keppler next showed that the earth moved around the sun in certain curves termed ellipses. Finally, Newton hit upon the wide-embracing law of gravitation, which unifies all the known facts of astronomy.[A] All the earlier laws were correct, so far as they included all the knowledge of the age in which they were proposed, but were insufficient to include the new discoveries. [Footnote A: See The Grammar of Science, Pearson, pp. 117, 118.] Laws of nature are, therefore, man's simplest and most comprehensive expression of his knowledge of certain groups of natural phenomena. They are man-made, and subject to change as knowledge grows; but, as they change, they approach or should approach more and more nearly to the perfect law. Modern science is built upon the assumption that the relations between cause and effect are invariable, and that these relations may be grouped to form great natural laws, which express the modes by which the forces of the universe manifest themselves. [Sidenote: A miracle is a law not understood.] In this matter, science is frankly humble, and acknowledges that the region of the unknown is far greater than that of the known. Forces, relations and laws may exist as yet unknown to the world of science, which, used by a human or superhuman being, might to all appearances change well-established relations of known forces. That would be a miracle; but a miracle simply means a phenomenon not understood, in its cause and effect relations. It must also be admitted that men possess no absolute certainty that though certain forces, brought into a certain conjunction a thousand times, have produced the same effect, they will continue to do so. Should a variation occur, however, that also must be ascribed to an inherent property of the forces or conditions, or the existence of a law not understood.[A] There can be no chance in the operations of nature. This is a universe of law and order. [Footnote A: The Credentials of Science, the Warrant of Faith, Cooke, pp. 169, 170.] [Sidenote: Joseph Smith taught the invariable relation of cause and effect.]