The skilled astronomer was soon called upon to render a service to several of the Colonies. By means of astronomical instruments he did such accurate work in marking out the boundary between Delaware and Pennsylvania that Mason and Dixon later accepted his results, and he settled the dispute between New Jersey and New York as to the point where the forty-first degree of latitude touches the Hudson River. Perhaps, however, the achievement that won for him greatest fame was the observation, made in 1769, of the transit of Venus. The importance of the observation is evident from the facts that it provides the best means for calculating the distance between the heavenly bodies, which had never been satisfactorily made, and that the opportunity would not occur again for one hundred and five years. After months of preparation, which included the making of delicate instruments, Rittenhouse, one of a committee of three appointed by the American Philosophical Society, succeeded. In the words of Pennypacker, "The first approximately accurate results in the measurement of the spheres were given to the world, not by the schooled and salaried astronomers who watched from the magnificent royal observatories of Europe, but by unpaid amateurs and devotees to science in the youthful province of Pennsylvania." Benjamin Franklin found in him a kindred spirit, and the Philadelphian was frequently a visitor at the Norriton farmhouse. On Sunday the two friends often went to the old Norriton Presbyterian Church, which had been built on the corner of the Rittenhouse farm, within sight of the house. This church, which probably dates from 1698, is still standing in good repair. Some years after the successful observation of the transit of Venus brought fame to the American astronomer, he moved to Philadelphia. There, among other duties, he had charge of the State House clock. At the beginning of the Revolution the Council of Safety asked that he should "prepare moulds for the casting of clock weights, and send them to some iron furnace, and order a sufficient number to be immediately made for the purpose of exchanging them with the inhabitants of this city for their leaden clock weights." The leaden weights were needed for bullets. Later he was sent to survey the shores of the Delaware, to choose the best points for fortifications. When he became Engineer of the Council of Safety "he was called upon to arrange for casting cannon of iron and brass, to view the site for the erection of a Continental powder mill, to conduct experiments for rifling cannon and muskets, to fix upon a method of fastening a chain for the protection of the river, to superintend the manufacture of saltpeter, and to locate a magazine for military stores on the Wissahickon."