The earliest speculations about communication with extraterrestrial intelligence (CETI) involved contact with the inhabitants of other bodies of our Solar System, either our Moon or other planets. Astronomical theories in the nineteenth century made plausible a belief in the prevalence of life on non-terrestrial worlds circling our Sun. When the focus turned to the possibility of communicating with the potential denizens of these worlds, their relative closeness to the Earth made it conceivable that signals could be detected through optical telescopes.