The bright side of the Moon: The historical journey of our tidally locked satellite

Abstract The paper discusses a major shift in the (Western) science of our Moon. It reconsiders well-known developments, as the Aristotelian concept of distinct celestial and terrestrial worlds in which the Moon was one of seven perfect planetary spheres gave way to our modern view of a very Earth-like body, even endowed at one stage with oceans and continents. It examines the impact of pre-telescopic and telescopic observations in the period 1560 to 1660, notably those of Galileo Galilei and William Gilbert. Its new perspective is to present the strong evidence that supported the idea of the Moon as a planet. In large part this was because it is now known that aspects of the Moon’s orbit are unusual compared with the multiplicity of “moons” in the Solar System. It argues that a major reason was Aristotle’s influential argument for the theoretical impossibility of lunar libration. It concludes with the extraordinary work of William Gilbert (1544-1603) who, it is argued, was the true discoverer of libration because Galileo, who is often credited with it, failed to grasp the necessary dynamics of lunar motion. Notwithstanding, both played crucial roles in the emerging conception of our Moon as “another Earth”. Human activity is now degrading both bodies.