[Conceptions of Similarity of the Heavenly Bodies to the Earth and of Extraterrestrial Life in European Natural Philosophy during the First Half of the Seventeenth Century. Master's Thesis in General History. Department of History and Ethnology, University of Jyväskylä, Finland. 2003.]
The 17th century was a period of revolutionary development, as far as the natural sciences and scientific world view are concerned. This was especially the case in astronomy, where the Copernican, or heliocentric, theory gradually began to gain more supporters. Many of its proponents thought that, in contrast to the prevailing world view, there was no longer any reason to argue that the Earth differs from other celestial bodies. The majority of those who supported traditional cosmology felt the position of man at the top of creation to be threatened by the placement of the Earth among the planets, instead of the centre of the universe. Thus, this was not just a theoretical disagreement, but a fundamental contradiction in world view.
The idea of the Earth-likeness of the celestial bodies is usually followed by the question of extraterrestrial life. Indeed, several natural philosophers were intensively considering the matter, especially after the telescope had been introduced as an astronomical observation tool in 1609. This thesis explores mainly astronomical literature published in different European countries between 1596 and 1659, as well as some unpublished drawings of the Moon. The aim of the study is to examine the views of natural philosophers on the plurality of the worlds, namely the existence of many worlds similar to the Earth, and to refine the image of the scientific debate on the subject created by historical research to date. The approach followed in this thesis can be defined as the history of ideas and concepts in the cosmological world view.
From the sources studied it becomes clear that the question of Earth-likeness of the celestial bodies, and even the possibility of life on them, arose permanently on par with other scientific problems as early as the 1610s, even though the methods of the time did not provide satisfactory means to resolve the question. In the early stages, the subject was only discussed in passing, but gradually its weight seems to have increased. Those who welcomed the plurality of worlds also supported heliocentric cosmology, but not necessarily the other way around.
The writers' attention was chiefly directed to our nearest celestial body, the Moon. Texts describing the Moon's surface and its mapping, i.e. selenographical sources, tell a great deal about the perceptions of their authors. The sources of this study are divided into two groups: first, planetography, which emphasizes visual observation of the heavenly bodies; and second, speculative texts, where the question is approached mostly by means of reasoning. In addition to astronomical sightings, the most important arguments used in the works are Earth analogies, teleology, the tenets of Christianity, and appealing to previous writings. The preconceptions of natural philosophers played a decisive role in their interpretation of visual perceptions.