The typology evolution of lunar formations

When Galileo publishes his Sidereus Nuncius, a number of astronomers influenced by Aristotle considers the Moon as a perfect celestial body, spherical and without any roughness on its surface. It is in opposition to the “sublunary” world, imperfect and contingent. If Galileo is not the first who revealed the existence of the Moon’s surface, he is although the first who rejected this hypothesis by observing rigorously.

The sharing idea of the Moon as another Earth, made out of mountains and valleys, would have provoked theories that were often surprising. If we take notice of older theories for example, the lunar spots were thought to be a reflection of the Earth’s terrestrial geography. Other theories described the Moon as a sort of cloud with particular properties, or made of crystal. However, the great selenographies of the 17^th century, notably those from Hevelius and Riccioli will not discuss about the existence of this kind of rough topography.

After this change of paradigm, some questions are still unanswered. What is the nature of these large dark spots that are visible to the naked eye? Where do the mountains and circular craters originate? Does the Moon have an atmosphere?

Rapidly, the theory of lunar seas took form as a logical solution on the problem of the determination of the nature of dark regions of the Moon. With William Gilbert, we already find a significate nomenclature, which refers to islands, bays, seas: Insula Medilunaria, Sinus Magnus, Mare Medilunarium… However, Gilbert associate to the actual seas names of lands. In a similar way for Johannes Kepler (1571-1630), the darker zones corresponds to the lands and the clearer ones to the seas. Galileo is against this idea and prefer to defend the opposite idea: the darker regions corresponds to the seas, and the lighter regions to the lands. Somehow, he stays quite prudent with his theories on the Moon’s surface. After successfully gaining Kepler on his point of view, he writes in one of his letters addressed to a friend: “I do not believe that the Moon’s body is composed of soil and water”. Riccioli’s nomenclature will finally impose the “mare”, the seas. Today still, we talk about the Sea of Tranquility, Sea of Nectar or Ocean of Storms, even if these names does not refer in reality to hydrography.

J.H. Schröter attempts for the first time to deliver the lunar nomenclature with a certain influence with hydrography. During the end of the 18^th century, it is established that the water is not present on the Moon’s surface.

Schröter proposes neutral and descriptive terms that are closer to a geological approach: Bergketten (chain of mountains), Bergadern (ridges), Thalern (valleys), Flachen (plains), Einsenkungen (depressions).

As we have seen, it is the study of the craters’ shadows, observed under different levels of sunlight in function of the phases of the Moon, which have permitted them to be known. In this perspective, J.H. Schröter’s drawings are particularly interesting (lien vers Schröter). Once again, Schröter is revealed by the systematic use of two terms: rille and crater.

Before the theory of the impact of craters, other hypothesis has been formulated. One in particular: the origin of volcanic craters. The defense of this thesis finds itself in the exceptional work  of the Scottish James Nasmyth, The moon: considered as a planet, a world, and a satellite published in London, in 1874.

Having an education in engineer, James Nasmyth (1808-1890) conceived in first place work machines for the industry. Little by little, under the influence of an astronomer friend William Lassell, he creates telescopes and realizes his own observations. With the assistance of James Carpenter (1840-1899) from the Royal Observatory of Greenwich, Nasmyth develops his theory of the lunar craters’ volcanic origin by comparing with the structure of Mount Vesuvius. By following the same analogical reasoning, he also establish a hypothesis on the geological structures of the satellite by analyzing the wrinkles or the distribution of cracks on a glass globe (in this case, Nasmyth attempts to bring closer this image and Thycho’s crater ejactas.

Nasmyth also creates plaster models that will permit him to demonstrate his own conception of lunar landscape. The positive pictures of his models are gained by a printing process known as the Woodburytype. It allows the creation of images, with a great precision with a subtle contrast. Nasmyth publication will impress his contemporaries, from the large public to the scientific community. The magazine Nature will even celebrate his work that will influence the Moon’s representation and its landscape.

In the same background than Nasmyth, Robert Schindler (1850-1920), a Swiss amateur astronomer that publishes in 1905 Mécanique de la Lune. Like Nasmyth, Schindler believes that volcanic activity explains the cause of its surface. By boiling wax, Schindler obtains forms that evokes the Moon’s craters. His works is kept confidential, but Camille Flammarion (1842-1925) will congratulate him.[1]

The American geologist, Grove Carl Gilbert (1843-1918), will be the first who will announce the bombing theory.[2] As an expert in geology, Gilbert observe the Moon with the refracting telescope from the Naval Observatory of Washington and studies the craters’ shapes, which he will interpret as the result of an impact. Gilbert is also interested in the lunar “seas” and considers them as being the result of a greater impact, which have caused a sliding of lava. Ralph Baldwin’s (1912-2010) work The Face of the Moon, published in 1949, confirms those hypotheses without agreeing them firmly. Apollo’s mission will cease the debate by confirming definitely the theory of the craters’ meteorite origin.

Suspended for a long period, the questioning over the lunar atmosphere seems divided at the end of the 19^th century. Beer and Mädler state that the rich visible details on its surface is enough to demonstrate that it is without an atmosphere. The observations would have been interfered due to the gas, which would have been encircling the Moon. If this hypothesis is today confirmed (the Moon being encircled by a thin layer of gas, that cannot be qualified as an atmosphere). Some astronomers, such as Maurice Loewy and the American William Henry Pickering (1858-1938) have defended the opposite idea.