Tabula selenographica

This is a print by Doppelmayr and Homann of two understandings of the Moon. Now, it’s easy to guess that one of the depictions, perhaps the one on the left, is of the side facing the Earth and the other is the side not facing the Earth – but we know that one side of the Moon is always facing the Earth, and, at the time that these maps were prepared, in the mid-18th Century, humankind had no idea what the backside of the Moon looked like. So, what are these? Well, the depiction of the Moon on the left-hand side is a depiction that a mapmaker by the name of Hevelius created, and his depiction of the Moon is sufficiently accurate that it became the foundation for most of the science of Selenography[1] that would follow in the centuries afterward. On the right-hand side is a different depiction of the same facing side of the Moon, this one by Riccioli for a Franciscan priest, excuse me, a Jesuit priest by the name of Grimaldi. What’s interesting about this depiction is that its names are the ones that stuck, and the way of referring to the Moon is the way we refer to it today. So, for example, on the left-hand side, the Hevelius side, we have a large dark section called “Mediterraneum,” but that same area is depicted on the Riccioli side as “Mare Imbrium,” the term that is still used today in referring to that part of the Moon. Of course, the Riccioli terminology was in part based on the erroneous notion that there were actual seas, hence the term “Mare” or “Sea.” We now know that there is not surface water, at least not surface water of any significance, on the Moon, but those are the names that stuck.

What’s also interesting about both the depictions is although one side of the Moon is always facing the Earth, there is enough variation from month to month that a little bit more than one half of the Moon’s surface is visible from time-to-time from the Earth, hence the overlapping circles in both cases. As one can also see, there are some wonderful depictions of Cherubs – one Cherub holding on his shoulder and the other looking through, presumably, emulating the way in which the early viewers of the Moon used telescopes to learn more about the surface.

[1] Selenography can be defined as “the science of the physical features of the moon” or “lunar geography.” https://www.merriam-webster.com/dictionary/selenography Accessed 23 Mar. 2021.