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Moon

"So, let's start talking about the side of the Moon which faces us
- that's how the Italian scientist starts describing his first telescopic observations in Sidereus Nuncius -
On the one hand, the brighter part seems to surround the entire hemisphere as well as to sprinkle it with all its light; on the other hand, the darker one, like a cloud, obscures the face of the Moon, and it lets it appear spotty.
Now, these pretty obscure and quite big spots can be seen by anyone. They have been noticed by human kind every time; for these reasons we'll call them big or ancient spots, to distinguish them from other ones, which have smaller sizes ...
The smaller spots have been observed by none before us; thanks to our frequent inspections, we can state that the surface of the Moon is not flat and uniform at all and it is not perfectly spherical in shape either. Many philosophers thought that the Moon and also other celestial bodies were flat, uniform and spherical in shape. On the contrary, the surface of the Moon is not smooth, it is rough, it has a lot of cavities and projections of rocks, just like the surface of the Earth, which has chains of mountains and deep valleys."

Galileo's pictures of the Moon

[ Galileo's pictures of the Moon, from Sidereus Nuncius, 1610 ]
The years between the closing of the XVI Century and 1609 saw the birth of the telescope and they witnessed Galileo's observations. In those years, other people, some of them before Galileo, have observed the sky and, why not, the Moon with the new instruments.
Harriot's drawing of the Moon

[ Harriot's drawing of the Moon ]

For example, the mathematician and philosopher Thomas Harriot made this sketch of the Moon on July the 26th 1609, four months before the observations which are described in the Sidereus Nuncius by Galileo.

The quality of Galileo's telescope was not that good. Moreover, it couldn't make distant objects appear so much larger and nearer. Nevertheless, he was able to understand that the details, which he could see through his instrument, were projections and cavities, very similar to the mounts and the valleys which are present on Earth.
This is what makes Galileo's observations so astonishing. According to Aristotle and to the common ideas which were present at that time, since the Moon is a celestial body, it was thought to be perfectly spherical and made out of pure and incorruptible matter. The other way around was thought to be true for the Earth.
Consequently, Galileo was observing for the first time in man history things that none had ever seen before. This is very different from observing objects which have already been observed. Moreover, Galileo had no model which he could refer to.

 

 

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