[...] Certainly it is a great thing to add to the countless multitude of fixed stars visible hitherto by natural means and expose to our eyes innumerable others never seen before, which exceed tenfold the number of old and known ones. […] » 

« […] It is most beautiful and pleasing, to the eye to look upon the Lunar body, distant from us about sixty terrestrial diameters, from so near as if it were distant by only two of these measures, so that the diameter of the same Moon appears as if it were thirty times, the surface nine-hundred times, and the solid body about twenty-seven thousand times larger than when observed only with the naked eye. Anyone will then understand with the certainty of the senses that the Moon is by no means endowed with a smooth and polished surface, but is rough and uneven and, just as the face of the Earth itself, crowded everywhere with vast prominences, deep chasms and convolutions. […] »  
 

On the fourth or fifth day after conjunction, when the Moon displays herself to us with brilliant horns, the boundary dividing the bright from the dark part does not form a uniformly oval line, as would happen in a perfectly spherical solid, but is marked by an uneven, rough and very sinuous line.  

For several, as it were, bright excrescences extend beyond the border between light and darkness into the dark part and, on the other hand, little dark parts enter into the light.  

Indeed, a great number of small darkish spots, entirely separated from the dark part, are distributed everywhere over almost the entire region already bathed by the light of the Sun, except, at any rate, for that part affected by the large and ancient spots.  

We noticed, moreover, that all these small spots just mentioned always agree in this, that they have a dark part on the side toward the Sun; while on the side opposite the Sun they are crowned with brighter borders like shining ridges.  

And we have an almost entirely similar sight on Earth, around Sunrise, when the valleys are not yet bathed in light, but the surrounding mountains facing the Sun are already seen shining with light. And just as the shadows of the earthly valleys are diminished as the Sun climbs higher, so those lunar spots lose their darkness as the luminous part grows. » 
 

Now, on Earth, before Sunrise, aren’t the peaks of the highest mountains illuminated by the Sun’s rays while shadows still cover the plane? Doesn’t light grow, after a little while, until the middle and larger parts of the same mountains are illuminated, and finally, when the Sun has risen, aren’t the illuminations of plains and hills joined together?  

These differences between prominences and depressions in the Moon, however, seem to exceed the terrestrial roughness greatly, as we shall demonstrate below.  

Meanwhile, I would by no means be silent about something deserving notice, observed by me while the Moon was rushing toward first quadrature, the appearance of which is also shown in the above figure: for toward the lower horn a vast dark gulf projected into the bright part. As I observed this for a long time, I saw it very dark. Finally, after about two hours, a bit below the middle of this cavity a certain bright peak began to rise and, gradually growing, it assumed a triangular shape, still entirely removed and separated from the bright face. Presently, three other small points began to shine around it, as the Moon was about to set, this enlarged triangular shape, now made larger, joined together with the rest of the bright part, and like a huge promontory, surrounded by the three bright peaks already mentioned, it broke out into the dark gulf.   

Also, in the tips of both the upper and lower horns, some bright points emerged, entirely separated from the rest of the light, as shown in the picture above.  

And there was a great abundance of dark spots in both horns, especially in the lower one. Of these, those closer to the boundary between light and dark appeared larger and darker, while those farther away appeared less dark and more diluted.  

But as we have mentioned above, the dark part of the spot always faces the direction of the Sun and the brighter border surrounds the dark spot on the side turned away from the Sun and facing the dark part of the Moon.  

This lunar surface, which is decorated with spots, like the dark blue eyes in the tail of a peacock, is rendered similar to those small glass vessels which, plunged into cold water while still warm, crack and acquire a wavy surface, after which they are commonly called ice-glasses.  

The large [and ancient] spots of the Moon , however, when broken up in a similar manner, are not seen to be filled with depressions and prominences, but rather to be even and uniform; for they are only here and there sprinkled with some brighter little places. Thus if anyone wanted to resuscitate the old opinion of the Pythagoreans that the Moon is, as it were, another Earth, its brighter part would represent the land surface while its darker part would more appropriately represent the water surface. Indeed, for me, there has never been any doubt that when the terrestrial globe, bathed in sunlight, is observed, from a distance, the land surface will present itself brighter to the view and water surface darker.  

Moreover, in the Moon the large spots are seen to be lower than the brighter areas; for in her waxing as well as waning, on the border between light and dark, there is always a prominence here or there around these large spots, next to the brighter parts, as we have taken care to show in the figures; and the edges of the said spots are not only lower, but more uniform and not broken by creases or roughnesses.  

Indeed, the brighter part stands out very much near the ancient spots, so that both before the first and near the second quadrature, some huge projections arise around a certain spot in the upper, northern part of the Moon, both above and below it, as the adjoining figures show. »

Then, when the bright surface has decreased in size, as soon as almost this entire spot is covered in darkness, brighter ridges of mountains rise loftily out of the darkness. The following figures clearly demonstrate this double appearance. »

But in the manner of the other spots, its shaded part faces the Sun, while its bright part is situated toward the dark part of the Moon, which, I advise for the third time, is to be esteemed as a very strong argument for the roughnesses and unevennesses scattered over the entire brighter region of the Moon. Its darker spots are always those that border on the boundary between light and dark, while the farther ones appear both smaller and less dark, so that finally, when the Moon, is at opposition and full, the darkness of the depressions differs from the brightness of the prominences by a modest and quite small degree. 

These things we have reviewed are observed in the brighter regions of the Moon. In the large spots, however, such a difference between depressions and prominences is not seen to be the same, as we are driven to conclude by necessity in the brighter part on account of the change of shapes caused by the changing illumination of the Sun’s rays as it regards the Moon from many different positions. In the large spots there are some darkish areas, as we have shown in the figures, but yet those always have the same appearance and their darkness is not increased or abated. Rather, they appear, with a very slight difference, now a little darker, now a little lighter, as the Sun’s rays fall on them more or less obliquely. Moreover they join with nearby parts of the spots in a gentle bond, their boundaries mingling and running together.  

Things happen differently, however, in the spots occupying the brighter part of the Moon, for like sheer cliffs sprinkled with rough and jagged rocks, these are divided by a line which sharply separates shadow from light.  

Moreover, in those larger spots certain other brighter areas – indeed, some very bright ones – are seen. But the appearance of these and the darker ones is always the same, with no change in shape, light, or shadow. It is thus known for certain and beyond doubt that they appear this way because of a real dissimilarity of parts and not merely because of inequalities in the shapes of their parts and shadows moving diversely because of the varying illumination by the Sun. This does happen beautifully in the other, smaller, spots occupying the brighter part of the Moon.  

Day by day these are altered, increased, diminished and destroyed, since they only derive from the shadows of rising prominences.  

But I sense that many people are affected by great doubt in this matter and are so occupied by the grave difficulty that they are driven to call into doubt the conclusion already explained and confirmed by so many appearances. For if that part of the Moon’s surface which more brilliantly reflects the solar rays is filled with innumerable contortions, that is, elevations and depressions, why is it that in the waxing Moon the limb facing west, in the waning Moon the eastern limb, and in the full Moon the entire periphery are seen not uneven, rough and sinuous, but exactly round and circular and not jagged with prominences and depressions? And especially because the entire edge consists of the brighter lunar substance which, we have said, is entirely bumpy and covered with depressions, for none of the large [ancient] spots reach the very edge, but all are seen to be clustered far from the periphery. 

Since these appearances present an opportunity for such serious doubt, I shall put forward a double cause for them and therefore a double explanation of the doubt. First, if the prominences and depressions in the lunar body were spread only along the single circular periphery outlining the hemisphere seen by us, then the Moon could indeed, nay it would have to, show itself to us in the shape of, as it were, a toothed wheel, that is, bumpy and bounded by a sinuous outline. If, however, there were not just one chain of prominences distributed only along a single circumference, but rather very many rows of mountains with their clefts and sinuosities were arranged about the outer circuit of the Moon – and these not only in the visible hemisphere but also in the one turned away from us (yet near the boundary between the hemispheres) – then the eye, observing from afar, could by no means perceive the distinction between the prominences and depressions. For the interruptions in the mountains arranged in the same circle or the same chain are hidden by the interposition of row upon row of other prominences, and especially if the eye of the observer is located on the same line with the peaks of those prominences.  

Thus on Earth the ridges of many mountains close together appear to be arranged in a flat surface if the observer is far away and situated at the same altitude. So [also] in a billowy sea the high tips of the waves appear stretched out in the same plane, even though between the waves there are very many troughs and gulfs so deep that not only the keels but also the upper decks, masts and sails of tall ships are hidden. Since, therefore, in the Moon itself and around its perimeter there is a complex arrangement of prominences and depressions, and the eye, observing from afar, is located in about the same plane as their peaks, it should be surprising to no one that, with the visual rays skimming them, they show themselves in an even and not at all wavy line. […] » 

« […] From the appearances already explained, I think it is sufficiently clear that the brighter surface of the Moon is sprinkled all over with prominences and depressions. It remains for us to speak of their magnitudes, demonstrating that the terrestrial roughnesses are far smaller than the lunar ones. I say smaller, speaking absolutely, not merely in proportion to the sizes of their globes. This is clearly shown on the following manner. 

As has often been observed by me, with the Moon in various aspects to the Sun, some peaks within the dark part of the Moon appear drenched in light, although very far from the boundary line of the light. Comparing their distance from that boundary line to the entire lunar diameter, I found that this interval sometimes exceeds the twentieth part of the diameter. […] »

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Bibliography: Galileo Galilei - Sidereus Nuncius - translated and commented by Albert Van Helden - University of Chicago Press