1610: On 7 January 1610, Galileo wrote his first letter describing telescopic observations.
In his observations, he saw the Moon's cratered surface. He wrote, "...it is seen that the Moon is most evidently not at all of an even, smooth and regular surface, as a great many people believe of it and of the other heavenly bodies, but on the contrary it is rough and unequal. In short it is shown to be such that sane reasoning cannot conclude otherwise than that it is full of prominences and cavities similar, but much larger, to the mountains and valleys spread over Earth's surface."
Galileo went on to describe the phenomena in considerable detail, rehearsing, as it were, the observations and conclusions he was to publish more elaborately a few months later in Sidereus Nuncius.