Mountains on the moon

'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 shaped spherical solid, but is marked by an uneven, rough, and very sinuous line, as the figure shows.... what causes even greater wonder is that very many bright points appear within the dark part of the Moon... gradually these are increased in size and brightness [and...] joined with the rest of the bright part.... Now, on Earth, before sunrise, aren’t the peaks of the highest mountains illuminated by the Sun’s rays while shadows still cover the plain?'

Source: Galileo Galilei, Sidereus Nuncius (1610), fols. C1v-C2r, B4r, C2v (from archive.org); trans. Albert Van Helden (University of Chicago, 1989).

Supplementary resources

Galileo's original drawings of the moon, 'the first realistic depiction of the Moon in history', upon which these engravings were based, are preserved in Florence, Biblioteca Nazionale Centrale (Florence), Ms. Gal. 48, and reproduced by the Museo Galileo.

The manner in which Galileo calculated the height of the mountains on the moon is explained in an instructive brief video from the Museo Galileo (below).

The drawings of the Moon in Galileo's Sidereus nuncius touched of the rapid development of 'selenographia', which is treated further in the next cluster.
 

Galileo's Telescope.mp4