Sketch in crayon by James Nasmyth showing craters under Tycho and dated 4th May 1846.

Nasmyth observed the Moon nightly, making carefully annotated sketches of specific features, such as these craters. As the Moon orbited the earth, different angles allowed him to judge the height of lunar mountains and depth of the craters. Working with plaster, he then modelled detailed sections of the surface in three dimensions, and photographed these under strong sunlight. He published the resulting images in a seminal work 'The Moon' with James Carpenter, 1874.