Nasmyth retired from business in 1856 when he was 48 years old, as he said "I have now enough of this world's goods: let younger men have their chance". He settled down near Penshurst, Kent, where he renamed his retirement home "Hammerfield" and happily pursued his various hobbies including astronomy. He built his own 20-inch reflecting telescope, in the process inventing the Nasmyth focus, and made detailed observations of the Moon. He co-wrote "The Moon: Considered as a Planet, a World, and a Satellite" (1874) with James Carpenter (1840–1899). This book contains an interesting series of "lunar" photographs: because photography was not yet advanced enough to take pictures at very high magnification directly of the Moon itself, Nasmyth built plaster relief scale models based on his visual observations of the Moon and then photographed the models under electric illumination, replicating the shadows of the topographic contours he observed on the Moon. A crater on the Moon is named after him.