Nongsugak, an observatory built by Hong Daeyong in the yard of his residence, provides a rare example in the history of astronomy in pre-modern Korea of a Confucian scholar engaging in a systematic program of astronomical observation outside the government bureaucracy. This paper examines Hong’s program of astronomy and its place in his broader pursuit of literati learning. Particular emphasis is placed on the ways in which Hong intertwined two kinds of endeavor: first, the material practice of building instruments and the observatory, and second, the socio-literary practice of exchanging letters, poems, and essays about his observatory with his fellow literati. This illustrates how he culturally legitimized a new concept of a literati mathematician, an emerging figure in mid-eighteenth-century Korea. It also shows how Hong’s scholarly program and the role of astronomy within it evolved in the 1760s and 70s to increasingly emphasize the importance of calculation and observation as essential means of reforming the metaphysically oriented Confucian scholarship of the time.