John William Draper (November 20, 1811 – February 27, 1882) was an American astronomer and chemist, best known for his efforts to measure the density of stars and provide a quantitative measurement of their distance from the Earth. His early career was devoted primarily to physical chemistry, and he became interested in astronomy while working on a new process for tanning leather. He became an expert observer of the 1845 transit of Venus and subsequently became one of the founders of modern photometry, creating new techniques to measure the brightness and position of stars and planets. He also made important contributions to spectroscopy, photometry, precision mechanics, thermodynamics, and geophysics
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After his work in astronomy, he was involved in the development of photography.