Template:Are You Sure/September 25

A diagram of Jupiter (B) eclipsing its moon Io (DC) as viewed from different points in earth's orbit around the sun. From Olaf (Ole) Roemer, "Demonstration tovchant le mouvement de la lumiere trouvé par M. Römer de l' Academie Royale des Sciences," December 7, 1676.

• ... that astronomer and instrument maker Ole Rømer made the first quantitative measurements of the speed of light?

• ... that mathematician and theologian George Salmon was in regular and frequent communication with Arthur Cayley and J. J. Sylvester during the 1850s, and that the three of them together with a small number of other mathematicians (including Charles Hermite) developed a system for dealing with n-dimensional algebra and geometry?