#OTD in 1735.
Mathematical calculations suggest that it is on this day that dwarf planet Pluto moved inside the orbit of Neptune for the last time before 1979.
For almost 50 years, Pluto was thought to be larger than Mercury, but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately and to determine that it was much smaller than initial estimates.