Dr. Robert S. Harrington, former head of the US Naval Observatory, calculated several parameters of Planet X and its orbit. Harrington started from the perturbations in the orbits of Neptune and Uranus, knowing that Pluto could not be responsible for them. The observations he used were supplied by the Nautical Almanac Office of the US Naval Observatory and go back as far as 1833 for Uranus and 1846 for Neptune.

See the "Harrington Paper".