During his tenure as the seventh Astronomer Royal, the Royal Observatory at Greenwich became a model of efficiency for positional astronomy. He belittled pure research, however, which made Greenwich a late starter in the fields of spectroscopy and astrophysics. The transit telescope he installed at Greenwich in 1851 now defines the position of 0° longitude on Earth. Airy’s only significant astronomical discovery was of irregularities in the orbits of Venus and the Earth. In 1854, by making gravity measurements at the top and bottom of a mineshaft, he estimated the Earth’s mass.