Positional astronomy was one of the primary functions of Greenwich for the Admiralty. The Astronomer Royal Airy was an advocate of this and the transit circle instrument he had installed in 1851 was used for a century for positional astronomy. One of the difficulties with positional astronomy, is accounting for the refraction of light through Earth's atmosphere. Sources of error include the precision of the instrumentation, and then there has to be accounting for precession, nutation, and aberration. Sources of error in the instrument have to be tracked down and accounted for to produced more accurate results.