Royal Observatory, Cape of Good Hope Spencer Jones, was active in an international project for determining the solar parallax through observations of the minor planet Eros. In the second half of the twentieth century Alan Cousins set up very precise southern standards for UBV and introduced a widely used system of VRI photometry that enjoyed international recognition for precision. In 1977 the occultation of the star SAO 158687 was observed by Joseph Churms from the former Royal Observatory and these observations provided needed confirmation of the Uranian rings discovered from the Kuiper aeroplane by Elliot et al. During the 19th century the Observatory was regarded as
Stability of the Solar System that keeps the far side of the Moon away from the Earth. Mercury is in a 3:2 spin–orbit resonance. The planets' orbits are chaotic over longer timescales, such that the whole Solar System possesses a Lyapunov time in the range of 2–230 million years. In all cases this means that the position of a planet along its orbit ultimately becomes impossible to predict with any certainty (so, for example, the timing of winter and summer become uncertain), but in some cases the orbits themselves may change dramatically. Such chaos manifests most strongly as changes in eccentricity, with some planets' orbits