An unusual story can be found at www.esa.int/export/esaSC/SEM0TLSXXXG_index_0.html (hat tipped towards member Gary Gilligan) from the European Space Agency web site, 10th February 2012. ESAs Venus Express spacecraft has discovered Venus is rotating a little slower than when it was previously measured in the 1990s by NASAs Magellan Orbiter spacecraft. The measurements are said to be precise and detailed and the idea appears to be to find out if Venus has a liquid or a solid core. What might cause it to slow? It has a dense atmosphere which could influence the rotation rate, it is argued. It seems even on earth the atmosphere, largely the wind and tides, may affect rotation – but this is in the region of milliseconds a year. On venus, over the course of the last 16 years the length of day has changed by 6 and a half minutes a year, a total of 104 minutes overall. Did it occur gradually, the assumption, or did it happen more rapidly?
It has also been suggested there could be exchanges of angular momentum between Venus and the Earth when the two planets are relatively close to each other. This sounds suspiciously like some of the ideas expounded by Peter Fairlie-Clark at recent Study Group meetings in London.