The rotation of Mercury
DOI: 10.1063/1.3048010
The planet Mercury does not have a rotation period equal to its orbital period. The aesthetically satisfying belief that the planet was gravitationally locked in so that it always turned the same face to the sun has been received doctrine for more than 80 years and is enshrined in almanacs and reference books. It is not so according to work done during the past year. Both radar and optical observations now appear to agree on a rotational period somewhere around 58 days or about two thirds of the orbital period (88 days). This kind of ratio, which means that Mercury turns opposite faces to the sun at successive perihelion passages, is unique so far in the solar system. Lately a theory has been put forward to explain the stability of the ratio 1:2/3.
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