1. The rotation period of Venus
Because of the thick cloud cover on the surface of Venus, its rotation period remained unknown for a long time. The discovery of the red Jupiter spot, and the superficial structure of Mars made possible for Jean-Domenica Cassini (1625-1712) to determine with a good precision the period of rotation of Jupiter and Mars since 1664-1666. But the exercise proves more difficult for Venus, not easily observable during several hours in continuity, and without apparent marks on its surface. The first observations of Jean-Domenica Cassini in 1666 and 1667 suggested him a period of less than one day, but did not allow to distinguish if the planet was in rotation or libration. A few years later, following observations carried out in 1726-1727, Francesco Bianchini (1662-1729) proposes a period of 24 days and 8 hours, whereas Jacques Cassini (1677-1756) finds 23h 15mn after a new analysis of the observations of his father, and shows that a period of 23h 20mn makes it possible to also account for the observations of Bianchini. The 23 hours period is confirmed by Francesco de Vico (1805-1848) which obtains the incredibly precise value of 23h 21mn 21.934s, after an observation campaign carried out between 1839 and 1841, during which he could follow planet in full day. The controversy will continue during more than one century between the partisans of Cassini for which the period of rotation was close to 23 h, and those of Bianchini which preferred to see planet turning around its axis in 24 days.
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Figure.1:Observation of Venus by Jean-Domenica Cassini in February and April 1667. "Until the 28 day of April, I could not notice any shiny spot similar to that which I had already seen, but this day, fifteen minutes before the rising of the Sun, I started to re-examine the disc of this Planet, for which half or approximately appeared at the time enlightened, a shiny spot located near the section" (Gallica/BNF). |
On the other hand, Giovanni Virginio Schiaparelli (1835-1910), during its observations of 1877-1878, does not note significant variation of the rotation of planet, and concludes that its period of rotation cannot be of 23h, but must be much longer. He proposes then in 1890 a period of 224,7 days for Venus rotation, corresponding to a state of synchronous rotation with its orbital motion around the Sun, just like the Moon carries out a rotation on itself in the same time as its revolution around the Earth. In this configuration, for a Venusian, the Sun appears fixed in the sky, and half of planet is plunged in the night, while other half profits of a perpetual sun.
The proposal of Schiaparelli renewed the controversy, and the observations of Venus continued. Camille Flammarion from its observatory of Juvisy observes the planet between 1887 and 1894, and publishes its conclusions in 1894. He does not manage to determine with precision the period of rotation, but concludes "that it is not very distant from twenty-four hours". He observes even polar caps on Venus which enable him to say that the axis of planet is not very tilted...
In fact, the true period of rotation of Venus will be discovered only in 1962, thanks to the radar observations carried out by the Jet Propulsion Laboratory (the USA) which make it possible to be freed from the cloud cover. These observations will show, at the great surprise of astronomers, that this planet has a retrograde rotation with a 243,0 days period (Goldstein, 1964, Carpenter, 1964). This period of rotation, associated to a 224,7 days orbital period leads to a duration of the Venusian "day" of 116 terrestrial days, with a Sun which rises to the west and sets in the east.