Note 29 — Extract from the Preface of Kepler’s Dioptrics. Augsburg, 1611.
Edward Stafford Carlos translation
The first scientific determination of the period of the rotation of Venus was made by Dominique Cassini in 1666, from observations of spots on the planet, and concluded to be about 24 hours; but in 1726 Bianchini deduced a period of 24 d. 8 h. from similar observations. The true period is considered to be 23 h. 21 m., determined by Schroeter by a series of observations lasting from 1788 to 1793 on the periodicity of the deformation of the horns of Venus.—(Arago, Astronomie Populaire, 1854.)
Kepler’s statements can only be regarded as anticipations of phenomena not yet actually observed.