Transits of Venus and the Astronomical Unit: four centuries of increasing precision

Only seven transits of Venus have been observed and studied up to now since the first astronomical observations with the telescope: 1639, 1761-69, 1874-82 and 2004-12. The measurement of the Astronomical Unit has been one of the main goal of the study of this rare phenomenon, as well as the identification of the planetary atmosphere and its properties, the understanding of the black drop phenomenon and the measurement of the solar diameter. The transit of 1639 observed by J. Horrocks is presented in more detail.