The transits of Venus are very rare astronomical phenomena. When the Sun, Venus and Earth are in a near-perfect alignment we can see, with the appropriate instruments, a small black disk – the silhouette of Venus – crossing the bright disk of the Sun. Like Venus, Mercury – the other planet with an orbit inside the Earth’s orbit – also crosses in front of the sun. But due to the peculiarities of its orbit, the transits of Mercury are more frequent, recording on average 13 per century. The transits of Venus occur in pairs in eight year intervals. In turn, each of these pairs occurs, alternately, every 105.5 and 121.5 years. Humanity has only observed this phenomenon six times: in 1639, 1761, 1769, 1874, 1882 and 2004. The last transit of Venus occurred on the morning of June 8th, 2004 and was fully visible in mainland Portugal (only its beginning was not observed in the Azores and Madeira), for more than six hours.
In 1760, the French astronomer Joseph-Nicolas Delisle organized an international campaign to observe the transit of Venus that would take place on June 6th the following year. Through his extensive network of correspondents, he encouraged the astronomers of his time to participate in the venture to find the average distance between the Earth and the sun. Some historians argue that this was the first major scientific initiative of international cooperation. The plan was to measure the small variations of Venus’ position on the Sun’s disk when viewed from different places on Earth – this variation would then calculate the distance between our planet and the king star, and the distances between the planets, using the laws formulated by Johannes Kepler in the early seventeenth century. Four Portuguese astronomers, or astronomers working in Portugal, answered the call: Oratorian Father Teodoro de Almeida, in Porto; Miguel Ciera at the Colégio dos Nobres, in Lisbon; Soares de Barros, in St. Genevieve, Paris, and the Jesuit missionary João Loureiro, in Cochinchina, the southern region of Vietnam, so named by Portuguese sailors. Teodoro de Almeida published his data in the Mémoires de Mathemátique et de Physique, Académie Royale des Sciences, in 1774. Today he is remembered as one of the most prominent observers, from among more than 120 who experienced the phenomenon around the globe. The Portuguese Oratorian was also an active populariser of the new natural philosophy, especially through his work Philosophical Recreation, published in ten volumes over the second half of the eighteenth century. However, despite having become known as a publisher, Teodoro de Almeida was also a practitioner of astronomical observations. The observation of the transits of Venus would be abandoned in determining the distance between the Earth and the Sun after other techniques proved to be more stringent. Today, however – and despite not being visible in 2012 in Portugal – telescopes placed in space, thousands of amateur astronomers and millions of prying eyes will not miss the opportunity to study, record, or simply enjoy a phenomenon that will not be repeated in our lifetime. It’s just that the next Venus transit will only be seen from the Earth on December 11th, 2117.Technical Details Issue Date: 27.06.2012 Designer: Atelier Acácio Santos / Elizabete Fonseca Printer: offset Colours: 4 Colours Size: 40 x 30,6 mm Values: 2.00