On Monday, May 9th, 2016, the inner planet Mercury will eclipse with the Sun. There was a time when astronomers were observing this curious phenomenon of a black pinhead crossing the Sun’s surface with much more rigor than today.
As Mercury transits were only noticeable since the invention of the telescope in 1608, the first observation of such an event occurred in 1639. Observations of Mercury transits were not only important for obtaining a better understanding of Mercury’s trajectory in its orbit around the Sun, in the 18th-century the transits of the inner planets (Mercury and Venus) were regarded as the best (if not sole) opportunity to determine the Solar parallax to an acceptable level of accuracy. In those days the solar parallax or the mean distance between the Sun and the Earth was regarded as bench¬mark for the size of the solar system, or even more: the universe.
It was the English astronomer Edmund Halley who in 1691 called the attention of his fellow scholars to the possibility of such measurements, designing in the next years a mathematical method which could be used by his astronomical successors, long after the time of his own death. Where Halley disregarded the transits of Mercury in this respect (he focused primarily on the much rarer transits of Venus (which would occur in 1761 and 1769), later astronomers made an effort to use his method much earlier, during the Mercury transits of 1743 and 1753.
In this seminar I will present an overview of the efforts made by 17th- and 18th-century astrono¬mers to observe Mercury transits and to use the collected data for the purposes described above. Some emphasis will be given to efforts of this kind made on Dutch soil.