Of all the discoveries made during the first phase of telescopic astronomical discoveries perhaps the more impactful was the discovery by various observers of the phases of Venus, which showed that Venus in fact orbited the Sun and not the Earth. This was the first real empirical proof that a pure geocentric system of astronomy was not possible. Because Mercury displayed the same behaviour as Venus in never moving more than a few degrees from the Sun as viewed from the Earth it was assumed, by analogy, that Mercury also orbited the Sun and not the Earth. This observed behaviour of Venus and Mercury had already led Martianus Capella (fl. c. 410–420) in late antiquity to hypothesise that both of them orbited the Sun and not the Earth. The Capellan system was widespread and well known in Europe during the Middle Ages. Unfortunately due to its comparatively small size and distance from the Earth, and the inadequacies of the most used Dutch or Galilean telescopes it would be almost three decades before anybody succeeded in observing the phases of Mercury.
The knowledge that Venus orbited the sun and the assumption that Mercury also did led to the possibility of observing a so-called transit of one or other of them, i.e. the passage of the planet across the face of the Sun, as observed from the Earth. What would appear to be simply simple is in fact complicated by various factors. Principally the orbits of the Earth and Venus and/or Mercury around the Sun do not actually lie in the same plane. Taken Venus, for example, its orbit is tilted with respect to the Earth’s this means that most times when Venus is between the Earth and the Sun, it mostly passes above or below it rather than in front of it. Transits of Mercury take place approximately 13 or 14 times per century. Transits of Venus are less frequent taking place over a 243 year-cycle, two transits separated by eight years followed by long gaps of 121.5 and 105.5 years.
In order to be able to observe transits of the inner planets an astronomer requires detailed, accurate planetary tables, which allow them to predict the occurrence of the transit. Following the discovery of the phases of Venus it was Johannes Kepler (1571–1630), who, with the Rudolphine Tables (1627), first provided a set of planetary tables accurate enough to predict a transit of either Venus or Mercury.
According to Kepler’s calculations there should have been transits of both Mercury and Venus in 1631. Kepler did his best to draw astronomers’ attention to these occurrences with his De raris mirisque Anni 1631 including anadmonitio ad astronomos (1629), because their observation would they would help astronomers to settle the question of the true size of the planetary orbits, which up till then known relative to the Earth’s distance from the Sun, an unknown distance, and to determine the angular sizes of Mercury and Venus.
Kepler died in 1630 but his son in law, the astronomer Jacob Bartsch (c. 1600–1633) published a printed pamphlet advising European astronomers of Kepler’s information.
Strangely there is no information that Bartsch observed the Transit of Mercury on 7 November 1631 but at least four other European astronomers did. The first was the German astronomer, astrologer, physician and calendar writer, Johann Rudrauf (1588–1654), known Johannes Remus Quietanus, a correspondent of Galileo, Kepler and Johannes Faber. Rudrauf observed the transit from Rufach in the Alsace border region between Germany and France, where he was town physician. The second was the Swiss Jesuit astronomer Johann Baptist Cysat (c. 1587–1657), who had been a student and assistant of Christoph Scheiner (c. 1573–1650) at the University of Ingolstadt and became his successor as professor of mathematics in 1618. Cysat observed the transit from Innsbruck in Austria.
The third observer was the French philosopher, astronomer and mathematician, Pierre Gassendi (1592–1655), who observed the transit from Paris. A fourth unknown astronomer observed the transit from Ingolstadt.
Gassendi’s account of the transit was the most widely read and studied so he is usually credited with being the first to observe a transit of Mercury. On 6 December 1631 Gassendi tried to observe the transit of Venus that had been predicted by Kepler unaware that because it took place during the night it wasn’t visible from Europe.
Kepler’s tables predicted no further transits of either Mercury or Venus for the seventeenth century so theoretically nobody would have set out to observe one. However, a young, self taught, English astronomer, Jeremiah Horrocks (1618–1641)did something quite extraordinary. Studying Kepler’s Rudolphine Tables Horrocks’ realised there would be another transit of Venus on 24 November 1639 (os) (4 December (ns)). Horrocks and his friend and fellow amateur astronomer William Crabtree (1610–1644) both observed the transit that Horrocks had predicted, Crabtree from Manchester and Horrocks from Much Hoole by Preston about 40 miles north of Manchester.
Both of them made their observations by projecting the telescopic image on to a sheet of paper. Horrocks wrote an extensive report of their observations calculating both the size of Venus and the distance between the Earth and the Sun. Unfortunately Horrocks died in 1641 just 23 years old and his report of the first ever observation of a transit of Venus, Venus in sole visa, was first published was first published by the astronomer Johannes Hevelius (1611–1687)in Danzig in 1662.
In 1639 the Italian Jesuit astronomer Giovanni Battista Zupi (1589–1650) succeeded in observing the orbital phases of Mercury using an astronomical telescope (two convex lenses) constructed by the Neapolitan astronomer, mathematician and telescope maker Francesco Fontana (1585–c. 1656).
Zupi and Fontana often observed together and Zupi’s observations of the phases of Mercury were published in Fontana’s Novae Colestium in 1646.
The observations of the transits of Mercury and Venus, and the phases of Mercury was final empirical proof, if it was needed, that both of the inner planets orbited the Sun and not the Earth nailing down the lid on the coffin of a pure geocentric, astronomical system. However, by the time these observations were made the majority of the astronomical community had already decided that only Kepler’s elliptical system or a Tychonic system with diurnal rotation were acceptable as the true system. The observations were of course compatible with both systems, so a final decision was not yet possible.