On 9 May the astronomers of Europe, and other regions, having screwed their sun filters onto their telescopes, will settle down to observe a transit of Mercury. For any not familiar with astronomical jargon that is when the planet Mercury crosses the face of the sun.
Neither as rare nor as spectacular as the similar transits of Venus, it will still be regarded as a major event in the astronomical calendar. Transits of Venus occur in pairs separated by eight years approximately once every one hundred and twenty years. The last pair was in 2004 and 2012. The cycle of occurrences of transits of Mercury is much more complex but there will be a total of fourteen in the twenty-first century with next Monday’s being the third. Because Mercury is much smaller than Venus and much further from the Earth, unlike a transit of Venus which can be observed with the naked-eye (taking the necessary precautions against the sunlight of course), a transit of Mercury can only be observed with a telescope. The French astronomer, Pierre Gassendi, was the first person to observe a transit of Mercury in 1631 but this historic event was preceded by a couple of thousand years of speculation about the orbital path of the Messenger of the Gods.
Both Mercury and Venus when viewed from the Earth never appear to move very far away from the sun leading some astronomers in antiquity to suggest the so-called Heracleidian of Egyptian system in which the two planets orbited the sun whilst the sun orbited the earth in a geocentric system. Thanks to the De nuptiis of Martianus Cappella (fl. 410-420 CE) this partial helio-geocentric model was well known and moderately popular in the Middle Ages, so the idea that Mercury and Venus orbit the sun was not new when Tycho Brahe suggested it in his full helio-geocentric system, in which all the planets, except the moon, orbit the sun which in turn orbits the earth.
In 1608 Hans Lipperhey invented the telescope and within a very short time various astronomers began to use it to observe the heavens. In November 1610 Benedetto Castelli (1578–1643) wrote to Galileo reminding him that Copernicus had predicted that Venus would have phases like the moon in a heliocentric system.
On 11 December Galileo wrote to Kepler informing him that he had discovered those phases, famously putting the information into an anagram, which Kepler failed to decode properly. Galileo was not alone in making these observations, Thomas Harriot in England, Simon Marius in Germany and Giovanni Paolo Lembo in Rome all independently discovered the phases proving that Venus did indeed orbit the sun and by analogy Mercury probably did as well. The telescopes in the early seventeenth century were not powerful enough to resolve the phases of Mercury.
That Venus and Mercury had been shown to orbit the sun was not a proof of heliocentricity, as this was also the case in the Heracleidian as well as various Tychonic and semi-Tychonic systems but it did mean that theoretically it should be possible to observe a transit of one or the other of them. Due to the fact that the orbits of the earth, Venus and Mercury do not all lie in the same plane but are all slightly tilted with respect to each other a visible transit does not occur by every orbit but as mentioned above at semi regular irregular intervals and in order to observe such a transit someone first had to calculate when they would take place. This task was carried out by Johannes Kepler in his Rudolphine Tables based on Tycho Brahe’s observations and published in 1627.
Using the information supplied by Kepler’s tables Gassendi tried to observe a transit of Venus in 1631 unaware that it would take place at nighttime for an observer in Europe. Kepler’s table lacked this level of accuracy. However earlier in the same year, on 7 November, Gassendi had become the first person to observe a transit of Mercury. The first observation of a transit of Venus was made by Jeremiah Horrocks in 1639. Gassendi was very initially cautious in going public with his discovery because his measurements of the size of the planet showed it to be much smaller than previous estimates. However three further transit observations in the seventeenth century, Jeremy Shakerly 1651, Christiaan Huygens 1661 and Edmund Halley 1677, confirmed Gassendi’s first observations and measurements.
Observation of transits of Mercury have long since become routine but that won’t stop the amateur and professional astronomers on next Monday putting up their telescopes to follow the tracks of the Messenger of the Gods as he plods his way across the sun.
 For a fuller description of the discovery of the phases of Venus and its significance for the history of heliocentricity see my post The Phases of Venus and Heliocentricity: A Rough Guide.