During the first phase of telescopic astronomy at the end of the first decade of the 17th century one of the most important discoveries was of the phases of Venus, made independently by Galileo, Simon Marius, Thomas Harriot and Giovanni Paolo Lembo in about 1611. The phases that they all observed and recorded meant that Venus definitely orbited the sun and not the earth as claimed in the Ptolemaic model of the solar system. It was this observation that actually sounded the death knell for the Ptolemaic system but was not a direct confirmation of the heliocentric system as it was also compatible with both the so-called Heracleidian and Tychonic systems. In the former both Mercury and Venus orbit the sun whilst in the latter all the planets orbit the sun, which in turn orbits the earth. With the demise of the purely geocentric Ptolemaic system both of these systems were serious competitors to the Copernican and Keplerian heliocentric systems as I have discussed in detail elsewhere. The Heracleidian system was a product on antiquity and resulted from the observation that when Mercury and Venus were visible in the heavens they never strayed far from the sun leading to the speculation that they actually orbit the sun and not the earth.
The discovery of the phases of Venus was a proof that it orbited the sun and strengthened the assumption that Mercury also did so. The next stage of the process would of course be an observation of one or both of these planets crossing the face of the sun in its orbit. Such an observation would require an accurate prediction of the time of such a transit, which itself was dependent on accurate tables of the planetary orbits. In 1627 Johannes Kepler delivered the necessary tables with the publication of his Rudolphine Tables calculated from the thirty years worth of raw observational data accumulated by Tycho Brahe at the end of the 16th century. Using his own tables Kepler calculated a transit of Mercury for the 7th of November and a transit of Venus for 7th of December in 1631.
What was now needed was an astronomer with the necessary patience and luck to be the first to observe such an event and supply a further confirmation that the two planets did indeed orbit the sun. The patience was required because Kepler’s calculations were not accurate enough to predict the exact time of transit so the astronomer had to observe over a very long period time with the hope of catching the transit. Luck was required to deliver good observing conditions at the required time. The man who took up the challenge was the Jesuit educated French astronomer Pierre Gassendi (1592 – 1655). On the 7th November 1631 Gassendi became the first person ever to observe a planetary transit in the solar system. On the 7th of December his luck deserted him in his attempt to observe the Venus transit as this took place in the night in Paris his observational position.
At first Gassendi’s achievement went almost unnoticed because he didn’t actually really believe in it himself. According to the then concept of the solar system and the relative sizes of the heavenly bodies Mercury should have been much larger than the miniscule black spot that Gassendi observed. Having failed in his attempt to observe the 1631 Transit of Venus the honour of being first would go to the young English astronomer Jeremiah Horrocks in 1639, a transit of which Gassendi was not aware. Horrocks’ observations were first published by Hevelius in the 1660s and it is first here that the 17th century astronomical community really became aware of Gassendi’s achievement and his and Horrocks’ observations became an important step in the acceptance of heliocentricity.