On 7th Nov 1631 – Pierre Gassendi, priest, mathematician and astronomer, made the first observation of the transit of a planet, Mercury.
From the Adelaide Review:
The planets orbit the Sun in roughly the same plane and every so often planets closer to the Sun will pass between it and the outer planets. From Earth, there are only two possible candidates for this phenomenon; Mercury and Venus. This is called a transit and the inner planet will appear as a dark spot moving across the much larger disc of the Sun.
Transits of Mercury occur once or twice each decade. This high frequency is due to Mercury being so close to the Sun and orbiting so rapidly (a ‘year’ on Mercury lasts just under 88 days). But Mercury is tiny and observing its transit requires some skill. The first recorded Transit of Mercury was in 1631 by the French astronomer Pierre Gassendi who was working from a prediction by Johannes Kepler.
And more about Pierre Gassendi, who is better known as a philosopher, by Peter King of Oxford University:
Gassendi was born in Champtercier, near Digne in Provence; he was educated first at Digne, then at home, and finally at the Universities of Aix-en-Provence and Avignon, where he studied philosophy and theology. He received his doctorate in philosophy from Avignon in 1614, and was ordained a priest in 1615. His intellectual promise was recognised early, for at sixteen he taught rhetoric at Digne, and at nineteen he was appointed to teach philosophy at Aix. Somewhat unwillingly he accepted a professorship in mathematics at the Collège Royale in Paris (for which he was recommended by Cardinal Richelieu). In Paris he met and befriended Marin Mersenne, and became part of his extensive network, corresponding with Galileo and Kepler among many prominent scientists. This helped him to develop his interest in astronomy; he was an excellent observer, and an acute defender of the Copernican system against its critics (though he seems not to have fully accepted it himself). In 1631 he became the first astronomer to observe the transit of Mercury across the Sun (as predicted by Kepler).
Mersenne encouraged him to abandon maths and science in favour of philosophy, and Gassendi became the author of one of the sets of Objections to Descartes’ Meditations. This wasn’t a happy experience for him, though; his Objections were published without his agreement (and without his more detailed responses to Descartes’ Replies, the “Instances”). Not only that, but he was the only Objector to be named, and Descartes’ replies to him were particularly abrasive. Gassendi later expanded his Objections and the Instances into a book,Disquisitio Metaphysica (1644).
His disagreements with Descartes were wide-ranging; aside from a number of important issues in their scientific theories about the nature of the world, they differed deeply concerning the nature of philosophical and scientific method. Although Gassendi shared much with Descartes, including an opposition to the Aristotelianism of the time, he was best known as a champion of Epicurus, whose philosophy he developed in a way that attempted to bring it into line with Christian thought. At the centre of this position is a mechanistic, atomistic view of the world, though Gassendi added to it a belief in the immortality of a spiritual soul which lay outside the physical. Nevertheless, he rejected both Descartes’ argument for dualism and his account of the relationship between mind and body.
The chief disagreement between the two philosophers, though, was epistemological. Gassendi especially rejected Descartes’ use of universal doubt, and his appeal to knowledge gained through reason alone. Systematic doubt was acceptable, but to doubt everything was unreasonable – indeed, not clearly possible. As a special case of that, perhaps, he held the Aristotelians and Descartes alike guilty of rejecting the help of as wide a range of writers as possible: Epicurus of course, but also Plato, Democritus, and other ancient writers. Descartes had thrown out the baby with the bath-water; in rejecting the appeals to authority of the Aristotelians, he’d gone too far the other way, and tried to do everything unaided. As for reaching knowledge through the use of reason alone, Gassendi’s mature view was that, though reason must play its part, all knowledge must start with the senses (though in his earlier work he’d taken a much stronger sceptical approach to sense-based knowledge).