If Jean-Sylvan Bailly had stuck to looking through his telescope he might well have lived to enjoy a long and honourable old age. Instead, he became involved in politics in that most dangerous of places, Paris during the Revolution. And thus it was that he found himself ascending the steps of the guillotine 205 years ago tomorrow, November 12th, 1793.
Bailly was born in Paris in September 1736. Developing an interest in astronomy acquired in early life, he earned for himself in due course a reputation as one of the leading European scientists of his day, noted for his work on comets and celestial mapping. He wrote several important treatises on the subject and is particularly remembered for Histoire d'Astronomie, a classic of its kind.
But then came the urge to participate in public life. Caught up in the spirit of those revolutionary times, Bailly became a prominent member of the Third Estate, the representatives of the common people who met as part of the Estates-General at Versailles in 1789. When the Third Estate, angry at their treatment by the king and aristocracy, declared themselves a National Assembly, Jean-Sylvan Bailly was elected as its president.
It was in this capacity in June 1789 that Bailly orchestrated one of the most dramatic events of the incipient Revolution: he presided over the Tennis Court Oath, by which the members of the assembly, one by one in front of him, swore " . . . never to separate, and to meet whenever circumstances demand, until the Constitution of the Kingdom has been firmly established and consolidated".
But as in the case of many others, Bailly in due course became a victim of the monster he had helped to create. By July 1791 he was mayor of Paris, and in that capacity incurred widespread odium on the 17th of that month by allowing the National Guard to open fire on a group of republicans petitioning on the Champ de Mars. He was later arrested by the Jacobins, charged with conspiracy against the Revolution, and consigned into the custody of Madame Guillotine.
But Bailly has an unusual if apt memorial. It is not a statue near the Eiffel Tower, nor a bust upon a plinth beside some tree-lined boulevard in Paris; with the right equipment it is visible anywhere on Earth at certain times of the month as a vast walled plain, over 180 miles in diameter, near the south-west edge of the visible surface of the moon. It is a distinctive feature of our natural satellite to which we have given the name of "Crater Bailly".