Four hundred years ago this month, Galileo Galilei presented his eight-powered telescope to the Venetian Senate. He was soon working with a 20-powered telescope, and later that year, he proved that the moon's surface was rough, contrary to the prevailing view. Galileo went on to become one of the most recognized names in scientific history. But why do we call him by his first name only?
Because that's how he referred to himself. At the time of Galileo's birth in 1564, surnames were optional in Italy. In daily interactions, an Italian would use the name his parents gave him at birth—what we'd now call a first name—and, if further clarification were required, add on his father's name (like di Antonio, or "son of Antonio"), his birthplace (Romano, or "from Rome"), his occupation (Panettiere, * meaning "baker"), or a traditional family surname (if one existed, like Galilei).
The Italian astronomer's name is unusually confusing because both Galileo and Galilei were surnames used by his family for generations. (An equivalent might be "William Williams.") This was not a particularly common practice at the time. Moreover, the name Galileo itself, although not completely unique, was quite rare. This is part of the reason we continue to use his first name only—it's unambiguous.
In Renaissance Italy, individuals didn't even stick with the same second, or identifying, name throughout their lives. Many used their family surnames one day and place of birth the next, depending on the circumstances. Take Leonardo da Vinci. Because Vinci was a very small town, calling himself Leonardo from the town of Vinci left little room for confusion—unless, of course, he was in Vinci at the time. (Leonardo was a common name.) In that case, the artist would probably have called himself Leonardo di ser Piero da Vinci, making reference to his father. Once he became famous, he often signed his name simply "Leonardo." Galileo referred to himself sometimes by first name only, sometimes as Galileo Galilei, and sometimes as Galileo Galilei Linceo (a nod to his alliance with a progressive group of scientists, which served, in part, as a kind of honorific). Also, because the convention was so casual, some individuals weren't consistent with spelling or construction. Negri, Negro, de Negro, or Neri might all refer to the same person.
The governments of the various Italian city-states eventually grew frustrated by their citizens' constantly shifting last names—without standardization, it was difficult to levy taxes or enforce military registration requirements. Beginning in Galileo's lifetime, therefore, laws swept through Italy requiring parents to record both first and last names for their children. If a family had a traditional surname, they usually used that. If not, they resorted to town of origin or occupation, and then these names were passed down through the generations. For the first time, a person named da Vinci might not actually be from Vinci. A man named Ferrari might not be a blacksmith. Italians also had to record their names upon marriage and death with either church or state authorities, depending on the area. Italy was a bit of a latecomer in this regard. Many nearby countries, like France and Germany, had systematized surnames generations earlier. This is probably why we don't refer to Johannes Kepler, Galileo's colleague and regular correspondent, or Nicolaus Copernicus, who pre-dated Galileo, as Johannes and Nicolaus.
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Explainer thanks Valeria Finucci of Duke University, Meredith Gill of the University of Maryland, and Owen Gingerich of Harvard University.
Correction, Aug. 21, 2009: The original version of this article misspelled the Italian word for baker. ( Return to the corrected sentence.)