This morning one of the sources I consult to remind me of anniversaries of discoveries, births and deaths in the history of science had two entries concerning the father and son Friesian Renaissance astronomers David and Johannes Fabricius. According to this source David was born 9th March 1564 and Johannes made his first observation of sunspots on 9th March 1611. Oh I thought in my half-awake state what a nice birthday present for his father! Then I woke up. Hang on a minute I have read just about everything that has ever been written about the Fabricii and I’ve blogged about David and about Johannes’ sunspot observations how come I’ve never noticed this before? After all it’s pretty eye-catching that Johannes made his major discovery on his father’s birthday, except that he didn’t. So what’s wrong here? Is one or even both of the dates wrong? No they are, taken separately, both right. What we have here is a bad case of calendrical confusion.
David Fabricius was indeed born on 9th March however his son first observed sunspots on 27th February. The Fabricii lived in a Protestant German state, which didn’t adopt the Gregorian calendar until 1700. Both dates are on the Julian calendar or as historians say Old Style (OS). The source I consulted this morning had left David’s birthdate Old Style but converted Johannes’ sunspot discovery to the Gregorian calendar, or as historians say New Style (NS), by adding ten days; thus producing a nice, but actually fictitious, coincidence of dates.
Anybody doing history in the early modern period should always check, which calendar is being used when a historical date is given. I personally dislike the, I think largely American, habit of converting Old Style dates to New style. As I have pointed out on my last Newton birthday post Newton was born on Christmas day and always celebrated his birthday on Christmas day so to now claim that he was born on 4th January is a historical perversion. When giving European dates in the period during which two calendars were in use then the historian is obligated to state clearly whether a given date is Old Style or New Style and to mix Old Style and New Style not only within one article but even within one family is, to put it mildly, absolutely taboo!