Decimal Points Were Used 150 Years Earlier Than Previously Thought

According to a new paper recently published in the journal Historia Mathematica, decimal points have been used 150 years earlier than previously thought.

Until now, it was believed that the first example of decimal points used for calculation purposes was in 1593 in astronomy-related work Astrolabium by German mathematician Christopher Clavius. However, according to Glen Van Brummelen, math historian at Canada’s Trinity Western University, Clavius borrowed the concept of decimal points from Venetian merchant Giovanni Bianchini.

Van Brummelen found that Bianchini used a decimal point on multiple instances in a manuscript known as Tabulae primi mobilis B, which provided instructions on calculating the coordinates of planets. The document is dated to the 1440s, meaning that Bianchini’s use of decimal points preceded Clavius for some 150 years.

“He was using the decimal point, actually, in two different contexts. We don’t exactly know which was first, but probably he was using it in conjunction with surveying instruments to find distances across fields or altitudes of buildings and so on. But then, what we hadn’t been entirely clear on is that he ended up borrowing this for his work in astrology,” Van Brummelen said in an interview with NPR.

Researchers have previously been puzzled by the fact that Clavius only used decimal points in Astrolabium, one of his lesser-known manuscripts, but disregarded them in later works. According to Van Brummelen, this would make much more sense if he was just borrowing the idea from someone else.

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