The earliest example I've seen of 2π is in a 1763 letter from Thomas Bayes (the paper that appeared in the Royal Society proceedings directly after the one that

The earliest example I've seen of 2π is in a 1763 letter from Thomas Bayes (the paper that appeared in the Royal Society proceedings directly after the one that's famous). He used c for the circumference of a circle whose radius is unity. If you've ever used Stirling's approximation, this is the paper that first points out that it's a divergent series. Scan of original (it's also on JSTOR): http://www.york.ac.uk/depts/maths/histstat/letter.pdf http://www.york.ac.uk/depts/maths/histstat/letter.pdf With modern typesetting and an explanation: http://www.stat.ucla.edu/history/letter.pdf http://www.stat.ucla.edu/history/letter.pdf (I don't seriously think we should change from pi to tau.)