By Jacob Aron Needles falling on paper will calculate pi, everyone’s favourite mathematical constant, on the day that bears its name. Pi day is 14 March, or 3.14 as Americans might write it. The constant – the ratio of a circle’s circumference to its diameter – is a string of numbers beginning 3.14159, although it goes on forever. Computers have crunched pi to trillions of digits. But this pi day, beginning at 1.59 pm GMT, Marcus du Sautoy of the University of Oxford will run Pi Day Live, an online experiment to get people to calculate the constant using a 200-year-old method called Buffon’s needle, which anyone can take part in. The technique involves dropping a needle onto paper marked with evenly spaced parallel lines that are further apart than the needle’s length. The probability of the needle crossing a line is linked to the value of pi. A single needle drop won’t give an accurate estimate, so the idea is to pool results from as many participants as possible. When the Pi Day Live team tried Buffon’s needle themselves with a total of 100 drops, they got 2.63 as an estimate for pi, which would make for some pretty wonky circles. Du Sautoy hopes scaling up the experiment online will improve results. “There will be errors in everyone’s calculation because it is a probabilistic method, but the more data we get coming in, the more we’ll home in on pi,” du Sautoy says. The experiment will also look at other low-tech ways of measuring pi, such as counting marbles crammed into a roughly circular shape, a method thought to have been used by the ancient Egyptians to compare a circle’s diameter to its area. “Pi is so much part of our mathematical heritage,” says du Sautoy. “Most people can rattle off a few decimal places.” Why is pi so universally beloved? After all, no one celebrates e day to commemorate 2.71, the constant that is the base of natural logarithms. It’s partly because 71 February doesn’t fit nicely with our calendar – but other mathematically themed days have failed to take off as well. “I tried to introduce the perfect day, June 28th,” says du Sautoy, chosen for the first two perfect numbers – numbers that are the sum of their factors (28 = 1 + 2 + 4 + 7 + 14, for instance). But nothing has the popularity of pi. Perhaps it is just because circles are so important, both culturally and scientifically. “Pi is historically one of the very first numbers mathematicians started trying to calculate and explore,” says du Sautoy. So why not have a go yourself this pi day?