Question: How many digits in the pi number have you been able to memorise?

Alexandre Borovik answered on 16 Jun 2020:

I remember pi as 3.1415926 — mostly because of the poetic rhythm of digit when they are pronounced in Russian, my native tongue. I hardly ever use pi; if I do, pi = 3 suffices. I was lucky that at school, I was taught the art of quick back of an envelope, or even mental, estimates for rough approximate answers to physics problems, at university I was taught to do rough estimates for problems in economics; not much practical use, but it helps when I read newspapers. I explained that because I remember one curious thing about pi: it is pretty close to the square root of acceleration of gravity on the surface of Earth. And this is explained by history: meter as a standard unit of length was proposed in the 18th century and defined as the length L of a pendulum with the half period T = pi * sqrt[L/g] of small oscillations T = 1 second (at the latitude of Paris). After some discussion it was replaced by 1/20,000 part of the length of the meridian through Paris, which was easier to measure with high precision, but happened to be approximately the same. If we substitute T= 1 and L= 1, we see that 1 = \pi*sqrt[1/g], and, measured in second and meters, pi = sqrt[g].