  Studies of ancient bronze artifacts can be useful in understanding the
progression of human knowledge of mathematics and geometry. Here I discuss the
decoration composed by several circles and spirals of the Langstrup belt disk,
an artifact of the Bronze Age found in Denmark. I am showing by measurements of
diameters and distances of spirals, that the artist who made the decoration
knew some approximations by rational numbers of the number pi, the
dimensionless physical quantity representing the ratio of circumference to
diameter.
