Fibonacci spiral and Phi, computer artwork. The constant Phi, written to 866 decimal places, is calculated as (1 + square root of five, divided by 2). Also seen is a spiral within squares whose sides decrease in length by a factor of 0.168. This number (phi) is derived from the golden ratio Phi (1.618). The spiral is formed by drawing a quarter circle in each box. It resembles that seen in nature, such as the shape of a snail's shell or the segmentation of a pine cone. The Fibonacci spiral was named after the Italian mathematician Leonardo Fibonacci (1170- 1230), and is also known as an equiangular or logarithmic spiral.