We construct a one-dimensional dual theory that effectively describes the sector of the (2+1)D flat gravity phase space near a flat space cosmology saddle labeled by definite mass and angular momentum. This Schwarzian-type action describes the dynamics of the (pseudo-)Goldstone bosons of BMS3 algebra on a circle as the symmetry is spontaneously and anomalously broken. This 1D theory, living on the celestial circle, provides the first explicit construction of a celestial dual theory in (2+1)D. We use it to calculate the semiclassical entropy of flat space cosmologies and find perfect agreement with existing literature. Published by the American Physical Society 2024