  Let G=ASL(2,R) be the affine special linear group of the plane, and set
Gamma=ASL(2,Z). Building on recent work of Str\"ombergsson we prove a rate of
equidistribution for the orbits of a certain 1-dimensional unipotent flow of
Gamma\G, which projects to a closed horocycle in the unit tangent bundle to the
modular surface. We use this to answer a question of Elkies and McMullen by
making effective the convergence of the gap distribution of \sqrt{n} modulo 1.
