Latitude represents the measure of a central angle with vertex at the center of the earth, its initial side passing through a point on the equator, and its terminal side passing through the given location. (See the figure.) Cities A and $\mathrm{B}$ are on a north-south line. City $\mathrm{A}$ is located at $30^{\circ} \mathrm{N}$ and City $\mathrm{B}$ is located at $52^{\circ} \mathrm{N}$ . If the radius of the earth is
approximately $6,400$ kilometers, find $d$ , the distance between the two cities along the circumference of the earth. Assume that the earth is a perfect sphere.