I am supposed to be finding the radius of the circle that New York City lies and rotates on given that its latitude is 40.7128$^\circ$ N (according to Google). The radius of the Earth, for this problem, would be 3960 miles.
I was finding the radius by $3960 \times \cos(40.7128)$, however, that results in a negative radius. How am I supposed to be interpreting this result? Any help will be appreciated.