using geokit gem, can I determine if latitude & longitude are within a certain radius of another latitude and longitude

I'm using the geokit-rails gem (http://ift.tt/1fHZ9NA). I have a list of latitude and longitudes. I also have a list of the latitude and longitudes of some major cities that my app is concerned with.

I'd like to know how I can find out if a given latitude & longitude is within a 30 mile radius of one of the major cities I have latitude and longitude for.

I don't want to use acts_as_mappable + find. This is a one time lookup, I really don't want to change my DB tables for this.