https://github.com/collectiveidea/graticule/issues/issue/12

graticule.rb

module Graticule
  module Distance
    class Spherical < DistanceFormula
      def self.to_sql(options)
        options = {
          :units => :miles,
          :latitude_column => 'latitude',
          :longitude_column => 'longitude'
        }.merge(options)
        %{(ACOS(LEAST(1,
            SIN(RADIANS(#{options[:latitude]})) *
            SIN(RADIANS(#{options[:latitude_column]})) +
            COS(RADIANS(#{options[:latitude]})) *
            COS(RADIANS(#{options[:latitude_column]})) *
            COS(RADIANS(#{options[:longitude_column]}) - RADIANS(#{options[:longitude]}))
          )) * #{Graticule::Distance::EARTH_RADIUS[options[:units].to_sym]})
        }.gsub("\n", '').squeeze(" ")
      end
    end
  end
end