def distance(other)
      earth_radius = 6371
      Δlat = radians(other.latitude - latitude)
      Δlong = radians(other.longitude - longitude)
      a = sin(Δlat / 2) * sin(Δlat / 2) +
          cos(radians(latitude)) * cos(radians(other.latitude)) *
          sin(Δlong / 2) * sin(Δlong / 2)
      c = 2 * atan2(√(a), √(1 - a))
      earth_radius * c
    end
def distance(other)
      earth_radius = 6371
      Δlat = radians(other.latitude - latitude)
      Δlong = radians(other.longitude - longitude)
      a = sin(Δlat / 2) * sin(Δlat / 2) +
          cos(radians(latitude)) * cos(radians(other.latitude)) *
          sin(Δlong / 2) * sin(Δlong / 2)
      c = 2 * atan2(√(a), √(1 - a))
      earth_radius * c
    end
Computes the distance from this postcode to another postcode using the
 Haversine formula

 @return [Float]