spherical_point_at_distance.rb

  def point_at_distance(center, distance, bearing=180) 
    pi = Math::PI

    r_e = 6378137.0 #meters
    d = distance/r_e
    lat1 = (pi/180) * center.latitude
    lng1 = (pi/180) * center.longitude

    tc = (pi/180) * bearing
    y = Math.asin(Math.sin(lat1)*Math.cos(d)+Math.cos(lat1)*Math.sin(d)*Math.cos(tc))
    dlng = Math.atan2(
            Math.sin(tc)*Math.sin(d)*Math.cos(lat1),
            Math.cos(d) - Math.sin(lat1)*Math.sin(y)
          )
    x = ((lng1-dlng+pi) % (2*pi)) - pi

    lat = y * (180/pi)
    lng = x * (180/pi)
    center.factory.point(lat,lng)
  end
end