code: function interpolate(t) {
      var A = Math.sin(d - (t *= d)) / sd,
          B = Math.sin(t) / sd,
          x = A * cy0 * cx0 + B * cy1 * cx1,
          y = A * cy0 * sx0 + B * cy1 * sx1,
          z = A * sy0       + B * sy1;
      return [
        Math.atan2(y, x) / d3_geo_radians,
        Math.atan2(z, Math.sqrt(x * x + y * y)) / d3_geo_radians
      ];
    }
docs: From http://williams.best.vwh.net/avform.htm#Intermediate