d3.geo.greatArc = function() {

  var source = d3_geo_greatArcSource,

      target = d3_geo_greatArcTarget,

      precision = 6 * d3_geo_radians;

  function greatArc() {

    var a = typeof source === "function" ? source.apply(this, arguments) : source,

        b = typeof target === "function" ? target.apply(this, arguments) : target,

        i = d3_geo_greatArcInterpolate(a, b),

        dt = precision / i.d,

        t = 0,

        coordinates = [a];

    while ((t += dt) < 1) coordinates.push(i(t));

    coordinates.push(b);

    return {

      type: "LineString",

      coordinates: coordinates

    };

  }

  // Length returned in radians; multiply by radius for distance.

  greatArc.distance = function() {

    var a = typeof source === "function" ? source.apply(this, arguments) : source,

        b = typeof target === "function" ? target.apply(this, arguments) : target;

     return d3_geo_greatArcInterpolate(a, b).d;

  };

  greatArc.source = function(x) {

    if (!arguments.length) return source;

    source = x;

    return greatArc;

  };

  greatArc.target = function(x) {

    if (!arguments.length) return target;

    target = x;

    return greatArc;

  };

  // Precision is specified in degrees.

  greatArc.precision = function(x) {

    if (!arguments.length) return precision / d3_geo_radians;

    precision = x * d3_geo_radians;

    return greatArc;

  };

  return greatArc;

};

function d3_geo_greatArcSource(d) {

  return d.source;

}

function d3_geo_greatArcTarget(d) {

  return d.target;

}

function d3_geo_greatArcInterpolate(a, b) {

  var x0 = a[0] * d3_geo_radians, cx0 = Math.cos(x0), sx0 = Math.sin(x0),

      y0 = a[1] * d3_geo_radians, cy0 = Math.cos(y0), sy0 = Math.sin(y0),

      x1 = b[0] * d3_geo_radians, cx1 = Math.cos(x1), sx1 = Math.sin(x1),

      y1 = b[1] * d3_geo_radians, cy1 = Math.cos(y1), sy1 = Math.sin(y1),

      d = interpolate.d = Math.acos(Math.max(-1, Math.min(1, sy0 * sy1 + cy0 * cy1 * Math.cos(x1 - x0)))),

      sd = Math.sin(d);

  // From http://williams.best.vwh.net/avform.htm#Intermediate

  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

    ];

  }

  return interpolate;

}