I am having some troubles using the geometry at google maps. I am getting the following error:
Unable to get property 'spherical' of undefined or null reference.

This is some code from w3c with some additions of mine.

var x=new google.maps.LatLng(52.395715,4.888916);

var stavanger=new google.maps.LatLng(58.983991,5.734863);

var amsterdam=new google.maps.LatLng(52.395715,4.888916);

var london=new google.maps.LatLng(51.508742,-0.120850);

function initialize()

{

var mapProp = {

  center:x,

  zoom:4,

  mapTypeId:google.maps.MapTypeId.ROADMAP

};



var map=new google.maps.Map(document.getElementById(googleMap),mapProp);

var myTrip=[stavanger,amsterdam,london];

var flightPath=new google.maps.Polyline({

    path:myTrip,

    strokeColor:#0000FF,

    strokeOpacity:0.8,

    strokeWeight:2

    });

flightPath.setMap(map);



try{

    var markerpos = google.maps.geometry.spherical.interpolate(flightPath.getAt(0), flightPath.getAt(1), .5);

    // also tried //

    //var Spherical = google.maps.geometry.spherical;

    //var markerpos = Spherical.interpolate(flightPath.getAt(0), flightPath.getAt(1), .5);

    //var markerpos = google.maps.geometry.spherical.interpolate(amsterdam, london, .5);

}

catch(ex){alert(ex);}



var marker = new google.maps.Marker({position: markerpos,

    map: map,  clickable: false

});

}

What is wrong with the above code?