Updating google maps marker in real time

Google maps marker latlng position wont update.

  Timer.periodic(Duration(seconds: 1), (timer) {
  allMarkers.add((Marker(
  markerId: MarkerId('marker'),
  draggable: true,
  onTap: () {},
  infoWindow: InfoWindow(
  title: "ISS",
  snippet: "Current Location: $lat, $long",
  onTap: () {}),
  position: LatLng(lat, long))));
    });
 });

i expect the coordinates to update every second but instead it only uses the first initial latlng position that the api fetches.