def get_geometry(lon, lat, size):
    """Returns a geo json geometry.
    
    Give lon, lat, and size in degrees.
    """
    geo_json_geometry = {
      "type": "Polygon",
      "coordinates": [
        [
          [lon-size/2, lat-size/2],
          [lon-size/2, lat+size/2],
          [lon+size/2, lat+size/2],
          [lon+size/2, lat-size/2],
          [lon-size/2, lat-size/2]
        ]
      ]
    }
    return geo_json_geometry