diff --git a/panorama-viewer.js b/panorama-viewer.js
index e29f506..458abcc 100644
--- a/panorama-viewer.js
+++ b/panorama-viewer.js
@@ -159,6 +159,11 @@
 				case "mousemove":
 					θy += event.movementX * 0.005;
 					θx += event.movementY * 0.005;
+					if (θx < -Math.PI / 2) {
+						θx = -Math.PI / 2;
+					} else if (θx > Math.PI / 2) {
+						θx = Math.PI / 2;
+					}
 					break
 			}
 			return PanState.panning;