diff --git a/README.md b/README.md
index b4b5c09..7234bc4 100644
--- a/README.md
+++ b/README.md
@@ -24,6 +24,11 @@ navigator.geolocation.getCurrentPosition(function(position) {
    	var sunset = new Date().sunset(position.coords.latitude, position.coords.longitude);
 });
 
+//You can set a custom zenith to get twilight times. 
+//See the source for available zenith values and their meanings.
+//Get civil dusk for Forks, WA
+var twilight =  new Date().sunset(47.95, -124.38, Date.zenith.CIVIL);
+
 ```
 
 By Matt Kane (@ascorbic). Copyright © 2012 Triggertrap Ltd. All Rights Reserved.
diff --git a/sun.js b/sun.js
index 3db78a2..a07d787 100644
--- a/sun.js
+++ b/sun.js
@@ -27,9 +27,23 @@ Date.prototype.sunset = function(latitude, longitude, zenith) {
 	return this.sunriseSet(latitude, longitude, false, zenith);
 }
 
+Date.zenith = {
+    /** Astronomical sunrise/set is when the sun is 18 degrees below the horizon. */
+     ASTRONOMICAL: 108,
+
+     /** Nautical sunrise/set is when the sun is 12 degrees below the horizon. */
+     NAUTICAL: 102,
+
+     /** Civil sunrise/set (dawn/dusk) is when the sun is 6 degrees below the horizon. */
+     CIVIL: 96,
+
+     /** Official sunrise/set is when the sun is 50' below the horizon. */
+     OFFICIAL: 90.8333
+}
+
 Date.prototype.sunriseSet = function(latitude, longitude, sunrise, zenith) {
 	if(!zenith) {
-		zenith = 90.8333;
+		zenith = Date.zenith.OFFICIAL;
 	}