The light we see from the moon is the light it reflects from the sun. Whenever a crescent shape, either light or dark, can be seen on the moon's surface it gives an observer the ability to determine a southern direction. By sighting a line that connects the endpoints of the crescent to the horizon it indicates a southern heading. Also, if the moon rises before sundown, the illuminated side indicates the west. If the moon rises after midnight, the illuminated side indicates the east. This provides a navigator with a rough east-west reference in the night sky.