What was found
AT CRUISE ALTITUDE (FL 340), AT NIGHT, CAPTAIN NOTED A REFLECTION OF THE MOON ABOVE HIS HEAD IN THE COCKPIT. UPON INVESTIGATION THE PILOT DISCOVERED THE LT AFT COCKPIT WINDOW HAD A SEMI-CIRCULAR CRACK APPROX 8" LONG BY APPROX ,5" WIDE. THE CREW DECLARED AN EMERGENCY AND REQUESTED AN EMERGENCY DESCENT. THE DESCENT WAS UNEVENTFUL AND THE ACFT MADE AN UNSCHEDULED LANDING. THE ACFT REMAINED PRESSURIZED DURING THE DESCENT AND LANDING. MX REPLACED THE DEFECTIVE WINDOW AND THE ACFT WAS RETURNED TO SERVICE. A CAUSE FOR THE FAILURE OF THE WINDOW COULD NOT BE DETERMINED.