On 17 Jun 66, at 2142 hours, local time, (redacted) eheran, Ran, on an international flight, I observed a strange phenomenon on the horizon. As we were approaching the Rudashur beacon, located approximately 25 miles WSW of Mehrabed Airport, Tehran, we suddenly observed a brilliant white sphere approximateing the coloration and intensity of a full bright moon. The sphere appeared suddenly and at the first sighting was approximately three times the size of a full moon. At the moment, we were flying at approximately 18,0000 feet on a course of 070 degrees at a speed of about 700 knots. The sighting occurred just as we prepared to fly a circular pattern over the Rudeshur beacon prior to descending to a lower altitude and heading for Mahrebad Airport. We observed the sighting throughout the time we circled the Rudeshur beacon, a time we lated calculated to be of four to five minutes duration. I would estimate the position of the sphere to be on a heading of 010 degrees and deep within the Soviet Union, perhaps north of 60 degrees latitude. During the four to give minutes that I observed the sphere, it continued to grow in size as its intensity diminished. Toward the end of this period it became very faint and its enormous size seemed to full the sky. The base of the sphere appeared to rest on the horizon throughout the period it was observed, indicating that the center of the sphere was rising during the time that it was expanding. The weather conditions were excellent and the unusually clear sky afforded unlimited visibility. 2. Another airliner at a slightly lower altitude also obvserved the phenomenon and we exchanged remarks about it. On landing at Mahrabad Airport some 20 minutes later, we discovered that the phenomenon was not seen from the ground except possibly during the very last stages. At that point, since the intensity had so markedly diminished, it would go unnoticed unless one were alerted to look for it. The reason for its being obscured from the ground is attributed to the abrupt rise of the very high mountain range north of the city in the direction of the phenomenon. 3. I don't know what caused this strange occurrence which was observed. I had never seen anything resembling it in over 20 years of flying experience. One theory advanced to me was that the sphere was a "false moon" and that such a phenomenon was seen near Rome, Italy. My feeling in that the sphere was formed by gasses ??ating from a man-made explosion. These gases forming the sphere may have been illuminated by the sun from the back side. If this were the case, it would account for the diminishing intensity as the density of the gases decreased with the expansion of the sphere.