The "Prophecy Corner" Website describes an unsual huge shining cross shape over Jerusalem, seen by St Cyril in 351 AD, and mentions a possible scientific explanation:
At 9 AM in 351 AD, the great luminous cross was seen over Golgotha on Pentecost. "Philostorgius and the Alexandrian chronicle affirm, that this cross of light was encircled with a large rainbow." Bishop Cyril wrote the emperor about the miracle. After describing it, he said, "The whole city, penetrated alike with awe and with joy at this portent, ran immediately to the church, all with one voice giving praise to our Lord Jesus Christ, the only Son of God." He ended his letter with wishes "that the emperor may always glorify the holy and consubstantial Trinity." The faithful considered the miracle a sign that victory over Arianism was coming, and it probably was. Church historians Socrates, Sozomen, Theophanes, Eutychius, John of Nice, and Glycas also wrote about it. Some soldiers that saw the cross in the sky were frightened of what it might mean.
Parhelion is a bright spot that sometimes appears on either side of the sun, often on a luminous halo. White parhelia are believed to form from light that is reflected off of atmospheric ice crystals; colored parhelia are believed to form from light that is refracted by atmospheric ice crystals. Multiple parhelia can often be seen at the same time. An anthelion (meaning opposite the sun) is a rare optical phenomenon appearing on the parhelic circle opposite to the Sun.
Britannica says that parhelion is "also called Mock Sun, or Sun Dog, atmospheric optical phenomenon appearing in the sky as luminous spots 22° on each side of the Sun and at the same elevation as the Sun. Usually, the edges closest to the Sun will appear reddish. Other colours are occasionally visible, but more often the outer portions of each spot appear whitish."
Th miraculous cross of 351 must have also been seen by people outside of Jerusalem. "Gallus (Caesar) arrived in Antioch on May 7, 351. Socrates (Hist. eccl. 2.28.2) says his adventus was marked by the appearance of a cross in the sky." http://prophecycorner.theforeverfamily.com/procon1406.html
Here is an example of a parhelion:
One problem I see with calling it a parhelion is that generally the images I see of parhelions are ones taken over snowy, cold, or wintry areas. And the description of Parhelions in general seems to be that they are caused by reactions to icy particles, although it's true that sometimes icy hail can fall in places in summer.
Edwin Gifford comments on the "Writings of Cyril. The Catechetical Lectures" with skepticism:
The beginning of Cyril's Episcopate was marked by the appearance of a bright Cross in the sky, about nine o'clock in the morning of Whitsunday, the 7th of May, 351 a.d. Brighter than the sun, it hung over the hill of Golgotha, and extended to Mount Olivet, being visible for many hours. The whole population of Jerusalem, citizens and foreigners, Christians and Pagans, young and old, flocked to the Church, singing the praises of Christ, and hailing the phenomenon as a sign from heaven confirming the truth of the Christian religion.
Cyril regarded the occasion as favourable for announcing to the Emperor Constantius the commencement of his Episcopate; and in his extant letter described the sign as a proof of God's favour towards the Empire and its Christian ruler. The piety of his father Constantine had been rewarded by the discovery of the true Cross and the Holy places: and now the greater devotion of the Son had won a more signal manifestation of Divine approval... the phenomenon is too strongly attested by the historians of the period to be called in question. While, therefore, we must reject Cyril's explanation, we have no reason to suspect him of intentional misrepresentation. A parhelion, or other remarkable phenomenon, of which the natural cause was at that time unknown, might well appear "to minds excited by the struggle between the Christian Faith and a fast-declining heathenism to be a miraculous manifestation of the symbol of Redemption, intended to establish the Faith and to confute its gainsayers."
A Dictionary of Christian Biography, by Henry Wace
Some time before the battle [Constantine the Great] must have received the letter from St. Cyril of Jerusalem, describing a cross of light which appeared " on May 7, about the third hour," " above the holy Golgotha and stretching as far as the holy mount of Olives," and seen by the whole city. St. Cyril praises Constantius and reports this marvel as an encouragement to him in his campaign.
The first year of Cvril's episcopate was rendered memorable by the appearance. May 7, 35 1, of a remarkable parhelion, or other atmospheric phenomenon, over Jerusalem, which was regarded as a miraculous manifestation of the symbol of redemption intended to establish the faith and confute gainsayers, and produced great excitement in the city. The churches were thronged with worshippers, and many Jews and Gentiles were converted to the faith. So important did the phenomenon appear to Cyril that he wrote to the emperor Constantius describing it. This letter has been perserved...
This is actually a feast day in the Orthodox Church, and John Sanidopoulos' blog "Mystagogy" gives excerpts from several Church historians of the time, as well as the following feast day hymns:
Commemoration of the Precious Cross that appeared in the sky over Jerusalem in 351 A.D. (Feast Day - May 7th)Apolytikion in the First ToneSo what do you think: Was St Cyril of Jerusalem's Cross a parhelion or other normal phenomenon that occasionally happens?
The image of Thy Cross at this time shone brighter than the sun, when Thou didst spread it out from the holy Mount of Olives to Calvary; and in making plain Thy might which is therein, O Savior, Thou didst also thereby strengthen the faithful. Keep us always in peace, by the intercessions of the Theotokos, O Christ our God, and save us.Kontakion in the Fourth Tone
Making its rays to shine above in the heavens, the spotless Cross dawned on the earth, bright with splendour; for it had opened Heaven, which was shut of old. Granted the effulgence of its divine operation, we are surely guided to the unwaning resplendence. In battles we possess it as a true weapon of peace and a trophy invincible. http://www.johnsanidopoulos.com/2009/05/cross-that-appeared-in-sky-over.html