TRADITION HAS it that it was on this day in May, AD 330, that the Emperor Constantine declared Byzantium, later called Constantinople and even later Istanbul, to be "New Rome", and thenceforth capital of the Roman Empire.
The move to the east was the culmination of a long series of events whose turning point had been the battle of Milvian Bridge near Rome, some 18 tumultuous years before. A strange thing happened on that battle field, on which Constantine defeated Maxentius, his arch rival for the Imperial laurel wreath, and meteorologists think they know the reason why.
According to Constantine's biographer Eusebius, on the evening before the battle a great cross appeared in the sky. Later, we are told, Christ appeared to Constantine as he slept, and said: "Hoc signo vince". "By this sign win your victory".
Immediately Constantine ordered the Christian monogram - the letters chi and rho, the first two in Christ's name - emblazoned on his standards.
There are cynics who say that the alleged vision was a clever ploy, and that Constantine adopted the insignia merely to secure the loyalty of his army - which it seems was largely Christian. But there is no disputing the Sign of the Cross in the sky: it is well documented, and was seen by the entire army.
Some meteorologists on hearing this story have recalled that strange optical effects occur when the sun is blow in the sky, and when simultaneously a thin cloud of ice crystals lies between the sun and an observer. The effects are particularly dramatic if by virtue of their being a very special shape, the ice crystals are orientated all in the same direction. They act like millions of tiny mirrors to reflect the sun's rays in a number of specific patterns.
In such circumstances, there may occur simultaneously two phenomena called respectively a sun pillar and a parhelic circle. A sun pillar is a vertical line of light, which appears in the sky passing vertically through the sun; the parhelic circle is a corresponding horizontal line, again passing through the sun, but stretching - in its perfect form - right around the full 360 degrees parallel to, and some distance above, the horizon.
Perhaps Constantine's legendary cross comprised a sun pillar and the appropriate segment of a parhelic circle?
In any event, Constantine won the battle, and indeed the war, that day at Milvian Bridge. In due course, as we know, he became a Christian, institutionalised the Christian Church within the Roman Empire, and reigned relatively wisely in Byzantium until his death in AD 337 at the age of 65.