When meteorology altered the course of history (or maybe not)

By Bob Plant

The Battle of Milvian Bridge was fought on 28 October in the year 312 CE. The atmospheric conditions there on that day may have had a critical influence on the course of human history ever since. It’s a defensible opinion. Or they may not have been all that important: that’s a defensible opinion too. On the other hand, perhaps nothing in the least interesting happened, at least nothing of a meteorological nature. Again, that’s entirely plausible. This is a very longstanding and very much ongoing controversy. I’ll try to explain it …

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Figure 1. A painting of the Battle of Milvian Bridge by Guilio Romano

The Roman empire by the third century had become difficult to control, with civil war becoming increasingly common and internal conflicts becoming increasingly destructive. The emperor Diocletian had tried to stabilize matters by formally dividing the running of the Eastern and Western halves of the empire, each run by its own emperor and junior emperor. This worked fairly well; the frontiers were strengthened and the tax system better organized. However, Diocletian’s abdication due to illness in 305 precipitated yet another succession struggle and civil war.

The ruthless chancer who eventually emerged victorious from this particular mess was the emperor Constantine, who began his bid in 306 in York and completed it by 324. Along the way, the battle of the Milvian Bridge in 312 was fought on the outskirts of Rome against the army of Maxentius, himself a recent usurper but supposedly recognized by Constantine as his superior and the emperor for the Western half of the empire. We’ll come back to the battle in a moment but first we should emphasize why Constantine’s victory matters. He went on to found the city of Constantinople (modern Istanbul) and shifted the imperial capital there. This was important in cementing the division into the eastern and western empires which did so much to shape the last two millennia of European and east-Asian history. But even more far-reaching was that he instigated Christianity as the state religion. This proceeded piecemeal, starting by relaxing and removing the persecutions and proscriptions of the latter part of Diocletian’s time (e.g. the Edict of Milan in 313) but ultimately establishing distinct legal and political advantages for Christians. The consequences of those changes have been enormous. 

It’s not clear whether Constantine’s actions on religion and the state were motivated by his own political calculations, by his genuine religious convictions or by some scrambled mixture of the two. If you were to take a guess anywhere along that spectrum it would not take long to find reputable historians making strongly-expressed arguments in support of it. Nonetheless, he was baptized on his death bed, and consistently professed to be a believer well before that, so it is safe to assume that he was at least a partial convert. An important but deeply controversial question for historians is when and how that conversion happened. And that brings us back to Milvian Bridge.

On the night of the battle Constantine apparently experienced a miraculous dream and around noon on the day itself a miraculous vision. A dream is somewhat difficult to verify or falsify of course, and not really our interest here. The vision was of “a cross of light in the heavens, above the sun, and bearing the inscription, CONQUER BY THIS”. The vision has been considered by many as being pivotal in his conversion, and in helping him to inspire the troops to victory on the day. What are we to make of this? 

We should note where this account actually comes from. It appears in the writings of Eusebius around two decades later, and apparently his source is that he was told so “long afterwards” by none other than the emperor himself. Now, Eusebius is not exactly considered the most reliable of writers on various matters for various reasons, and it is not difficult to find reasons to be cautious. The event is conspicuously absent in an account by Lactantius, for example, despite the fact that there would have been potentially thousands of witnesses to it associated with a full-scale battle on the edge of a major city. On the other hand, why invent something if there are potentially thousands of witnesses who might contradict it? Indeed many historians since, however credulous about miracles, have accepted that there may just be something in it: i.e., supposing that there may have been some natural atmospheric phenomenon, just possibly viewed with, shall we say … a little licence. This line of argument goes back several hundred years itself and is far from settled. To give a flavour of these sober and dispassionate scholarly debates, here is a quote from a very lengthy footnote in Potter’s (2013) biography of the emperor, “For further support of Weiss’s view [claiming a solar halo] see Barnes (2011), though I should note that refusal to accept Weiss’s view does not necessarily indicate an attachment to the Nazi party as is implied in his discussion.”

The meteorological explanation usually put forwards in modern historical articles is that it may have been a “sun dog”. Figure 2  is a very typical picture put forwards to support the notion, taken from the wikipedia page about the battle:

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Figure 2. Two ‘sun dogs’ (parhelia). Source: Wikipedia.

Perhaps it does look like a plausible explanation, allowing for, shall we say … a little licence. But let’s be a little more careful. If we try searching online for pictures of sun dogs we can easily find many beautiful photographs, but we see relatively little that might resemble the description.

What is a sun dog anyway? A highly-recommended guide to atmospheric optical phenomena is provided by the atoptics.co.uk website. A sun dog, or parhelion, is rather common and occurs when light is scattered by ice crystals in the shape of hexagonal plates that are suitably oriented, with the hexagonal faces aligned close to the horizontal. For a more vertically-extended display, or a “tall sundog”, it is helpful if the plates are not quite horizontal but are wobbling slightly as they fall. However, too much wobble, more than a degree or so, and the halo is lost. Less common, but a little bit more like the description, is a sun pillar which requires the ice crystals to be consistently somewhat tilted and a low angle of the sun. A rare event would be to combine both upper and lower pillars with a substantial horizontally-extended halo such as a parhelic arc which arises from reflections from the vertical faces of the crystal – creating something which can indeed look like a cross. This is not easily achieved, however, and it may be worth adding that Rome around noon in late October is a most unlikely time and place to be able to catch it. To get a sense of just how delicate the conditions would need to be, there is a fun halo simulator available from atoptics that you can play around with and see if you can manage to generate something like the image.

There are entire books and countless articles on this event from historians. And for that matter there are entire books on atmospheric optics. If you really want to develop an informed opinion, you have a lot of reading to do! I’ve simply tried to give a short introduction as a non-expert for non-experts. But I thought it may be interesting for the meteorologically-minded to know something of how and why the possible appearance of an atmospheric optical phenomenon has been such a hotly-debated question.

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