As you may have surmised, I have a healthy collection (my wife might say an unhealthy collection) of books about science and astronomy collected over the years. Some of my favorite science books are long out of date — written in the 19th or early 20th century — and I recently discovered the availability of many of these gems online in the public domain. It was the reading of one of these old texts — "Astronomical Curiosities: Facts and Fallacies", by J. Ellard Gore that started me on the brief journey of discovery that I would like to relate this week.
Gore's book is an odd collection of observations and facts, many of which have obtained full explanation in the years since its publication in 1906. Having read other excellent texts from this age, and back into the 19th century, I have come to view this time period as the start of all of the modern techniques of the science of astronomy, and indeed the level of understanding of the physics behind astronomical phenomena was very advanced by the 1880s. Gore himself was a very well established author of popular astronomical texts, and was well aware of the science as it then existed.
One area that remained rather poorly understood was the nature and content of space lying within the solar system between the planets and the sun. Where we now understand that space to be almost a complete vacuum, other than a relatively small number of asteroids, comets, and insignificant bits of rock, the possibility of "meteoric material" and "nebulous clouds" of large size and vast extent were considered possibilities at the time this book was written.
Several instances of "sun darkenings" are listed by Gore, including references from ancient Rome that would be best described as mythical (for example, a darkening for two weeks after the death of Julius Caesar), and several from the Dark Ages (pun kinda intended), which I would dismiss as unreliable. However, Gore also included the following:
"A remarkable instance of sun-darkening recorded in Europe occurred on May 22, 1870, when the sun's light was observed to be considerably reduced in a cloudless sky in the west of Ireland, by the late John Birmingham, at Greenwich on the 23rd, and on the same date but at a later hour, in northeastern France... A similar phenomenon was observed in New England on September 6, 1881".
This caught my attention as very odd — that two recent events of an overall darkening of the sun for an extended period of time, without a predicted eclipse, had gone unexplained and presumably forgotten by later researchers (since I had never heard of these events). And so I began my quest to see if I could discover an explanation, using only one tool: The Internet.
I started with the earlier date. The first thing to do was to confirm Gore's story. A quick search for the date and "sun" brought up an article from 1877 in the "Observatory" journal mentioning this event as possible evidence for the "nebular theory" for the formation of the solar system, perhaps showing that some residual of the cloud from which the solar system formed still remained in the solar system. (The theory was largely correct, the conclusion wrong). So this verified that the darkening had indeed occurred.
So, I asked myself, what could really cause the sun to dim? Very slight changes in our sun's output do occur on a regular cycle of about 11 years, but at a level that only a solar astronomer would notice. Longer period changes also occur — causing ice ages and warm periods - but a one or two day noticeable dimming would seem impossible if our understanding of the sun and solar system is not badly mistaken. So I considered more Earthly explanations.
A quick check showed that no massive volcanic eruption occurred in 1870. But that thought led me to think about fires... big fires. Searching for the date and "fire" led nowhere, so I just went straight to a Wikipedia list of "largest fires", and Shazam! There was a massive brush and forest fire in the Saguenay River Valley in Quebec on May 19, 1870. Nearly a million acres burned in a single day. A massive smoke cloud would take about 3 days to cross the Atlantic in the jet stream, and would arrive in the stratosphere over western Ireland where the darkening was first observed, high enough in the atmosphere to not appear as a “cloud”. Mystery solved!
While I was at it, I noticed another massive fire was listed for Michigan in 1881... But first things first - could I confirm the sun darkening in New England in September, 1881? This time the search was very easy to complete, as this day was known as "The Yellow Day" throughout New England for years after the event. In "A History of New England Storms" from 1891, the day is described as dark without clouds, but with a general yellow dim light that caused the landscape to appear very odd, and the persistent smell of smoke. In this case, the connection to a forest fire was made right away, but knowledge of the actual location of the fire never made it into the accounts of The Yellow Day in the 1800s.
In fact, the fire occurred in Michigan, again burning a million acres in a day, but this time with more tragic results. Some 282 people perished in this fast moving blaze, and about 14000 were left homeless. In this case the connection between the Yellow Day and this fire is now well known, though at the time of the event the two events were seen as unrelated. There was great excitement in some apocalyptic religious sects, others thought it was connected to President Garfield's coming death; the scientific community wondered if Earth had passed through a comet's tail.
For a moment, think about the difference between these occurrences from the 1800's and how the same events would transpire today. Within hours of the start of these forest fires, anyone on the planet could be informed of the unfolding disaster. The dimming of the sky would be predicted long before it occurred - no one but the most out of touch would be surprised at the loss of sunlight. And the tragic loss of life and property would be a mere fraction of the losses sustained in these historical fires.
And while you are musing on the modern marvels of communication, consider the vast information store that is the Internet. This small adventure has shown me the truly unlimited possibilities of continually available, distributed knowledge. It took me far less than an hour to resolve the mystery of the 1870 sun darkening event - an admittedly forgotten oddity in the meteorological (I won't say astronomical) history that quite possibly had never been explained.