The "Dark Day" phenomenon is amoung the earliest reported natural phenomenons in the world. It has many causes, including cloud cover, smoke from
forrest fires, and ash from volcanos. Sometimes the cause is more unusual, and sometimes it's a mystery. I've included many examples, some of which
fall into the "mysterious" category.
On April 12, 1910, a darkness settled for two hours on Chicago, causing widespread terror among people who thought it was something to do with
Halley`s Comet. The Weather Department said it was a combination of rain, wind and smoke.
On April 30, 1971, an ominous overcast at Jacksonville, Florida, blotted out almost all daylight for half an hour. Streetlights came on and birds went
to roost. There was no thunder and no rain, but there was a line of thunderstorms just north of the city.
On October 24, 1933, there was "midnight at mid-day" in London, when a "high fog" caused total darkness. An Imperial Airways pilot said that "the
pall over London at mid-day looked like a huge black mushroom completely shrouding the city".
On June 30, 1861, a great comet did obscure the Sun as the Earth passed through its tail two-thirds of its length from the nucleus. E. J. Lowe, in
England, recorded that "the sky had a yellow, auroral, glare-like look, and the sun, though shining, gave but a feeble light." There was "a
singular yellow phosphorescent glare, very like diffused Aurora Borealis, yet being daylight such Aurora would scarcely be noticeable."
On February 12, 1106, according to Erman and J. R. Hind, there was a "sun-darkening" which was accompanied by meteors. A few days earlier, on
February 5, a great comet had been seen near the Sun.
In AD 542, according to Hector Boetius, "The sun appeared about noondays, all wholly of a bloody colour. The element appeared full of bright stars to
every man's sight, continually, for the space of two days together."
On April 23, 1547, in England, France and Germany, "the Sun appeared for three days as if it were suffused by blood while at the same time many stars
were visible at noon."
In AD 934, according to a Portuguese historian, the Sun lost its ordinary light for several months. Then "an opening in the sky seemed to take place,
with many flashes of lightning, and the full blaze of sunshine was suddenly restored." This last sounds like a description of the end of a total
eclipse; but, of course, eclipses do not last for months.
On September 29, 1091, Schnurrer says that "there was a darkening of the Sun which lasted three hours, and after which it had a peculiar colour which
occasioned great alarm."
According to a Notes and Queries correspondent of 1857, a total darkness at noon which lasted for hours enveloped Amsterdam on a summer day about
1800. The day was fine, the air was calm, and there was no fog. Many people were drowned by falling into the canals.
At Pernambuco, Brazil, on April 11, 1860, about noon in a cloudless sky, "suddenly the light of the Sun was diminished". The darkness increased, and
the planet Venus shone brilliantly. A corona appeared round the Sun. (This may have been a diffraction corona, not the solar corona). The darkness
lasted several minutes. Total eclipses for 1860 were January 23 and July 18.