A dark day is not a metaphor. It is a rare natural phenomenon in which the light of the Sun fails for some reason to reach the Earth, and what should be daylight becomes darkness. Day darkness can vary in intensity from gloom to total blackness.
Dark days are among the earliest recorded natural events. The darkness of Exodus, which lasted three days, is an example. Another is the "cloud" which caused the Sun to disappear while the King of the Persians was besieging Larissa in c. 556 BC. After the death of Julius Caesar in 44 BC, the Sun was dimmed for a whole year, and the air was cold and misty. The darkness at the Crucifixion, which lasted only an afternoon, remains a mystery.
Dark days have a number of causes, some prosaic, others mysterious.
Obviously clouds can hide the Sun, and the thicker the cloud, the darker the gloom. However, darkness in daylight is so unsettling that, even when the cause is evidently a cloud, it can still cause alarm. 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.
A common winter feature of London and other large cities in the days before smokeless zones was what was called, euphemistically, "high fog". During an anticyclone, an inversion formed over the city, trapping fog and smoke beneath it. The Sun turned yellow, then red, and sometimes disappeared. 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".
A 19th-century artist’s impression of the 1780 dark day
Smoke from burning forests and other large fires can cause darkness hundreds of miles away. This is the type of dark day that often causes terror, as it can happen in fair weather, with no obvious cause. The most famous example is the New England Black Friday of May 19, 1780. Candles were lighted at noon in Providence; in N.E. Massachusetts print could not be read outdoors for several hours; in Worcester, "a sickly melancholy gloom overcast the face of Nature". Soot-coloured rain fell, and this was enough to reveal the cause of the darkness to most people. A Massachusetts farmer disagreed, saying that to attribute the darkness to the "smoke of burnt leaves" was absurd; it was time to repent, as "the day of the Lord draws nigh".
Sometimes desert dust raised by a storm is lifted into the upper air, and, like smoke, carried for hundreds of miles. An event of this sort caused terror in Baghdad on May 20, 1857. A dust storm had been blowing all day, when at about 5 pm, a darkness set in that was "deeper than the darkest night". Panic gripped the city. "People looked for each other, to see the end of the world together." After about five minutes, "the black darkness was succeeded by a red, lurid gloom" and "a dense volume of red sand fell".
Volcanic dust (low level)
The most well-known example of this type of darkness is that described by Pliny the Younger, who was trapped near the erupting Vesuvius on August 24, AD 79. He said that at one point the darkness was "not the dark of a moonless or cloudy night, but as if the lamp had been put out in a closed room." People trying to escape the volcano shouted and screamed, in an effort to recognise each other by their voices.
Volcanic dust (high level)
A great volcanic eruption can send dust into the stratosphere and around the world, causing solar obscurations over a vast area. In AD 536, in Italy, "for the entire year the Sun sent forth his rays without his usual brilliance, like the Moon". In Mesopotamia, the Sun was darkened for 18 months. "Each day it shone for about four hours, and still this light was only a feeble shadow…the fruits did not ripen and the wine tasted like sour grapes". This may have been the result of a huge eruption from the New Britain volcano Rabaul. (There is a possibility that it was the result of an asteroid strike in South America, whose craters have recently been discovered).
All these dark days have been the result of obscurations within the Earth's atmosphere. There are some dark days, however, that may be caused by agents beyond the Earth.
Eclipse of Sun by Moon
The horizon during the total solar eclipse of 11 August, 1999, at Totes, France. Photo by L.J. Chatfield
During a central solar eclipse, for a few minutes, the Moon completely hides the face of the Sun. However, there is never complete darkness, as the Sun's outer atmosphere, the corona, provides some light. How much light there is depends on the extent of the corona, which varies during the solar activity cycle. Observers at a total eclipse on July 28, 1851 in Norway said that the darkness was not similar to that of night. Mountains 15 miles away were faintly visible, and large print could just be read. The effect was still awe-inspiring. One astronomer wrote; "The awful appearance of the terribly black cloudy portions of the sky, and the intensely gloomy look of the remaining portion, cannot easily be forgotten." One old Norwegian woman was, however, unimpressed. During totality, she lit a candle and continued with her work. During the total eclipse of April 22, 1715, in London, three planets and several stars were visible. The astronomer De Louville grumbled, "it was a piece of good fortune at this time to have found in London an interval of clear sky."
Eclipse darkness at Bue Island on the coast of Norway, 28 July, 1851. Drawing by Piazzi Smyth
Eclipse of Sun by other body
Theoretically, an asteroid might pass close enough to the Earth to eclipse the Sun for a few seconds. It is not known if this has ever actually happened.
There seems little doubt that 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.
Day darkness with stars
There are a few accounts of dark days, outside solar eclipses, in which the stars were seen. Whatever the cause might have been on these occasions, it must have originated outside our atmosphere, otherwise the stars would have been obscured with 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." The famous astronomer Kepler thought this must have been due to "the wide diffusion of some cometary matter". Others said it was a Hohenrauch (high smoke), "notwithstanding the visibility of stars".
There are some accounts of dark days which are so peculiar, or which have so few details, that it is impossible to say what their cause was. 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. The most mysterious dark day must be the one described with irritating brevity by Charles Fort. 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.