The Bible makes numerous references to celestial signs. Charles Taze Russel took these seriously and looked for visible fulfillments of these prophecies. In 1780, a combination of clouds, fog, and smoke from forest fires caused a great darkness that made it necessary to light candles in some places. For most of us, this is just just an interesting historical curiosity. For Charles Taze Russel, it was a fulfillment of Bible prophecy! Consider his comments in Studies in the Scriptures IV - Battle of Armageddon, page 585.
"On May 19, 1780 (still 'in those days,' the 1260 years of Papal power, but after that power had begun to wane and the brunt of the tribulation had passed) a phenomenal darkening of the sun occurred, for which scientists of that time and since have never been able to account.. This unaccountable day, except as a sign from the Lord, is reckoned to have extended over 320,000 square miles - an area about twenty-five times the size of Palestine, to which the signs of the first advent were limited. Indeed, the fact that these signs were chiefly confined to the New England and Middle States need not surprise us, when we remember that the first movement amongst the 'Virgins'* (Matt. 25:1-5) was chiefly in the same locality."
See also the article on 1833.