The facts are fully covered by the statement in the dictionary, "The true cause of this remarkable phenomenon is not known."
What we do know is that the Saviour's prophecy declared, "Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light." And when the time for it came, the sign appeared.
Contemporary Records
Though the comparatively small-sized newspapers of the day were crowded with news of the progress of the Revolutionary War, then raging, no little space was given to reports and discussions of this remarkable darkening of the sun.
A correspondent of the Boston Gazette and Country Journal (of May 29, 1780) reported observations made at Ipswich Hamlet, Mass., "by several gentlemen of liberal education:"
"About eleven o'clock the darkness was such as to demand our attention, and put us upon making observations. At half past eleven, in a room with three windows, twenty-four panes each, all open toward the southeast and south, large print could not be read by persons of good eyes.
"About twelve o'clock, the windows being still open, a candle cast a shade so well defined on the wall, as that profiles were taken with as much ease as they could have been in the night.
"About one o'clock a glint of light which had continued to this time in the east, shut in, and the darkness was greater than it had been for any time before.... We dined about two, the windows all open, and two candles burning on the table.
"In the time of the greatest darkness some of the ... fowls went to their roost. Cocks crowed in answer to one another as they commonly do in the night. Woodcocks, which are night birds, whistled as they do only in the dark. Frogs peeped. In short, there was the appearance of midnight at noonday.
"About three o'clock the light in the west increased, the motion of the clouds [became] more quick, their color higher and more brassy than at any time before. There appeared to be quick flashes or coruscations, not unlike the aurora borealis.... About half past four our company, which had passed an unexpected night very cheerfully together, broke up."