Nor the comet that came unannounced out of the north, flaring in heaven;
Nor the strange huge meteor procession, dazzling and clear, shooting over our heads,
(A moment, a moment long, it sail’d its balls of unearthly light over our heads,
Then departed, dropt in the night, and was gone;)
The comet is clearly Comet 1860 III, the Great Comet of 1860. But what were the meteors? Historians have suggested that Whitman was describing the Leonid meteor shower of 1833, or perhaps the one in 1858. More recent scholars have suggested he was describing a fireball that passed overhead on the morning of November 15, 1859. But forensic astronomer Don Olson and his collaborators show in the June issue of Sky & Telescope that Whitman was actually describing a rare phenomenon called a "meteor procession" that occurred on July 20, 1860.
Olson and his colleagues ruled out the Leonids; these are multi-hour events, not the brief one of the poem, and 1833 and 1858 are not the years the poem covers. And though the fireball of 1859 fits the timeline, Whitman described a process of "balls of unearthly light," not a single one.
The solution to the mystery came from the back of a catalog from an exhibition of paintings by Frederick Edwin Church, a member of the Hudson River School of artists. Olson noticed similarities between the painting and Whitman's poem: The Meteor of 1860 depicted a line of several fireballs blazing across the New York sky. A search through newspapers, journals and other publications revealed hundreds of accounts from July 20, 1860 of a phenomenon known as a meteor procession—when a meteor grazes the Earth's atmosphere and fragments into smaller meteors all traveling in the same path—exactly what Whitman had described. It could be seen from Vermont to Virginia, from the Great Lakes to the Atlantic Ocean, in upstate New York where Church painted, and in New York City where Whitman lived.
Though the event garnered much public attention—Olson found hundreds of eyewitness accounts—it was mostly forgotten by the 20th century, perhaps overshadowed by the tremendous events that shortly followed. But Whitman and the thousands of other people who saw the meteor procession witnessed something truly special. Olson knows of only three other meteor processions in history. Now there are four.
(In related news, check out this story about a group of meteorologists who how Olson's group and some scientists from the UK tracked down where Claude Monet stood to create his painting Waterloo Bridge in 1903.)