VESTO SLIPHER'S OBSERVATIONAL DISCOVERY OF THE EXPANSION OF THE UNIVERSE: THE UNPUBLISHED, PARADIGM-SHATTERING MANUSCRIPT READ TO THE 1914 EVANSTON MEETING OF THE AMERICAN ASTRONOMICAL SOCIETY.

Lowell Observatory astronomer Vesto M. Slipher discovered the systematic redshifts of galaxies ('spiral nebulae' then) during the miracle years 1912 -1914. After presenting his findings on 15 spirals at the Seventeenth Meeting of the American Astronomical Society in August 1914, in Evanston, Illinois, the audience, with young Edwin Hubble in it, erupted in an unprecedented standing ovation. He had discovered what soon would be recognized as the expansion of the Universe. Slipher was reading from an unpublished manuscript (MS) now residing in the Lowell Observatory Archives. Here we present it as a full facsimile of the historic original. It reveals that Slipher clearly recognized then the radical nature of his discovery. He described the spirals as "fleeing", "leaving," and "receding" from the Milky Way at unheard of velocities, and ventured that the data might support the controversial "island universe" hypothesis. In contrast, in his follow-on formal publications, he dropped those robust descriptors in favor of the tepid term "scattering", concerned by the anisotropic distribution of the spirals and our peculiar motion among them. Words aside, from the outset the data spoke for him. By the end of his radial velocity program in 1921, 36 of his 41 spirals were shown to be redshifted--clear observational evidence for expansion. In this paper, we review and analyze the Slipher MS in the historical context of the controversy over the nature of the spirals, Percival Lowell's charge to study them spectrographically, Slipher's pioneering methods, his revolutionary data, his evolving interpretations, and why his monumental achievement is underrecognized to this day. Now that this MS is placed in the public domain it further fortifies Slipher's uncommemorated priority for the observational discovery of the expansion of the Universe. [ABSTRACT FROM AUTHOR]