how we first measured a galaxy's speed

The Mechanism

*Vesto Melvin Slipher* (born Mulberry, Indiana, 11 November 1875; died Flagstaff, Arizona, 8 November 1969, aged 93) was an American astronomer at the *Lowell Observatory* in *Flagstaff, Arizona*, whose September 1912 photographic-spectrographic observation of the *Andromeda nebula* (now known as the *Andromeda Galaxy*, M31) was the *first measurement of the radial velocity of any object outside the Milky Way*, and whose subsequent two-year programme of measuring the radial velocities of fifteen *spiral nebulae* — published as Slipher, V.M., "Spectrographic observations of nebulae," *Popular Astronomy* 23: 21-24 (1915) — provided the dataset that *Edwin Hubble* would use in 1929 to establish the linear redshift-distance relation now known as the *Hubble-Lemaître-Slipher law* (the basis of modern cosmic expansion). Slipher took an undergraduate degree at *Indiana University* in 1901 and was recruited the same year by *Percival Lowell* — the wealthy Boston-Brahmin amateur astronomer who had founded the Lowell Observatory in *1894* on a 7,180-foot mesa west of Flagstaff to search for canals on Mars — to operate the observatory's *24-inch Alvan Clark refractor*. Slipher would spend the next 53 years at the observatory; from 1916 to 1952 he served as its director. Lowell's original research programme — to image and map the *canali* on Mars — was abandoned by Slipher in the early 1900s in favour of *stellar and nebular spectroscopy*, the new technique for measuring stellar properties from the absorption lines in starlight. In *1909* Slipher commissioned a new high-dispersion *single-prism spectrograph* of his own design from the instrument-maker *John A. Brashear*, optimised for low-surface-brightness extended objects. *Spiral nebulae* — diffuse spiral-shaped patches of light scattered across the sky, most famously the *Great Nebula in Andromeda* — were in 1912 of unknown nature: most astronomers presumed them to be gaseous nebulae within our own Milky Way (the so-called *island universe* hypothesis that they were independent stellar systems remained an open question, not settled until Hubble's 1923-25 distance measurements). Slipher's idea was to *photograph the spectrum* of a spiral nebula and look for Doppler shifts of its absorption lines, which would establish its radial velocity. The first attempts in 1910 and 1911 failed: Andromeda is so faint that even with the 24-inch refractor and the new high-throughput spectrograph, a usable exposure required *more total integration time than a single night could provide*. Slipher developed a technique of *re-acquiring the same nebula on consecutive nights* with the same plate left in the spectrograph cassette, cumulatively building up a spectrum over multiple nights. The *Andromeda observations* were made on the nights of *17, 18, 19, and 20 September 1912* — four consecutive nights of clear Arizona winter sky — with a single plate left in the spectrograph through the daytime hours between exposures. He developed the plate on the morning of *21 September* and measured it through January 1913. The *Hβ, Hγ, Hδ, and Ca II K* absorption lines from Andromeda were *all blue-shifted by approximately 0.1 nm relative to the laboratory rest-frame wavelengths* — corresponding by Doppler's formula to an *approach velocity of approximately 300 km/s* (modern value: −110 km/s as the Sun moves around the Galaxy; the difference is the Sun's own orbital motion within the Milky Way). It was the *fastest radial velocity ever measured for any astronomical object* at the time — three times the highest stellar value previously recorded. Slipher reported the result in *Slipher, V.M., "The radial velocity of the Andromeda Nebula," Lowell Observatory Bulletin* 1(58): 56-57 (1913). He continued the programme through 1914, by which point he had measured velocities of 14 additional spiral nebulae — *12 redshifted, 2 blue-shifted*. He presented the full result at the *17th meeting of the American Astronomical Society* in *Evanston, Illinois* on *17 August 1914*. The audience — including the senior Harvard astronomer *Edward Charles Pickering*, the Princeton astronomer *Henry Norris Russell*, and a young observatory assistant named *Edwin Hubble*, then 24 years old — gave him a *standing ovation*. By 1923 Slipher had extended the programme to *41 spiral nebulae*, with *36 of them showing redshifts* and velocities up to *1,800 km/s*. The data were so striking and so puzzling that they motivated subsequent theoretical work: *Willem de Sitter* in 1917 worked out a *de Sitter cosmological solution* of Einstein's general-relativity equations in which empty space drives radial recession; *Georges Lemaître* in *1927* — independently of Hubble — derived the linear redshift-distance law from a different set of cosmological assumptions and quoted *Slipher's velocities* explicitly; *Edwin Hubble* in *1929* combined Slipher's velocities (acknowledged in Hubble's paper as "the data on radial velocities of extra-galactic nebulae kindly furnished by Dr Slipher") with his own *Cepheid-variable distance measurements* (made possible by the 1923 distance to Andromeda) to derive the law *v = H₀d* — what was called the *Hubble law* through 2018 and now, since the *2018 IAU resolution*, is called the *Hubble-Lemaître law*. Slipher's name is conspicuously absent from the modern textbook attribution of cosmic expansion — although recent historical scholarship by *Michael Way* (NASA Goddard, *Journal of Astronomical History and Heritage* 27: 796, 2024) has identified an *unpublished 1914 paradigm-shattering manuscript* by Slipher, read at the AAS Evanston meeting but never submitted to a journal, in which Slipher already proposes the *systematic recession of nebulae* as a global property of the universe — a result that, if published in 1914 instead of presented orally and forgotten, would have given Slipher priority over both Hubble and Lemaître. Slipher continued at Lowell until his retirement in 1954 as director emeritus. The *Pluto-discovery photographic plates* — taken by the Lowell staff member *Clyde Tombaugh* under Slipher's directorship in *January-February 1930* — were the most-celebrated single result of the observatory in Slipher's tenure, but Slipher's own observational legacy is the radial-velocity programme. He died in *Flagstaff, Arizona* on *8 November 1969*, three days short of his 94th birthday. The 24-inch Clark refractor and the 1909 Brashear single-prism spectrograph that took the September 1912 plates are *still on display* at the *Lowell Observatory* in Flagstaff. *Every redshift survey* in modern cosmology — from the *2-degree Field Galaxy Redshift Survey* of 1997-2002 to the *Sloan Digital Sky Survey* to *DESI* in 2025 — is a direct descendant of the four-night exposure Vesto Slipher made on the Andromeda nebula in September 1912.