Revisionist History?

It is not common for any other astronomers to be mentioned along with Edwin Hubble as being responsible for figuring out how the distance to a galaxy is related to its recession velocity. However, Hubble did not work alone and many other astronomers deserve credit for establishing the distance--redshift relationship.

The initial observations were made by Vesto M. Slipher, an American astronomer, just prior to World War I. Slipher observed the spectra of some nearby galaxies and noted that the spectral features observed for almost all of the galaxies were redshifted to longer wavelengths.

The Dutch astronomer Willem de Sitter interpreted the "spiral nebulae's" redshifts as due to Doppler effect. de Sitter incorporated the early observations in his 1917 cosmological model of an expanding universe based on Einstein’s (then new) general theory of relativity. For a time, the red shifts were known as the "de Sitter effect."

The tendency for "spiral nebulae" to have redshifted spectra was presented by Herber Curtis in the 1920 Shapley-Curtis debate as evidence that the "spiral nebulae" could not be ordinary galactic nebulae (But the evidence was not convincing enough for Curtis to be judged as the winner of the debate).

In 1923, using rough estimates of the distances to these galaxies based on their apparent size, the German Carl Wirtz suggested that Slipher’s redshift measurements provided evidence for a velocity-distance law --- i.e., that redshift is proportional to distance.

Milton Humason, using the 100 inch telescope at Mt. Wilson Observatory in the late 1920's, painstakingly worked to obtain reliable measurements of the distances to these galaxies. Humason collaborated with Hubble, providing important distance measurements later used by Hubble in his important 1929 paper.