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a ladder of measurements, one rung leading to the next. First he used Cepheid variables, his most reliable yardstick, to directly obtain the distances to six relatively nearby galaxies; then he judged the magnitude of the brightest stars in those galaxies. Figuring such stars were similarly bright in other, more distant galaxies, he proceeded to use them as standard candles. He sought out these radiant stars in more far-off galaxies—fourteen in all—and estimated each galaxy's distance based on the stars' apparent luminosities. Then, taking all twenty of these galaxies into account (the first six and the subsequent fourteen), he estimated the brightness for an average galaxy and used that value for judging the distance to four more remote galaxies. Hubble's moving outward like this, rung by rung, was similar to Shapley's strategy for his globular cluster distance measurements, but here Hubble was making an even braver leap into distant space.

Hubble then paired each galaxy's distance with its measured velocity to see if there was a connection, some sort of organized flow in which the galaxies flew outward into the depths of space. Humason by then had redone a number of the redshifts, but when Hubble prepared his first paper on the findings (“A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae”), he primarily used Slipher's original measurements.

Hubble was more vigilant than usual in preparing this landmark 1929 publication, chiefly because of the checkered history of the subject. An earlier and rather clumsy attempt by the Polish-American mathematical physicist Ludwik Silberstein to find a relationship between a galaxy's distance and its redshift had been met with derision, especially by noted astronomers Knut Lundmark and Gustaf Ström-berg, who was Hubble's colleague at Mount Wilson. Silberstein had lumped globular clusters in with spiral nebulae, which led to a meaningless result. He was ridiculed for both his inept analysis and leaving out data that went against his prediction, which tainted everyone's outlook on the problem. To make sure this didn't happen to him, Hubble shrewdly sought out the advice of Silberstein's two harshest critics and specifically highlighted their contributions in his paper. “Mr. Strömberg has very kindly checked the general order of these values… Solutions of this sort have been published by Lundmark,” he wrote fawningly. Hubble knew he was dealing with a controversial finding, so he was taking every precaution. He was wooing potential enemies to his side. He didn't even like Lundmark, having earlier accused the man of plagiarizing his system for classifying the galaxies, and the point he had Strömberg verify was so simple it scarcely needed checking. As it was, Hubble held up publication of his data to make sure he had nailed down every argument, as well as gathered data on even fainter galaxies so he and Humason could quickly publish a follow-up and prevent others from jumping into what Hubble considered his field. He was being both careful and cunning; he was not just introducing an idea but selling it hard. Hubble knew he had to make an air tight case in order to convince his more skeptical colleagues. “There is more to the advance of science than new observations and new theories,” historian Norriss Hetherington has noted. “Ultimately, people must be persuaded.”

According to Hetherington, Hubble presented his first data on the problem as if he were standing before a judge and jury—again, not surprising given his legal training. Hubble even had witnesses. With Hubble citing their assistance, Strömberg and Lundmark were brought forward to serve as objective bystanders to verify his competence. What Hubble saw was a definite pattern to the galaxies' retreat, a rule that was simple and yet so elegant. The velocity of the galaxies was found to steadily increase—rise in a linear fashion, as scientists say—as astronomers peered ever deeper into space. At double the distance, a galaxy's speed doubles as well. A galaxy 10 million light-years away travels twice as fast