The Day We Found the Universe - Marcia Bartusiak [46]
Vesto Slipher using the spectrograph mounted on Lowell Observatory's
24-inch refracting telescope (Lowell Observatory Archives)
But Slipher felt he needed to acquire an even better spectrum to peg the exact speed. It was an endeavor, he told Lowell, that “would doubtless impress all these observers as a quite hopeless undertaking, and maybe it is, but I want [to] make an attempt.”
He started the final measurement on December 29 at 7:35 p.m. and stayed with it until some clouds rolled in near midnight. On a scale from 1 to 10—1 being the worst, 10 the best—Lowell Observatory astronomers often joked that at 10 you can see the Moon, at 5 you can still see the telescope, and at 1 you can only feel the telescope. Fortunately, the sky was clear the following night, and he was able to collect additional light for nearly seven hours. Perhaps pressing his luck, he went into a third night, New Year's Eve. This time the weather was poor, and he had to finish up just before 1913 rang in. Yet, the additional attempt allowed him to squeeze one more hour of data onto his photographic plate.
Slipher had no time as yet to accurately measure this last plate, but he did a speedy check and right away knew that something was up. “I feel safe to say here that the velocity bids fair to come out unusually large,” he wrote Lowell right away. For Slipher to make such an impetuous claim at such an early stage was downright radical for a man normally so cautious. He must have been thrilled at what he had found.
Throughout January he focused on measuring all four of his plates more carefully, in order to gauge the velocity of Andromeda precisely. He did this by placing the plate of the nebula's spectrum in a “spectrocomparator,” which measured it against the standard spectrum—the rest frame. By turning a screw, he shifted one plate relative to the other. When the spectral lines at last matched, he recorded how much he had to shift the nebula plate to get it in line with the standard. The amount of shift established the velocity of the nebula. His calculations to convert the measured shift into a velocity filled page after loose page, with his figures neatly recorded in pencil. He started on January 7 and ended on the twenty-fourth.
The final result astonished Slipher. The Andromeda nebula was rushing toward Earth at the ridiculous speed of 300 kilometers per second (or around a million kilometers per hour), about ten times faster than Slipher had been expecting, given the average speed of a star in the Milky Way. Nebulae weren't supposed to act like this. Astrophysicists at the time generally believed that nebulae were rather slow cosmic creatures, plodding along at speeds far lower than stars. Instead, spiral nebulae seemed to be in a special class all to themselves. Andromeda was setting a cosmic speed record. In present-day terms, it's nearly forty times faster than a space shuttle in orbit.
Slipher, prudent as always, remeasured the plates he had just taken to make sure there was no error. He also sent a print of the spectrum to Edward Fath to obtain an independent check that the shift was real. In 1908, when Fath had taken his own spectrum of the Andromeda nebula at the Lick Observatory, he too had discovered a shift in its spectral lines. But at the time he simply wrote off the unexpected change as a likely malfunction of his spectrograph. It was the accepted wisdom that celestial objects simply did not move that swiftly. He heedlessly decided to brush aside the anomaly because, as he reported, “the shift has no direct bearing on the question for which an answer was sought.” Again, the hapless Fath missed his chance at making astronomical history. One can imagine his chagrin at receiving Slipher's print. He had seen the same spectral message as Slipher four years earlier, only to ignore it and not follow up.