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were taken by glass-topped buses to the U.S. Naval Observatory, in the northwest sector of the town, for a tour of the facility and a buffet luncheon in its stately main hall. Later that evening, New Year's Eve, “occurred an event which was marked on the program and celebrated by a number of the faithful,” Popular Astronomy recounted. As the clock struck twelve, astronomers happily changed to civil reckoning for determining the start of a day. No longer would the astronomical day begin at high noon, a tradition launched in the days of Ptolemy that often led to great bookkeeping confusion. Instead, it now began at midnight, just as it did for everyone else. “It will probably be remembered and noted long after other astronomical happenings of the current year are forgotten,” stated the magazine.

But a presentation made on Thursday, New Year's Day, ultimately overshadowed all other events at the meeting. Looking out their hotel windows that inaugural morning of 1925, convention-goers discovered a blanket of snow covering the city, enough to give holiday sleds a good tryout, reported the Washington Post. Despite the ongoing snowstorm, however, the astronomers kept to their schedule and walked the short distance to the newly constructed Corcoran Hall, on the nearby campus of George Washington University, for a joint session with the mathematicians and physicists of the AAAS. They first heard a talk on stellar evolution, followed by a lecture posing the question “Is the Universe Infinite?” which led to a lively discussion among the conferees. Then right before the noon break, a paper modestly titled “Cepheids in Spiral Nebulae” was presented to the assembled audience. Those not familiar with astronomy likely imagined it was a minor technical work, of interest only to a specialist. But the astronomers in the room immediately grasped its significance. For them, it was electrifying news. Despite its lackluster title, this paper was no less than the culmination of a centuries-long quest to understand the true nature and extent of the cosmos. January 1, 1925, was the day that astronomers were officially informed of the universe's discovery.

The author of the paper was thirty-five-year-old Edwin Hubble, a staff astronomer at the Mount Wilson Observatory, in southern California. Hubble had aimed Mount Wilson's 100-inch reflector, the largest telescope in its day, toward a pair of celestial clouds known as Andromeda and Triangulum, the only spiral nebulae in the nighttime sky that can be seen with the naked eye. By having access to significant telescopic power, Hubble was at last able to resolve individual stars in the outer regions of the two mistlike clouds, and to his surprise and delight some turned out to be Cepheids, special stars that methodically dim and brighten as if they were slow-blinking cosmic stoplights.

The signals revealed that our galaxy, the Milky Way, was not alone. The Cepheids were telling Hubble that the Andromeda and Triangulum nebulae were very distant, situated far beyond our galactic borders. Our celestial home was suddenly humbled, becoming just one of a multitude of galaxies residing in the vast gulfs of space. In one fell swoop, the visible universe was enlarged by an inconceivable factor, eventually trillions of times over. In more familiar terms, it's as if we had been confined to one square yard of Earth's surface, only to suddenly realize that there were now vast oceans and continents, cities and villages, mountains and deserts, previously unexplored and unanticipated beyond that single plug of sod. Hubble directed our eyes to billions of other galaxies—other Milky Ways formerly unknown—scattered like separate atoms through space and time, as far outward as telescopes could peer. Indications of the Milky Way's true place in the universe had been cropping up for years, but the evidence was indirect, conflicting, and controversial. Hubble stepped into the fray and finally provided the decisive proof. He confirmed an idea to everyone's satisfaction that beforehand had been on far shakier ground.