This is a story about the size of the universe, our place within it, and a womans breakthrough in a mans world. On the night of April 26, 1920 these themes came together in one of the most curious events in modern science, one that at the time was front-page news. Like any good story, this one has a curious background:

For astronomers, springtime is "galaxy season." In the Northern Hemisphere, the spring skies are dotted with thousands of distant galaxies, many of them easily seen in small telescopes as tiny smudges of light. These tiny smudges (known as "nebulae," Latin for "little clouds") had long been observed by astronomers, but until the 19^th century no one knew much about them.

Lord Rosse's telescope, seen today after renovations.

By modern standards Lord Rosses telescope was very primitive, but its large aperture revealed many things never before seen. One of its new revelations was that some of the "nebulae" had a spiral structure, with arms or lines curving away from the center. These then became known as "spiral nebulae," and as better telescopes were built, more of them were detected. By the turn of the 20^th century so many "spiral nebulae" were known that the question of what they might be was one of the great puzzles in astronomy.

There were two basic theories on the "spiral nebulae":

• That they were small and close by, and therefore probably gas clouds, perhaps in the process of condensing into solar systems like ours.
• That they were very far away and therefore large, perhaps other galaxies like our own.

Today this seems a rather silly debate -- of course there are other galaxies like our own -- but at the time that was not known. It was known that we live in a large cloud of millions of stars, but no one knew how big it was, or our position within it. As far as anyone knew, that cloud of millions of stars -- our galaxy -- was all there was, an "island universe" that contained all of creation.

The universe was thought to be perhaps 30,000 light-years across, which seemed huge, and our solar system was presumed to be somewhere near the center. But whether the spiral nebulae were inside or outside our galaxy, no one was sure.

The reason no one was sure about these "nebulae" was that there was no way to measure how far away they were and therefore no way to know if they were small or large. And it was here that, at a time when astronomy was exclusively the realm of men, a woman came to the rescue.

Henrietta Leavitt worked at the Harvard College Observatory, but not as an astronomer. She and a number of other women were assigned the drudgery of analyzing and cataloguing stars on photographic plates, a mind-numbing task that involved the microscopic comparison of millions of stars on photographs, looking for, among other things, tiny variations in brightness.

It was in cataloguing those variations that Henrietta Leavitt came up with a brilliant insight: She discovered that a certain type of variable star (known as a Cepheid variable) could be used as a "yardstick," a way to measure cosmic distances. These stars are peculiar because the rate at which their brightness varies is directly proportional to their true brightness. That means that by measuring their rate of variation we can know how bright they really are, then by measuring how bright they appear to us we can deduce how far away they must be. (Its a bit like observing headlights on a highway: Because you know how bright a headlight is, if you see a headlight dimly you know it must be far away; if you see it brightly you know it must be closer.)

With this discovery, Henrietta Leavitt had provided the key to many problems in astronomy. And one of the first to use that key was a self-confident young man, also from Harvard, who had made his mark with a new 60-inch (1.5-meter) telescope at the Mount Wilson Observatory in California.

Harlow Shapley immediately grasped the value of Leavitts discovery. He put it to use, along with the new scope, to measure the distances to globular clusters, which are small "beehives" of stars that orbit the center of our galaxy. Shapley found that the distances to these clusters indicate their center of orbit is not near us, but around a point tens of thousands of light-years away, implying that we are far from the center of the galaxy. From his measurements he also surmised that our galaxy -- our "island universe" -- was about 300,000 light-years across.

Shapleys results stunned the world: In a single stroke, he had proclaimed the universe at least 10 times as large as previously thought, and displaced humankind to its outer reaches.

In these conclusions Shapley put himself squarely at odds with many astronomers, including one of the senior statesmen of American astronomy, Heber Curtis. Curtis, an old-school academic so formal that he wore a suit and tie even when at the telescope, disagreed with Shapley on almost everything. He distrusted Shapleys use of variable stars to measure distances, he believed our galaxy (and indeed the whole universe) was much smaller than Shapley claimed and, unlike Shapley, he believed that our sun is near the center of our galaxy.

Harlow Shapley (left) and Heber Curtis.

Because of their strong disagreements, the National Academy of Sciences (NAS) set the stage for a major event: They invited Curtis and Shapley to debate at the NAS national meeting on the evening of April 26, 1920, with the luminaries of science in attendance, including Albert Einstein.

If this had been a Hollywood production, the young Shapley would have been the fiery speaker, his bold new ideas triumphing over the stuffy Curtis. But in fact it was Shapley who proved a rather dull speaker, monotonously reading a 19-page manuscript to an audience that at times seemed to be nodding asleep. His presentation, while full of exciting new ideas, was based on sketchy data and at times backed only with assumptions.

It was then Curtis turn, and in very deft style he proceeded not so much to present his own views but to shred Shapleys conclusions. Curtis spoke without notes and held the audiences attention as he sliced away at Shapleys evidence. He left few of Shapleys arguments standing, then in his closing remarks he brought up something Shapley had not expected: The old problem of the "spiral nebulae."

This was yet another area in which Curtis and Shapley disagreed: Curtis believed our galaxy to be small and the spiral nebulae to be other small galaxies. Shapley, who thought he had proved our galaxy to be huge, assumed the spirals were small gas clouds within our huge galaxy.

But Shapley was not prepared for this line of discussion, nor was he expert on the topic. In the heat of the debate he cited the observations of a close personal friend, Adrien van Maanen, as supporting his view that the spiral nebulae had to be small and therefore close-by. But van Maanens observations and evidence were flimsy, and in the end Curtis batted away Shapleys arguments with a dismissive one-liner: "There are some observations that are not worth a damn, and others that are not worth a damn. In my opinion, two damns are not better than one damn."

The audience broke up laughing, and the general conclusion in newspapers across the country was that Curtis had won the debate. But it was really a draw: Curtis, the champion of the old school, was right about the spiral nebulae being galaxies beyond our own, while young Shapley was right about our galaxy being much larger and that we are not at the center of it.

The basic questions Curtis and Shapley debated -- about the size of the universe and our place within it -- were eventually settled by Edwin Hubble, and once again it was Henriettas key that opened the door: Using a new and even bigger telescope, Hubble was able to detect some of Leavitts curious variable stars in the Andromeda Nebula, proving that this "spiral nebula" is not small or nearby, but a huge galaxy at a great distance, far outside our own galaxy.

In 1923, Edwin Hubble was examining photographic plates of the Andromeda nebula, which is in fact a large galaxy. Hubble located three novas, each marked with an "N.'' One of these, however, turned out to be a Cepheid variable -- a star that changes predictably in brightness -- and the "N'' was crossed out and the star was re-labeled "VAR!''

Today we know that billions of galaxies exist throughout the universe, and that the universe itself is far bigger than Curtis, Shapley, or even Hubble imagined. But it was that debate in 1920 that first focused the publics attention on the topic, and Hubble who finally proved that we are but a speck in a sea of specks in a cosmos larger than we ever imagined.