(Science Week, day 2!)

The physical universe was anthropocentric to primitive man. … To Ptolemy and his school, the universe was geocentric; but since the time of Copernicus the sun, as the dominating body of the solar system, has been considered to be at or near the center of the stellar realm. … Is not further evolution of our ideas probable?

That is from introductory remarks found in transcription of what is called "The Great Debate" between astronomers Harlow Shapley and Heber D Curtis. (All right – a lot of debates are called "The Great Debate," but this one's topic was as great as any when it occurred in 1921.)

We've heard tales of Copernicus and Galileo from centuries ago, when even the idea that the Earth was not the centre of the universe was dangerous to promote. (The deadly effects of this force known as religion remain present enough that I easily recall headlines about them from this week. Humanity is working on it.)

But how did the typical conception of the universe advance from those distant centuries through today, as we prepare to begin directly photographing the thousands of planets we've recently discovered in our area of what we've grown up regarding it simply factual to call our galaxy (as opposed to billions of others)?

Yesterday I was again fascinated by the idea that Darwin, who died nearing the year 1900, brought awareness to evolution by natural selection, but could never have known about genetics. (Natural selection, still well under two centuries ago? It's so recent!)

Even more recent was this apparently-noted debate, and I found it even more astonishing to read just the summary, found at the recommended home of the NASA-adjacent Astronomy Picture Of The Day, which states the positions of the aforementioned astronomers. The idea of a star-filled galaxy had become accepted, but those dim and distant spirals in the night sky were considered a type of nebula or cloud within it. One side of the debate: this was correct classification, though our sun, previously considered to be the centre of everything, was in fact further toward the edge. The other side: those distant "nebulae" were in fact other galaxies, and our sun is indeed the centre of ours.

Rather wonderfully, both figures would turn out right and wrong in other respects as well, and my crude summary of a summary fails to graze that complexity. Happily, the full transcription is available. I will be reading it, but my ankle-deep wade was more than inspiring enough to share today. Just tasting the time-and-place-setting lists of recent technological advances and discoveries swiftly paints an awestriking picture of what was known then; what I would stand to have known then, had the year 1921 been the latest accessible year to me.

• Summary of the debate
• The full transcription