The Earth Moves

UnknownEarly peoples might be forgiven had they viewed the stars as just a kind of wallpaper, without significance. Yet we always sensed something important out there, and struggled to understand it.

Unknown-2It was not stupid to think the heavens revolved around a stationary Earth. A few early theorizers saying otherwise were considered crackpots, and for sound reasons. If the Earth moved, why wasn’t everything on it jostled? And wouldn’t something thrown straight up fall at a distance? But the killer argument was parallax. If the Earth travelled, the stars should appear at different perspectives at different times. Yet they didn’t! Nobody realized how vastly distant the stars are, making the parallax effect infinitesimal.

While the heavens appeared to revolve in unison, a few stars didn’t follow the program, instead moving in seemingly crazy patterns. They were called “planets” (Greek for “wanderers”). This anomaly really bugged the ancients.

images-1Eventually the Second Century astronomer Ptolemy came up with a model with the stars moving on fixed spheres, but the planets using some complicated extra circles (“epicycles”) to account for their oddball movements. It was actually brilliant. But unfortunately, as astronomical observations got ever better, the scheme had to be continually rejiggered, growing ever more convoluted.



Copernicus thought of trying a radically different construct. If the Earth were a planet, circling the Sun, a lot of the complications went away. But he was reluctant to publish (he first held the book in his hands the day he died in 1543), partly because the calculations still wouldn’t work out. That was because Copernicus still assumed circular orbits.

images-3Then Johann Kepler took up the challenge. Kepler was obsessed by “the harmony of the spheres” — that in God’s perfect Heaven, everything went round in perfect circles. With access to Tyche Brahe’s immense store of accurate astronomical observations, for a decade Kepler bashed away at it, trying to somehow make the circles work. And then something truly amazing happened. Kepler realized he was wrong. He went back to it — and teased out the truth. The planets travel not in circles, but ellipses; their speeds vary with their closeness to the Sun; but for equal time intervals, they sweep out equal areas of their ellipses. (See picture.)

It was beautiful; it finally perfectly explained the movements; and it makes the hair stand up on the back of my neck to think that Kepler, despite craving a different  answer, could transcend his own preconceptions and figure it out.

Unknown-1Meanwhile, Galileo’s telescope proved Copernicus right about the Earth circling the Sun. The Church — having in 1600 burned the philosopher Giordano Bruno alive for saying so — browbeat Galileo into publicly denying it. “And yet it moves,” he supposedly grunted under his breath. And the Church was unable to suppress his book, Sidereus Nuncius (“The Starry Messenger”) which persuaded intelligent people who was right.

But we were not done yet. Why did the planets move as Kepler showed? What made them move at all?

Aristotle had theorized that anything moving had to be somehow pushed. But why a thrown object kept moving was a vexing puzzle for two millennia. Eventually, Galileo and Descartes developed the idea of inertia — anything moving keeps on moving unless something stops it (commonly, friction). And that movement would be in a straight line, unless something deflects the path. But why then didn’t the planets fly off in straight lines? What was deflecting them?

images-4It was a 23-year-old Isaac Newton who, in 1666, finally put it all together. What reconciled the theories of Copernicus, Kepler, and Galileo was yet another new idea — gravity. Of course we’d always known apples fall downward; but had never guessed this force was universal, acting even on planets. Newton worked out that gravity is proportional to mass and diminishes with the square of the distance between objects; and, voila, that this explained Kepler’s laws of planetary motion.

And so, at last, those little creatures who’d gazed with puzzlement at the cosmic wallpaper punched their way through to understand it. Again my neck hairs stand up.

Unknown-3Of course, even today, we still don’t know everything. Not even, in fact, why gravity does follow Newton’s law. Einstein got us closer, with the idea of mass bending space; you’ve seen the illustrations, with bowling bowls on mattresses. But that seems to me more metaphor than explanation; and physicists continue struggling to integrate gravity with the other fundamental forces to produce a “theory of everything.”

Yet the story I’ve told is the story of humanity growing up: our evolution from a mentality shaped by myth and superstition, steeped in mystery, to one of dispelling mystery by application of reason to observed reality. images-5I’ve read about it in Richard Tarnas’s eloquent book, The Passion of the Western Mind. And he points out that the new modern mindset was not just limited to science. Just as the old cosmology, tethered to religious dogmas, was replaced by a new rationalist view, so too everything in civilization, previously grounded in tradition-bound ideas of divine sanction — absolute monarchical power, aristocratic privilege, arbitrary laws, exploitive economics, etc. — could likewise be supplanted by new and better systems founded upon rationalist concepts of independent human dignity. And so it is coming to pass.


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4 Responses to “The Earth Moves”

  1. Anonymous Says:

    A concise and well written summary. It shows that a well written article is often better than a long winded book in conveying the point.

  2. Gregg Millett Says:

    Beautiful piece of writing (and thinking!)

  3. lennymaysay Says:

    Coincidentally came across something earlier that demonstrates that the earth revolves:

  4. rationaloptimist Says:

    I am not really sure what this little piece of video proves, if anything. But the Earth’s revolving is no longer in any serious doubt.

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