At the local viewing of the transit of Venus, I asked an astronomer named Lisa how people noticed a planet going in front of the Sun in the first place. (Surely they weren’t just staring at the sun all day?)

She told me:

  1. Edmund Halley predicted the transit of Venus. He died before being seen right, which seems sad, but we didn’t discuss that any further. Theory preceded observation. EDIT: Apparently Jeremiah Horrocks first wrote of the transit of Venus.
  2. The first observed transit of Venus killed that last free parameter to allow scientists to figure out the absolute distance from Earth to the Sun. (Previously they’d only known relative distances between planets.)
  3. I asked her the question I had formulated while watching Lawrence Krauss’ talk: how can you know, as in know-know, know know know, whether a star is bright or close?

    Her answer: astronomers make a lot of assumptions. (Ahhh, satisfaction.) In particular they assume that most stars are normal (Gaussian, not just usual). Well, that makes a lot of sense then.

  4. Nowadays another telescope is being built (thank you, government) that will triple the range within which relevant things can be seen, so we will be able to see to the centre of the Milky Way galaxy (and equal distance in the opposite direction) — and do so very precisely.

    So precisely that we will be able to measure parallax — the difference in how stars appear in winter versus summer, when we’re on opposite sides of the Sun — and obtain precise knowledge of where many, many stars are. (Tripling length means roughly times 3³ volume, so more like 20-30 times more stars’ positions will be known.)

  5. Now this is the kicker in your Popperian dirtsack. Ancient Greeks had the right theory (heliocentric solar system) but discarded it on the basis of experimental evidence!

    Never preach to me about progress-in-science when all you’ve heard is a one-liner about Popper and the communal acceptance of general relativity. Especially don’t follow it up by saying that “science” marches toward the Truth whilst “religion” thwarts its progress.

    According to Astronomer Lisa, it’s not true that the Greeks simply thought they and their Gods were at the centre of the Universe because they were egotistical. They reasoned to the geocentric conclusion based on quantitative evidence. How? They measured parallax. (Difference in stellar appearance from spring to fall, when we’re on opposite sides of the Sun.) EDIT: More by @rmathematicus, suggested by @sc_k. How did heliocentrism eventually triumph in the Renaissance?

    Given the insensitivity of their measurement tools at the time, the stars didn’t change positions at all when the Earth moved to the other side of the Sun. Based on that, they rejected the heliocentric hypothesis.

    If the Earth actually did move around the Sun, then the stars would logically have to appear different from one time to another. But they remain ever fixed in the same place in the Heavens, therefore the Earth must be still (geocentric).

I always told this story to myself as the gradual removal of anthropocentrism from the natural order. First we learn we’re not the centre of the Universe, then we’re not the only Galaxy, we’re not the only species that falls in love, we’re evolved by chance like everyone else, and so on. But that story is wrong. It doesn’t fit this bit of the history of ideas and I bet it doesn’t fit other bits of history either. I need a new story.


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