The planets

This morning, after being woken at 4am by the garbage trucks emptying bins in the street outside, I walked to the back of the apartment and saw a wonderful sight. The clouds had mostly cleared, and in the eastern sky three planets in close conjunction blazed over the city: Venus, Jupiter, and Mars.

The planets

The snapshot, taken with a handheld camera, doesn’t in any way capture the scene (indeed, it doesn’t capture Mars, the faintest of the three, at all).

This led me to think. In the ages of geocentric, Ptolemeian astronomy, did no clever person ever wonder about the huge difference between Mercury and Venus (which stay close to the sun) and the other planets (which wander over the whole zodiac)? Of course this can be (and was) explained with epicycles, but conceptually it is so much simpler to have a heliocentric model, with the orbits of Mercury and Venus inside that of the earth.

Also, was there no pre-telescopic astronomer sharp-eyed enough to observe the phases of Venus (a crucial piece of evidence for the heliocentric theory for Galileo, when he saw them through his telescope)?

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About Peter Cameron

I count all the things that need to be counted.
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4 Responses to The planets

  1. Wonderful photo!

  2. ari says:

    >This led me to think. In the ages of geocentric, Ptolemeian astronomy, did no clever person ever >wonder about the huge difference between Mercury and Venus (which stay close to the sun) and >the other planets (which wander over the whole zodiac)?
    Geocentric theories were the most prominebt but there were also alternative (but not so popular) theories see https://en.wikipedia.org/wiki/Aristarchus_of_Samos

  3. Peter says:

    It’s not that easy a theory to come up with, and the simple circle model is not more accurate than the better epicycle models (and those weren’t that complex). Coming up with the elliptical orbits, which does manage better prediction, is hard. Then, you probably aren’t thinking too much about the Sun always being close to some planets when you don’t see both at once (much).

    I think there are quite a lot of ideas which are obvious in hindsight, but clearly weren’t obvious beforehand. A couple of good examples are the Cartesian coordinate system, or the method of constructing a tangent which leads to differentiation. For that matter, ‘of course’ you should care about the order to which polynomials vanish, yet this one got ignored in algebraic geometry until the middle of the last century.

    • I certainly agree that it is obvious in hindsight. My implicit question was, really, did anyone before Copernicus point out the very different behaviour of Mercury and Venus from that of the other planets, and speculate on why this might have been so? I certainly don’t know enough of the history of science to answer this, but I never saw it discussed.

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