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Revealed Through Reason: The Phases of the Jovian Moons — 3 Comments

  1. If we assume that the elf hypothesis is not correct, the four new objects orbiting Jupiter are dark bodies (they are dark in the shadow of Jupiter) and so should show phases from Jupiter’s point of view. Therefore, they are moons of Jupiter. However, I find it interesting that it would appear that Locher also thought that Venus was a moon and not a planet as it, too, showed phases and thus was also a “dark body.” Quoting from above “And thus we have circling the sun a second ‘moon’ with an approximately annual cycle….”

    I do activities about how we classify things (or why Pluto is now a Dwarf Planet) by looking at the characteristics of a body in order to classify objects as planets, moons, etc. I never realized that at one person thought that a characteristic of a planet was that it was self-illuminating. Mars, Jupiter, and Saturn did not show phases, but now Venus showed phases, so it went from a planet to a moon!

  2. > I never realized that at one person thought
    >that a characteristic of a planet was that
    >it was self-illuminating.

    That’s right — keep in mind that the word “planet” comes from the Greek for “wanderer”. The planets were originally viewed as being, not worlds like earth, but “stars” that “wandered” (as opposed to fixed stars that remained in place in constellations) and thus might be self-illuminating (I need to throw in the caveat that there was debate over whether anything was self-illuminating, for some supposed that everything in the sky was illuminated by the sun). So the discovery of Venus’s phases showed that a planet like Venus behaved like the moon — Venus was a “moon” of sorts in Locher’s eyes.

  3. Pingback: Whewell’s Gazette: Year 3, Vol. #43 | Whewell's Ghost

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