Empirical Earth · How We Know — History & the Cosmos

The phases of Venus — what killed pure geocentrism

The claim

“Nobody can actually prove the planets orbit the Sun rather than the Earth.”

What is measured

Venus shows a full set of phases and changes size to match, impossible if it orbits Earth, inevitable if it circles the Sun inside our orbit. Galileo’s 1610 observation killed pure geocentrism. Parallax and aberration later sealed the moving Earth. Anyone with a small scope can repeat it.

What would show this is wrong

observing that Venus only ever shows crescent/new phases (never gibbous or full), or that its apparent size does not vary inversely with phase, either of which would match an Earth-centered orbit.

Sources

  1. Phases of Venus. Venus shows a full set of phases and an inversely varying apparent size as it orbits the Sun inside Earth’s orbit — a sequence impossible in the Ptolemaic geocentric model. link
  2. Galileo’s decisive observation (1610). Galileo’s telescopic crescent-to-full phases of Venus ruled out Ptolemy; the result remained compatible with the Tychonic system until stellar parallax link
  3. Greatest elongation of Venus. As an inner planet, Venus never appears more than about 47° from the Sun in the sky; its greatest elongation ranges from roughly 45.4° to 47.3°. It is therefore link
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