Geocentrism holds that Earth sits still at the center of the universe while the Sun, Moon, planets, and stars all orbit around it. Claudius Ptolemy's Almagest, completed around 150 CE, gave the model its most complete mathematical form, predicting planetary positions accurately enough that it dominated Western and Islamic astronomy for over 1,400 years. It began to give way after Copernicus published a heliocentric alternative in 1543, and lost the argument decisively once Galileo's telescope showed the phases of Venus in 1610 and nineteenth century measurements of stellar parallax directly confirmed that Earth moves. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/
Facts
Proposed YearSourced to the subject's own accountDated to Ptolemy's Almagest, the model's most complete and influential statement; geocentric ideas in Greek astronomy are centuries older still, so this year marks the model's definitive form, not its first appearance. Proposed BySourced to the subject's own accountClaudius Ptolemy, formalizing centuries of earlier Greek astronomy 1 FAQs
How did geocentrism explain planets that seem to move backward in the sky?
Planets moved on small circles called epicycles whose centres circled Earth, so their combined motion sometimes looked like a reversal.
Ptolemy's geocentric model, formulated around 150 CE, explained it with epicycles. Each planet was said to travel uniformly along a small circle called an epicycle, whose centre in turn moved around Earth along a larger circle called a deferent. Because one half of an epicycle runs counter to the general motion of the deferent, the combined motion sometimes appears to slow down or even reverse direction, which is the retrograde motion astronomers see from Earth. Ptolemy also enhanced the effect of eccentricity by having the epicycle's centre sweep out equal angles in equal times as seen from an off-centre point he called the equant.
Connections
Sources
1. Geocentrism (Wikipedia)
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