Empirical Earth · How We Know — History & the Cosmos

The astrolabe and the Prague clock — spherical astronomy you can hold

The claim

“Old astrolabes and astronomical clocks are geocentric, so the old view was a flat, central Earth.”

What is measured

The astrolabe turns spherical astronomy into a handheld brass computer (one that needs a different plate for every latitude, which only a round Earth requires), and the Prague clock has displayed that model in a public square since 1410. Geocentric in appearance, spherical in fact.

What would show this is wrong

a single astrolabe plate that reads correctly at every latitude (none does, which is why instruments carry several), or latitude fixed from a star’s altitude on a flat plane (the geometry does not close). Both are the opposite of how the instrument works.

Sources

  1. The astrolabe: stereographic projection, latitude plates & uses. Built on stereographic projection (Hipparchus, ~150 BC), the astrolabe carries a plate cut for each specific latitude and was link
  2. The Prague astronomical clock: an astrolabe dial since 1410. The Orloj’s astronomical dial is itself an astrolabe, using stereographic projection from the North Pole to show the Sun’s and Mo link
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