Giambattista Riccioli, Almagestum novum (Bologna, 1651; !New Almagest")

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Phases of VenusRiccioli, New Almagest

If Venus shows phases, does this prove that it revolves around the Sun?

Galileo!s I l Saggiatore (1623) contains Galileo!s earliest publishedillustration of the phases of Venus (left) which he had discoveredthrough his telescope. When Galileo looked at Venus throughthe telescope, he saw a full set of phases.

When Riccioli and others accepted Galileo's discovery of the phases ofVenus, did that mean that they were logicallycompelled to accept theCopernican system, in whichall the planets circle the Sun?

The frontispiece of Riccioli!s NewAlmagest depicts not two, but threemajor systems of the world.

1. The Ptolemaic system restsdiscarded (lower right corner)because of the phases of Venus andMercury (upper left corner). All"seeing Argus looks on, holding atelescope. Urania weighs in abalance the two chief worldsystems which remain:

2. The Copernican system appearsas the standard against whichalternatives must be measured.

3. Riccioli!s semi"Tychonic systemweighs in as the most warranted.

A comet and several telescopicdiscoveries (upper right corner)include the banded appearance ofJupiter and the ring of Saturn.

In the upper left corner (shownhere), Riccioli portrays Venus ♀and Mercury ☿ as they appearwhen on opposite sides of the Sun.The sides facing the Sun areilluminated. The sides turnedaway from the Sun are dark.

Do the phases of Venus prove thatVenus revolves around the Suninstead of around the Earth?

lynx-open-ed.org@lynx-open-ed

@ouhoscollection#galileosworldKerry Magruder and

Brent PurkapleGalileo, phases of Venus (1623)