Celestial mechanics. Coloured historical artwork depicting a medieval pilgrim looking out from the sky (blue) to see the mechanics of the heavens (upper left). Cogs (yellow) can be seen in the left corner, with tracks that the Sun, moon and stars move along. This is an example of the classical geocentric (Ptolemaic) worldview that dates back to Ancient Greek times. It was replaced by the heliocentric (Copernican) model, proposed by Nicolaus Copernicus in 1543. This woodcut is from The Atmosphere by Camille Flammarion, published in 1873.