heavens are perfectly spherical, and the upper regions are more divine than the lower. The stars
and planets are not composed of fire, but of the fifth element; their motion is due to that of spheres
to which they are attached. (All this appears in poetical form in Dante Paradiso.)
The four terrestrial elements are not eternal, but are generated out of each other--fire is absolutely
light, in the sense that its natural motion is upward; earth is absolutely heavy. Air is relatively
light, and water is relatively heavy.
This theory provided many difficulties for later ages. Comets, which were recognized as
destructible, had to be assigned to the sublunary sphere, but in the seventeenth century it was
found that they describe orbits round the sun, and are very seldom as near as the moon. Since the
natural motion of terrestrial bodies is rectilinear, it was held that a projectile fired horizontally will
move horizontally for a time, and then suddenly begin to fall vertically. Galileo's discovery that a
projectile moves in a parabola shocked his Aristotelian colleagues. Copernicus, Kepler, and
Galileo had to combat Aristotle as well as the Bible in establishing the view that the earth is not
the centre of the universe, but rotates once a day and goes round the sun once a year.
To come to a more general matter: Aristotelian physics is incompatible with Newton "First Law
of Motion," originally enunciated by Galileo. This law states that every body, left to itself, will, if
already in motion, continue to move in a straight line with uniform velocity. Thus outside causes
are required, not to account for motion, but to account for change of motion, either in velocity or
in direction. Circular motion, which Aristotle thought "natural" for the heavenly bodies, involves a
continual change in the direction of motion, and therefore requires a force directed towards the
centre of the circle, as in Newton's law of gravitation.
Finally: The view that the heavenly bodies are eternal and incorruptible has had to be abandoned.
The sun and stars have long lives, but do not live for ever. They are born from a nebula, and in the
end they either explode or die of cold. Nothing in the visible world is exempt from change and
decay; the Aristotelian belief to the contrary, though accepted by medieval Christians, is a product
of the pagan worship of sun and moon and planets.