Borelli , Giovanni Alfonso

(1608–1679) Italian mathematician and physiologist

Borelli was born in Naples. His mathematical training – he was professor at Messina and Pisa – led him to apply mathematical and mechanical laws to his two main interests, astronomy and animal physiology. He rightly explained muscular action and the movements of bones in terms of levers, and also carried out detailed studies of the flight mechanism of birds. However, his extension of such principles to internal organs, such as the heart, stomach, and lungs, overlooked the essential chemical actions that take place in these organs. Borelli's De motu animalium (1680; On the Movement of Animals), which includes his theory of blood circulation, is thus in part erroneous.

In astronomy, in his Theoricae mediceorum planetarum (Theory of the Medicean Planets; 1666), Borelli presented a novel and influential account of the motions of the Medicean satellites around their parent plant Jupiter. He accounted for their elliptical orbits in terms of two distinct motions. The first, ‘perpetual and uniform’, whereby the satellite is attracted rectilinearly to Jupiter as iron moves in a straight line to a magnet; the second, and “directly contrary…continually decreasing,” arises from the manner in which the satellite “is driven out from the sun by the force of its circular motion.” Newton was aware of Borelli's work and appreciated the originality of his approach, in the use of elliptical orbits and also his appreciation that orbital motion was complex.

Borelli was also one of the first astronomers, in his Del movimento della cometa di Decembre 1664 (1665), to propose, on the basis of observations and calculations, that comets also move in elliptical orbits. Earlier workers, including Kepler, had taken comets to be transient visitors passing through the solar system in a straight line. As the church opposed such views, Borelli chose to publish under the pseudonym Pier Maria Mutoli.