Francesco Maria Grimaldi

Swabia, Germany
1618-1663

Grimaldi was an Italian Jesuit priest, mathematician and physicist who taught at the Jesuit college in Bologna. Francesco Grimaldi was born into a well-off family. His father, Paride Grimaldi, was a silk merchant of noble birth. After the death of his first wife, he married Anna Cattani, who owned a chemist shop. Francesco was the fourth of his parents six sons, five of whom survived. Francesco and his brother Vincenzo both joined the Society of Jesus (the Jesuits) on 18 March 1632.

Grimaldi was taught by Giovanni Battista Riccioli. By 1640, he was assisting Riccioli with experiments. Grimaldi and Riccioli calibrated a pendulum by getting it to swing for 24 hours (measured by the star Arcturus crossing the meridian line). They used this 3 foot pendulum to calibrate a shorter pendulum to use in timing. Then Grimaldi dropped balls of wood and of lead from various heights from the Asinelli tower. Accuracy was obtained by getting a group of musical monks to chant in time with the swinging pendulum. The experiment did not confirm Galileo’s result for the lead ball always hit the ground before the wooden one when they fell from the same height. The discrepancy between the experiment and Galileo’s claim that they reached the bottom simultaneously was so great, Grimaldi was forced to conclude Galileo knew, but hid the knowledge, since it contradicted Galileo’s own thesis.

In astronomy, Grimaldi built and used instruments to measure the height of clouds  as well as lunar mountains. He drew an accurate moon map, or selenograph, which was published by Riccioli and now adorns the entrance to the National Air and Space Museum in Washington D.C. 

He also experimented with light. Grimaldi admitted the sun’s light into a dark room through a small hole. He noticed the breadths of the shadows of slender objects, as needles and hairs, on a screen, were much greater than they would have been if the rays of light had passed by them in straight lines. Also, the circle of light formed on a screen by the rays passing through a very small hole in a plate of lead was greater than it would be if the rays simply diverged. He concluded the light rays changed direction as they passed near the edges of objects. He found that the shadow of a small body was surrounded by three coloured streaks or bands which became narrower as they receded from the centre of the shadow. Where the light was strong, he saw similar coloured bands within the shadow: there appeared to be two or more of these, the number increasing in proportion as the shadow was farther from the body. He coined the word ‘diffraction’ to describe the effect, thereby becoming the first man to describe “diffraction bands”. Later physicists used his work as evidence that light was a wave. Isaac Newton acknolwedged his debt to Grimaldi, saying that his first knowledge of light’s refraction came from Grimaldi’s work. Newton would use Grimaldi’s foundational work to arrive at his own, more comprehensive, theory of light. 

When two holes were used, Grimaldi received the cones of light on a screen beyond the place where they overlapped each other. He noticed where both rays fell,, the screen was more strongly enlightened that it would have been by one cone of light; but he was surprised to find the penumbral portions which overlaid one another were darker than the corresponding portions in which there was no overlay. He therefore proposed ‘A body actually enlightened may become obscure by adding new light to that which it has already received.’ A crater on the moon is named for him.