Galileo Galilei Planetary Mystery Finally Solved

For many, Galileo Galilei is considered one of the most influential contributors to the scientific revolution of the 17th century. Born in Pisa in 1564, Galilei briefly studied medicine at the University of Pisa but left without completing his degree. He moved on to study mathematics and, by the time he died in 1642, his contributions made him one of the most well-known figures in all of Europe. Galilei’s works include using the telescope to examine the skies, seeing the mountains of the Moon and the moons of Jupiter, helping lay the foundation for mathematical physics, and advocating the Copernican system. Many historians believe Galilei to be one of the first true experimental scientists. However, there was one thing Galilei simply could not explain: Why did Venus appear larger than Jupiter to the naked eye but smaller when viewed through the telescope? Four hundred years later, scientists have finally been able to solve the ancient planetary mystery that stumped Galileo Galilei.

When Galilei began viewing bright planets through the telescope, he noticed many of them looked much different from when viewed with the naked eye. When viewed directly, the planet Venus seemed to have a fuzzy halo, or “radiant crown,” that made it appear eight to ten times larger than Jupiter when, in fact, Jupiter was about four times larger than Venus. Initially stumped by this apparent visual trickery, Galilei eventually assumed the switch was a result of refraction, postulating that it was perhaps due to moisture in the eye.

Many years later, Hermann von Helmholtz took Galilei’s work a step further. The German scholar applied mathematics to gain a better understanding of how the human eye perceives changes in levels of brightness at the edge of a light object against a dark background. Although he remained mystified as to the physical cause of the imbalance, von Helmholtz eventually referred to the visual disparity as “irradiation illusion.” Today, this term describes the visual illusion that makes lighter areas of an image appear larger than darker areas.

The illusion that stumped both Galilei and von Helmholtz is finally starting to make sense to neuroscientists at the State University of New York College of Optometry. In a recent article published online in the Proceedings of the National Academy of Sciences, the authors stated that they now know why the eye makes light areas appear larger than dark areas. Their findings lead many to believe that one of the oldest planetary mysteries, first discovered by Galileo Galilei, may finally be solved.

According to the article, “light and dark stimuli are separately processed by ON and OFF channels in the retina and visual thalamus.” Although many textbooks believed these responses were relatively balanced, recent research has uncovered what the authors called “a pronounced over representation of OFF responses in the cerebral cortex.” In addition, the “off” neurons responded to changes in illumination in a linear fashion, whereas the “on” neurons responded differently at the edges of the illumination. In other words, when the human eye perceives and processes light, it tends to deceptively enlarge bright objects that are set against a dark background. This perception is what makes white circles against a black background appear larger than same-sized black circles against a white background.

The eye’s tricky tendency to enlarge the edges of bright objects helps explain why the illustrious Venus appeared much larger to Galilei than the dim-by-comparison Jupiter. Although neuroscientists have finally been able to solve the planetary mystery that stumped Galilei and his successors, many continue to be amazed at how close the ancient star-gazer came to understanding much of what is still being discovered today.

By Katie Bloomstrom

LA Times
Proceedings of the National Academy of Sciences of the United States of America
Stanford Encyclopedia of Philosophy (1)
Stanford Encyclopedia of Philosophy (2)
TIME Science and Space

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