The Four Galilean Moons of Jupiter

Firing off a string of snapshots like a sports photographer at a NASCAR race, NASA’s Hubble Space Telescope captured a rare look at three of Jupiter’s largest moons parading across the banded face of the gas-giant planet: Europa, Callisto, and Io.

While watching the four Galilean moons of Jupiter buzzing around the planet recently, I was reminded of the historical role they have played in the advancement of science over the last 400 years.  Starting with their discovery by Galileo in 1610, their existence helped prove that the sun was the center of the solar system not the earth.  In addition, on December 27, 1612, as Galileo was sketching the positions of the four moons in his journal, he included a “fixed star” nearby which happened to be the planet Neptune.  That was 234 years before its official discovery in 1846!   This, of course, gave credence to the theory that the solar system is planar as all planets follow the ecliptic in their dance through the heavens.

Moons of solar system scaled to Earth’s Moon by NASA

In the late seventeenth century, Ole Roemer was working with Jean-Dominique Cassini at the Paris Observatory producing prediction tables for the occultations, eclipses and transits of the four Galilean moons around Jupiter.  Roemer noticed that their predictions were all off by approximately 22 minutes after a six-month time period and then the predictions would correct themselves after a year had transpired.  Roemer made an astounding leap of insight. He concluded that the time delay was caused by the light from the events being recorded having to travel the extra distance of the earth’s orbit around the sun as the earth went from being closer to Jupiter at opposition to much farther away at conjunction.  He calculated the speed of light to be 133,000 miles per second, which wasn’t that far off, considering that it was the first calculation of the speed of light ever made.

The prediction tables produced by Roemer and Cassini were designed to be used by surveyors to map the earth.  Since determining longitude required knowledge of the correct time at some reference location (Paris in this case), surveyors anywhere in the world could set their clocks using the prediction tables while sighting the moons of Jupiter through a portable telescope.  This was the only technique possible to determine longitude accurately before the invention of accurate chronometers which did not occur until the middle of the eighteenth century.  Unfortunately, this technique did not work at sea because the movement of the ship made it impossible to focus a telescope on Jupiter.

This “family portrait,” a composite of the Jovian system. By NASA

At the same time that Roemer and Cassini were working in Paris, Isaac Newton was developing his famous formula for the force of attraction between two bodies (universal gravitation).  Since gravity is the invisible string that pulls a body into a circular path, he was then able to equate centripetal force with gravity for satellites (moons, etc.) traveling around a central object.  When the constant in his formula was determined experimentally by Henry Cavendish in 1798, scientists were able to calculate the mass of Jupiter using any of the four Galilean moons as a reference.

So, the next time you gaze at Jupiter in a telescope and see the four moons in their dance around the planet, think of all the scientific knowledge gained by those distant bodies that Galileo discovered four hundred years ago.




Physics for Scientists and Engineers, Ninth Edition, 2014, Serway and Jewett

Coming of Age in the Milky Way, Timothy Ferris, 1988

Astronomy Today, Seventh Edition, Chaisson and McMillan, 2011

The Ring of Truth, Philip and Phylis Morrison, 1987