Essays, Science.

The moons of Jupiter

The fascination of astronomy for me, beyond the beauty of the heavens in the night sky, beyond the immense imponderables of the current model of a vast and ancient universe, is the idea that our understanding regarding the universe has been gleaned by observing from the Earth a few points of light in the sky. One illustration of this can be found in the history of our knowledge of the moons of Jupiter.

Astronomy before Copernicus

The early history of many civilizations reveal a record of careful observation of the heavens. The movement of the stars and the planets, of our Moon and Sun, were determined to be regular, and aligned with certain periods of time: the year, the seasons, the months and the days. The heavens at night provided signals for when to plant and harvest, for example. The recognition of heavenly timekeeping, so to speak, can be described as a vital piece of knowledge, a technology that supported the agricultural economy of the earliest civilizations.


Andreas Cellarius, Harmonia macrocosmica, 1661, plate 3 - orbium planetarum. PD-US. Click to view enlarged picture

The stars could only be viewed with a naked eye, and at such resolution, the stars relationship to each other, the distance between one star and another, did not vary. There was no observed parallax, or apparent shift of position of any one star against the background of other stars. As a consequence, Aristotle’s explanatory physical model of the heavens, a cosmology, described all of the stars as being the final background itself, very far away and equidistant from the earth, embedded in a “crystalline” shell or sphere, which rotated daily around the earth, producing the observed movement of the stars.  The planets, on the other hand, appeared to wander through the background of the stars relatively independent of each other, as did the Sun and Moon.  

Since there was little change observed over thousands of years in the heavens beyond the earth, the moon and beyond were considered immutable, the occasional supernova or comet aside, and because they could not be directly sampled, were considered to be incorruptible. Aristotle’s model had heavenly bodies moving in circles, and made of a unblemished substance.  The planets, Sun and Moon were accorded their own crystalline spheres in which they were embedded.  The universe was spherical,  and each of these shells were concentrically situated around the spherical earth, like a Russian babushka doll, one inside the other, rotating at different rates relative to each other. The Moon was embedded in the first sphere around the Earth, Mercury embedded in the sphere around the Moon, the next sphere containing Venus, followed by a sphere containing the Sun. The rest of the known planets followed: Mars, Jupiter, and Saturn, with the final sphere those of the fixed stars. 

Ptolemy’s astronomical synthesis a few hundred years later provided a geometrical model that could predict the motions of the Sun, Moon, and the known planets close to the limits of observations by the naked eye. It was a complex model, not in complete accord with Aristotle’s cosmology, and involved the artificial use of circles within circles to describe planetary retrograde motion, which is the apparent slowing and reversing of motion of the planets at different times in their nighttime movement through the background of fixed stars.

Copernicus creates a new model of the solar system


Copernicus, 1580. PD-US.

Change was nonetheless afoot.  Tyco Brahe refined his technique and instrumentation to improve the accuracy of naked eye observation, and in the course of his life of observation, made careful note of supernovas and comets, bringing more attention to change observed in the heavens, contrary to the Aristotelian idea of immutability. Kepler, using Brahe’s more accurate observations, divined a much more accurate astronomical model for planetary motion, adopting Copernicus’ heliocentric model, and adopted the use of ellipses rather than circles to both greatly simplify the model of planetary motion, and to significantly improve the accurate prediction of the motions of the planets.

Galileo points a telescope to the heavens

But it was Galileo, the first great mathematical scientist, an astute observer and a careful experimentalist, who thrust astronomy most firmly in its modern direction.  He began by building himself a telescope, recently invented by Dutch lensmakers, and directing it to the heavens.  What he saw was world-changing.  He saw much more evidence of change and “imperfection” in the heavens, and evidence of planetary behavior that could only be explained from a Copernican or heliocentric point of view.  

In 1610, Galileo trained his telescope on the surface of the Moon, a heavenly body considered without blemish, perfectly spherical, and he saw its more earth-like imperfections: He saw craters and mountains on the Moon!  When he looked closer at the Milky Way, he saw an enormous number of stars that had never been observed before.  When he observed the surface of the Sun, he observed dark imperfections (sunspots), which rotated to different positions over time, indicating also that the Sun rotated on its axis, something that the Copernican system also required the Earth to do.  

-UT Knoxville,

Expected phases of Venus, Copernicus v. Ptolemy . UT Knoxville.

Observing Venus, he noted over time that Venus showed a complete set of phases similar to the Moon, which he noted could only happen if Venus revolved around the Sun.  If the Sun was always farther away from the Earth and Venus, as the Ptolemaic system postulated, Venus would almost always be observed in a crescent phase.  This was the first empirical evidence that conclusively demonstrated that the Ptolemaic system was incorrect and the heliocentric theory correct. 

The moons of Jupiter


Galileo's observations of the Medician stars aka Moons of Jupiter, adapted from Sidereus Nuncius. Attrib: NASA, PD-USGOV.

Observing Jupiter, he saw four little points of light in close proximity to the planet, each day in a different position, sometimes seeing only three or two.  He surmised that these were moons revolving around Jupiter, sometimes obscured by going behind or across the front of the planet.  Galileo recorded the positions of the moons carefully, and found that the motions of these new satellites were highly predictable, as if they were moving steadily around Jupiter. The moons of Jupiter added additional evidence of other heavenly bodies inside of the celestial sphere, and indirectly supported Copernicus’s contention that the Moon was a satellite orbiting about the Earth, further contradicting the Ptolemaic system.  Galileo recognized that beyond the physics and cosmology, there were very practical possibilities in these moons.

Unlocking longitude

Ancient geographical maps have a common trait to the modern reader:  They all look misshapen, with lands and seas in correct positions relative to each other, but both land and seas are often grossly distorted from their actual size and shape on the surface of the Earth.  The distortion is most pronounced in the east-west direction, because the determination of longitude, which represents the east-west geographical coordinate of a position, was an insoluble conondrum during ancient times, up to Galileo’s age.  From ancient times, various methods were used to determine latitude by celestial navigation.  As maps came into more general use, these methods were employed to mark latitude, but longitude measurement required an accurate means to tell time, which was not readily available.


1594 map of the world. PD-US.

Book Book Review, Title Longitude, Author Dava Sobel, Rating 4.0,


Dava Sobel

Book Review

" Galileo was no sailor, but he knew of the longitude problem—as did every natural philosopher of his day. Over the next year he patiently observed the moons of Jupiter, calculating the orbital periods of these satellites, and counting the number of times the small bodies vanished behind the shadow of the giant in their midst. From the dance of his planetary moons, Galileo worked out a longitude solution. Eclipses of the moons of Jupiter, he claimed, occurred one thousand times annually—and so predictably that one could set a watch by them. He used his observations to create tables of each satellite’s expected disappearances and reappearances over the course of several months, and allowed himself dreams of glory, foreseeing the day when whole navies would float on his timetables of astronomical movements, known as ephemerides. . . . Galileo’s method for finding longitude at last became generally accepted after 1650—but only on land. Surveyors and cartographers used Galileo’s technique to redraw the world. And it was in the arena of mapmaking that the ability to determine longitude won its first great victory. Earlier maps had underestimated the distances to other continents and exaggerated the outlines of individual nations. Now global dimensions could be set, with authority, by the celestial spheres. Indeed, King Louis XIV of France, confronted with a revised map of his domain based on accurate longitude measurements, reportedly complained that he was losing more territory to his astronomers than to his enemies."(Longitude, by Dava Sobel, p. 24-27)  

Discovering the finite velocity of light

The speed of light was considered, prior to 1676, to be practically instantaneous.  But the Danish astronomer Ole Roemer, working to extend the accuracy of this method of measuring longitude, noticed that  "the eclipses of all four Jovian satellites would occur ahead of schedule when the Earth came closest to Jupiter in its orbit around the sun. Similarly, the eclipses fell behind the predicted schedules by several minutes when the Earth moved farthest from Jupiter. Roemer concluded, correctly, that the explanation lay in the velocity of light. The eclipses surely occurred with sidereal regularity, as astronomers claimed. But the time that those eclipses could be observed on Earth depended on the distance that the light from Jupiter’s moons had to travel across space. Until this realization, light was thought to get from place to place in a twinkling, with no finite velocity that could be measured by man. Roemer now recognized that earlier attempts to clock the speed of light had failed because the distances tested were too short. Galileo, for example, had tried in vain to time a light signal traveling from a lantern on one Italian hilltop to an observer on another. He never detected any difference in speed, no matter how far apart the hills he and his assistants climbed. But in Roemer’s present, albeit inadvertent, experiment, Earthbound astronomers were watching for the light of a moon to reemerge from the shadow of another world. Across these immense interplanetary distances, significant differences in the arrival times of light signals showed up. Roemer used the departures from predicted eclipse times to measure the speed of light for the first time in 1676. (He slightly underestimated the accepted modern value of 300,000 kilometers per second.) "(Longitude, by Dava Sobel, p. 29-30)  

This astronomical story of the moons of Jupiter was spun up from the simple strands of thin beams of light, which streamed from the heavens into the eyes of meticulous observers, for the first time through a slightly magnified tube, and added to a fabric of logical inferences to form a viable model of our solar system.  The current model into which it grew is real in the following sense: Immensely more powerful telescopes have since been trained on the Sun, the Moon and the planets, satellites have been dispatched to the planets for close observation, and humans have orbited and landed on the Moon, observing the spherical home planet of Earth from its surface, validating the details of a central Sun with planets moving around it in precise orbits, and moons circling these planets in similar fashion.

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