Galileo's Daughter

date Oct 29, 2014
authors Dava Sobel
reading time 16 mins

Galileo and Bruno

same year the Dominican friar Giordano Bruno was burned at the stake in Rome for insisting, among his many heresies and blasphemies, that the Earth traveled around the Sun, instead of remaining motionless at the center of the universe. In a world that did not yet know its place, Galileo would engage this same cosmic conflict with the Church, treading a dangerous path between the Heaven he revered as a good Catholic and the heavens he revealed through his telescope.

Galileo’s daughter

Virginia adopted the name Maria Celeste when she became a nun, in a gesture that acknowledged her father’s fascination with the stars.

Through the telescope

In 1609, when Suor Maria Celeste was still a child in Padua, Galileo had set a telescope in the garden behind his house and turned it skyward. Never-before-seen stars leaped out of the darkness to enhance familiar constellations; the nebulous Milky Way resolved into a swath of densely packed stars; mountains and valleys pockmarked the storied perfection of the Moon; and a retinue of four attendant bodies traveled regularly around Jupiter like a planetary system in miniature.


“I render infinite thanks to God,” Galileo intoned after those nights of wonder, “for being so kind as to make me alone the first observer of marvels kept hidden in obscurity for all previous centuries.”

Venus and Sun

He saw dark spots creeping continuously across the face of the Sun, and “the mother of loves,” as he called the planet Venus, cycling through phases from full to crescent, just as the Moon did.

From Earth to the Skies

Indeed, we should accept misfortune not only in thanks, but in infinite gratitude to Providence, which by such means detaches us from an excessive love for Earthly things and elevates our minds to the celestial and divine.”

Mathematics and Geometry

“But the book cannot be understood unless one first learns to comprehend the language and to read the alphabet in which it is composed. It is written in the language of mathematics, and its characters are triangles, circles, and other geometric figures, without which it is humanly impossible to understand a single word of it; without these, one wanders about in a dark labyrinth.”

Dressing pretentious

Galileo deemed official doctoral dress a pretentious nuisance, and he derided the toga in a three-hundred-line verse spoof that enjoyed wide readership in that college town. Any kind of clothing got in the way of men’s and women’s frank appraisals of each other’s attributes, he argued in ribald rhyme, while professional uniforms hid the true merits of character under a cloak of social standing.

Experimental proof

Many philosophers of the sixteenth century, unaccustomed to experimental proof, much preferred the wisdom of Aristotle to the antics of Galileo, which made him an unpopular figure at Pisa.

False prophecies - hindsight is 20/20?

Galileo had prepared many horoscopes, including one for his daughter Virginia at her birth in 1600, probably for the novelty of playing with astronomical positions, as he never expressed any faith in astrological predictions. In fact he remarked how the prophecies of astrologers could most clearly be seen after their fulfillment.

Making telescopes

Still Galileo continued to refine the optical design in subsequent attempts, and when autumn came with its early dark, he chanced to focus one of his telescopes on the face of the Moon. The jagged features that greeted him by surprise there spurred him to improve his skill at lens grinding to build even more powerful models to revolutionize the study of astronomy by probing the actual structure of the heavens, and to disprove Aristotle’s long unquestioned depiction of all celestial bodies as immutable perfect spheres.

Precise observations

All through that same autumn of 1610, with Venus visible in the evening sky, Galileo studied the planet’s changing size and shape. He kept a telescope trained on Jupiter, too, in a protracted struggle to ascertain the precise orbital periods of the four new satellites to further validate their reality.

Repeatable experiments by anyone are the basis of science

Galileo scrambled to build as many telescopes as he could for export to France, Spain, England, Poland, Austria, as well as for princes all around Italy. “In order to maintain and increase the renown of these discoveries,” he reasoned, “it appears to me necessary to have the truth seen and recognized, by means of the effect itself, by as many people as possible.”

Easy to dispel doubt

To dispel any possible doubt about his instrument’s veracity, Galileo also aimed the telescope point-blank at the exterior wall of the Lateran Church, where a chiseled inscription attributed to Pope Sixtus V could be easily read by all, though it stood over a mile away.

Nature is to be understood

Now I want them to see that just as Nature has given to them, as well as to philosophers, eyes with which to see her works, so she has also given them brains capable of penetrating and understanding them.”

The immobile universe

The cosmology of the sixteenth and seventeenth centuries, founded on the fourth-century-B.C. teachings of Aristotle and refined by the second-century Greek astronomer Claudius Ptolemy, made Earth the immobile hub. Around it, the Sun, the Moon, the five planets, and all the stars spun eternally, carried in perfectly circular paths by the motions of nested crystalline celestial spheres.

Motions of the heavens

Copernicus rationalized the motions of the heavens. He saved the enormous Sun the trouble of traipsing all the way around the smaller Earth from morning till evening. Likewise the vast distant realm of the stars could now lie still, instead of having to wheel overhead even more rapidly than the Sun every single day.

A stable solitude non-stressful day job that provides for innovation - Einstein, Matz, Ford and now Copernicus

his uncle, a bishop, helped secure Copernicus a lifetime appointment as a canon at the cathedral of Frombork. Serving forty years in that “most remote corner of the Earth,” with manageable duties and a comfortable pension, Copernicus created an alternate universe.


In a 1597 letter he wrote to a former colleague at Pisa, Galileo assessed the system of Copernicus as “much more probable than that other view of Aristotle and Ptolemy.” He expressed the same faith in Copernicus in a letter he wrote to Kepler later that year, regretting how “our teacher Copernicus, who though he will be of immortal fame to some, is yet by an infinite number (for such is the multitude of fools) laughed at and rejected.” Since the Copernican system remained just as absurd to popular opinion fifty years following its author’s demise, Galileo long maintained his public silence on the subject.

Challenge of the immutability

In 1604, five years prior to Galileo’s development of the telescope, the world beheld a never-before-seen star in the heavens. It was called “nova” for its newness.* It flared up near the constellation Sagittarius in October and stayed so prominent through November that Galileo had time to deliver three public lectures about the newcomer before it faded from bright view. The nova challenged the law of immutability in the heavens, a cherished tenet of the Aristotelian world order.

Disproved with observation

The roughness of the Moon, for example, showed that some of the features of Earth repeated themselves in the heavens. The motions of the Medicean stars demonstrated that satellites could orbit bodies other than the Earth. The phases of Venus argued that at least one planet must travel around the Sun. And the dark spots discovered on the Sun sullied the perfection of yet another heavenly sphere. “In that part of the sky which deserves to be considered the most pure and serene of all I mean in the very face of the sun,” Galileo reported, “these innumerable multitudes of dense, obscure, and foggy materials are discovered to be produced and dissolved continually in brief periods.”

Sun spots

Apelles upheld the idea that the dark spots must be many small stars circling the Sun. Galileo saw nothing starlike about them. To his mind, they more closely resembled clouds: “Sunspots are generated and decay in longer and shorter periods; some condense and others greatly expand from day to day; they change their shapes, and some of these are most irregular; here their obscurity is greater and there less. They must be simply enormous in bulk, being either on the Sun or very close to it.

We don’t know yet

But he quickly added: “I do not assert on this account that the spots are clouds of the same material as ours, or aqueous vapors raised from the Earth and attracted by the Sun. I merely say that we have no knowledge of anything that more closely resembles them. Let them be vapors or exhalations then, or clouds, or fumes sent out from the Sun’s globe or attracted there from other places; I do not decide on this and they may be any of a thousand other things not perceived by us.”

Sun’s rotation

As Copernicus viewed the sky, however, the passage of day to night resulted from the turning of the Earth. Galileo agreed with Copernicus that the Earth somehow drew this motion from the Sun. Galileo had further observed the Sun to have its own monthly rotation, which he discovered during his studies of sunspots.

Our own discovery

“I believe that the intention of Holy Writ was to persuade men of the truths necessary for salvation,” Galileo continued his letter to Castelli, “such as neither science nor any other means could render credible, but only the voice of the Holy Spirit. But I do not think it necessary to believe that the same God who gave us our senses, our speech, our intellect, would have put aside the use of these, to teach us instead such things as with their help we could find out for ourselves, particularly in the case of these sciences of which there is not the smallest mention in the Scriptures; and, above all, in astronomy, of which so little notice is taken that the names of none of the planets are mentioned.

Arts and Sciences

They seemed to forget that the increase of known truths stimulates the investigation, establishment, and growth of the arts; not their diminution or destruction.

Tides and questions

December 1615 thus brought Galileo to Rome brandishing new support for Copernicus derived from observations of the Earth, not the heavens. The tidal motions of the great oceans, Galileo believed, bore constant witness that the planet really did spin through space. If the Earth stood still, then what could make its waters rush to and fro, rising and falling at regular intervals along the coasts?


Galileo needed the evidence of the tides to support Copernicus because his astronomical findings to date had failed to prove the Earth’s motion. It was all very well to argue, as Galileo did, that a rotating, revolving Earth made for a more rational universe that asking the innumerable, enormous stars to fly daily around the Earth at fantastic speeds was like climbing to a cupola to view the countryside and then expecting the landscape to revolve around one’s head.

Observations contrary to the Bible

The pope summoned his theological adviser, Roberto Cardinal Bellarmino, the preeminent Jesuit intellectual who had served as inquisitor in the trial of Giordano Bruno. Cardinal Bellarmino, the “hammer of the heretics,” had once confided to Prince Cesi of the Lyncean Academy that he personally considered the opinion of Copernicus heretical, and the motion of the Earth contrary to the Bible.


To wit: As earnestly as men may seek to understand the workings of the universe, they must remember that God is not hampered by their limited logic that all observed effects may have been wrought by Him in any one of an infinite number of omnipotent ways, and these must ever evade mortal comprehension.

Scarcity –> Preciousness

What greater stupidity can be imagined than that of calling jewels, silver, and gold “precious,” and earth and soil “base”? People who do this ought to remember that if there were as great a scarcity of soil as of jewels or precious metals, there would not be a prince who would not spend a bushel of diamonds and rubies and a cartload of gold just to have enough earth to plant a jasmine in a little pot, or to sow an orange seed and watch it sprout, grow, and produce its handsome leaves, its fragrant flowers, and fine fruit. It is scarcity and plenty that make the vulgar take things to be precious or worthless; they call a diamond very beautiful because it is like pure water, and then would not exchange one for ten barrels of water.

We are not the center

And if not Galileo, then who would step forward to correct humanity’s self-centered view of the cosmos? Who better than Galileo to propound the most stunning reversal in perception ever to have jarred intelligent thought: We are not the center of the universe. The immobility of our world is an illusion. We spin. We speed through space. We circle the Sun. We live on a wandering star.


When you have observed all these things carefully (though there is no doubt that when the ship is standing still everything must happen in this way), have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still.

Gravitational force?

Remarkably, Galileo conceded through this early demonstration in relativity, no experiment performed with ordinary objects on the surface of the Earth could prove whether or not the world was actually turning. Only astronomical evidence and reasoning from simplicity could carry the argument. Thus the daily rotation of the Earth streamlined the hubbub of the universe and recognized the cosmic balance of power. For if the heavens really revolved with enough force to propel the vast bodies of the innumerable stars, how could the puny Earth resist the tide of all that turning?

Stars too far to exert any force?

Copernicus had pushed the stars away to unimaginable distances to explain their constancy in contrast to the planets. The reason the stars never seemed to rock this way or that as the Earth traveled all around the Sun over the course of the year, Copernicus explained, was that they lay too far away for any shift in position, or parallax, to be perceived.


“Besides,” Sagredo sputters at last in frustration with those who would limit the majesty of the universe, “what does it mean to say that the space between Saturn and the fixed stars, which these men call too vast and useless, is empty of world bodies? That we do not see them, perhaps? Then did the four satellites of Jupiter and the companions of Saturn come into the heavens when we began seeing them, and not before? Were there not innumerable other fixed stars before men began to see them? The nebulae were once only little white patches; have we with our telescopes made them become clusters of many bright and beautiful stars? Oh, the presumptuous, rash ignorance of mankind!”

Microbes and diastases

The causal connection, however, remained elusive. People continued to blame the plague on miasmas of swampy air, the full Moon, conjunctions of the planets, famine, fate, beggars, prostitutes, or Jews. Two hundred years before the germ theory of disease, no one realized that the plague was caused by microbes living in and on the ubiquitous black rats.

Galileo called for inquiry

Before the end of September, an official order reached the inquisitor at Florence, announcing that the Dialogue could no longer be sold (though it was already sold out) and demanding that the author appear before the Holy Office of the Inquisition during the month of October.

The struggle

There was only one trial of Galileo, and yet it seems there were a thousand the suppression of science by religion, the defense of individualism against authority, the clash between revolutionary and establishment, the challenge of radical new discoveries to ancient beliefs, the struggle against intolerance for freedom of thought and freedom of speech. No other process in the annals of canon or common law has ricocheted through history with more meanings, more consequences, more conjecture, more regrets.

Different approach

Galileo proposed to strike out on a different course to drop all Aristotelian talk of why things moved, and focus instead on the how, through painstaking observations and measurements. In this fashion, he had discovered and described phenomena that generations of earlier philosophers had not even noticed.

Discovery of breaking motion into parts

Galileo displays his singular insight in breaking motions into their separate components. For he shows that any cannonball fired from a mortar, for example, or any arrow shot from a bow, combines two vectors: the uniform forward thrust of the propulsion and the downward acceleration of free fall.

Quantity vs essences

While Aristotelian philosophers talked of essences and natural places, Galileo went after quantifiable entities such as time, distance, and acceleration.

Assistant to Galileo

Vincenzio Viviani, a Florentine youth of sixteen years with a remarkable aptitude for mathematics. His scholastic distinction had brought the boy to the attention of Grand Duke Ferdinando, who in turn commended him to Galileo as an assistant.

Pleasure of discoveries — reminds me to Feynman centuries later

indispositions and a decrepit age of 75 years, will not permit me to occupy myself in study. I shall therefore remain silent, and so pass what remains to me of my laborious life, satisfying myself in the pleasure I shall feel from the discoveries of other pilgrim minds.”