Interdisciplinary Studies in Letters & Science

Chabot College

Galileo's Letters on Sunspots

Page 95 - 144
Scott Hildreth

Pages
Quote
Interpretation/Comments

95
"I come now to [Apelles'] third letter, in which he speaks more positively about the postion, motion, and substance of the sunspots, concluding that they are stars not far removed from the body of the sun, and that they revolve about it in the manner of Mercury and Venus.

"Passing across the visible hemisphere of the sun in fifteen days, he says, the same spots ought to return every month, which does not happen. This argument would be conclusive if we could first be sure that these spots were permanent..."

"Now I think it is a very difficult thing - even impossible - to prove that they are permanent.... I am not affirming or denying that the spots are located on the sun; I merely say that it is insufficiently proved that they are not."

Note that Galileo is careful to question the assumptions made about the nature of sunspots.

What is the nature of proof needed to say categorically that sunspots are on the sun?

98
"It now remains for us to consider the judgment of Apelles concerning the essence and substance of these spots, which in sum is that they are neither clouds nor comets, but stars that go circling about the sun. I confess to Your Excellency that I am not yet sufficiently certain to affirm any positive conclusion about their nature.

"The substance of the spots might even be any of a thousand things unknown and unimaginable to us...."

"Therefore I see nothing discreditable to any philosopher in confdssing that he does not know, and cannot know, what the material of the solar spots may be.... I find in them nothing at all which does not resemble our own clouds."

Galileo continues to reason by analogy, just as he did relating the surface of the moon to the Earth by looking at shadows.

Here, he discredits Apelles' conclusion that the sunpots were orbiting planets, and compares them to clouds in their form, size, variation, motion, and changes.

100 "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."
Although Galileo is quite a different personality than Darwin, there are distinct similarities in how they approach their subjects scientifically.

How did Darwin deal with doubt about the certainty of his theories?

102 "For the shape of Saturn is thus..."

103 "Indeed, I hope that this new thing will turn out to be of admirable service in tuning for me some reed in this great discordant organ of our philosophy - an instrument on which I think I see many organists wearing themselves out trying vainly to get the whole thing into perfect harmony." Galileo as philosopher-scientist. Compare him to Darwin in his writing and poetic use of language.

106 "From special characteristics of this motion one may learn that the sun is absolutely spherical, that it rotates from west to east around its own center, carries the spots along with it in parallel circles, and completes an entire revolution in about one lunar month. Galileo summarizes what he had deduced from this observations.

107-108
"First, to see twenty or thirty spots at a time move with one common movement is a strong reason for believing that each doesnot go wandering about by itself, in the manner of the plnaets going around the sun.

"To begin with, the spots at their first appearance and final disappearnance near the edges of the sun generally seem to have very little breadth, but to have the same length."

"In the second place, one must observe the apparent travel of the spots day by day. The spaces passed by the same spot in equal times become always less as the spot is situated nearer the edge of the sun."

"A third thing which strongly confirms this conclusion may be deduced from the spaces between one spot and another.... The events are such that they could be met with only in circular motion made by different points on a rotating globe."
Galileo deduces from the observations of the sunspots that they are on the surface of the sun, that the sun rotates as a sphere, and that they follow regular patterns of motion around the sun.

113
"For I seem to have observed that physical bodies have physical inclnation to some motion (as heavy bodies downward), which motion is exercised by them through an intrinsic property and without need of a particular external mover."

"And therefore, all external impediments removed, a heavy body on a spherical surface concentric with the earth will be indifferent to rest and to movements toward any part of the horizon. And it will maintain itself in that state in which it has once been placed; that is, if placed in a state of rest, it will conserve that; and if placed in movement toward the west (for example), it will maintain itself in that movement."
Galileo predates Newton with this statement about "inertia" - the property of matter to resist changes in motion. Newton generalizes this into his first of three famous laws of motion.

118 "Now in order that we may harvest some fruit from the unexpected marvels that have remained hidden until this age of ours, it will be well if in the future we once again lend ear to those wise philosophers whose opinion of the celestial substance differed from Aristotle's." Galileo suggests an important lesson for all scientists - to not trust too strongly to our beliefs in what others have told us - even if they are illustrious. The belief in Aristotle's science held sway for 1500 years, and although much he believed was correct, other things were totallly wrong. Yet no one challenged him, so great was his reputation.

119 "Yet in this respect also we must recognize divine Providence, in that the means to such knowledge are very easy and may be speedily apprehended." Galileo suggests that science - and observation - is really open to all of us. Good science demands a multiplicity of observers, independently verifying discoveries.

124 "But if what we wish to fix in our minds is the apprehension of some properties of things, then it seems to me that we need not despair of our ability to acquire this respecting distant bodies just as well as those close at hand - and perhaps in some cases even more precisely in the former than in the latter. Who does not understand the periods and movements of the planets better than those of the waters of our various oceans?" Galileo understands the difficulties present with astronomy, where we cannot visit nor do direct experimentation upon the objects of our study. But he offers some rather optimistic thoughts about how that distance may not necessarily preclude our developing an understanding of the nature of those objects.

128 "But whatever te course of our lives we should receive them as the highest gift from the hand of God, in which equally reposed the power to do nothing whatever for us. 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." Is Galileo religious? Many believed him not at all religious, but perhaps they haven't really read these words?

140-141
"I believe that there are not a few Peripatetics on this side of the Alps who go about philosophizing without any desire to learn the truth and the causes of things, for they eny these new discoveries or jest about them, saying that they are illusions. It is about time for us to jest right back at these men and say they likewise have become invisible and inaudible."

"Still, one may not conclude as these men do from such a generalization; one must come down to the particular things observed in stars and in spots."
Galileo was never one to hold his opinion on those who practiced poor science. He maintained that to be sure, one should make observations of the phenomena in question, and decide upon the actual eveidence.

143-144 "Hence I stopped observing Saturn for more than two years. But in the past few days I returned to it and found it to be solitary, without its customary supporting stars, as as perfectly round and sharply bounded as Jupiter."
Galileo never learned the reason for this observation - that Saturn and its rings are tilted relative to its orbital plane, just like Earth, and consequently it is sometimes seen "edge on" and sometimes seen tilted significantly so that its rings are even more obvious."

Pages	Quote	Interpretation/Comments
95	"I come now to [Apelles'] third letter, in which he speaks more positively about the postion, motion, and substance of the sunspots, concluding that they are stars not far removed from the body of the sun, and that they revolve about it in the manner of Mercury and Venus. "Passing across the visible hemisphere of the sun in fifteen days, he says, the same spots ought to return every month, which does not happen. This argument would be conclusive if we could first be sure that these spots were permanent..." "Now I think it is a very difficult thing - even impossible - to prove that they are permanent.... I am not affirming or denying that the spots are located on the sun; I merely say that it is insufficiently proved that they are not."	Note that Galileo is careful to question the assumptions made about the nature of sunspots. What is the nature of proof needed to say categorically that sunspots are on the sun?
98	"It now remains for us to consider the judgment of Apelles concerning the essence and substance of these spots, which in sum is that they are neither clouds nor comets, but stars that go circling about the sun. I confess to Your Excellency that I am not yet sufficiently certain to affirm any positive conclusion about their nature. "The substance of the spots might even be any of a thousand things unknown and unimaginable to us...." "Therefore I see nothing discreditable to any philosopher in confdssing that he does not know, and cannot know, what the material of the solar spots may be.... I find in them nothing at all which does not resemble our own clouds."	Galileo continues to reason by analogy, just as he did relating the surface of the moon to the Earth by looking at shadows. Here, he discredits Apelles' conclusion that the sunpots were orbiting planets, and compares them to clouds in their form, size, variation, motion, and changes.
100	"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."	Although Galileo is quite a different personality than Darwin, there are distinct similarities in how they approach their subjects scientifically. How did Darwin deal with doubt about the certainty of his theories?
102	"For the shape of Saturn is thus..."
103	"Indeed, I hope that this new thing will turn out to be of admirable service in tuning for me some reed in this great discordant organ of our philosophy - an instrument on which I think I see many organists wearing themselves out trying vainly to get the whole thing into perfect harmony."	Galileo as philosopher-scientist. Compare him to Darwin in his writing and poetic use of language.
106	"From special characteristics of this motion one may learn that the sun is absolutely spherical, that it rotates from west to east around its own center, carries the spots along with it in parallel circles, and completes an entire revolution in about one lunar month.	Galileo summarizes what he had deduced from this observations.
107-108	"First, to see twenty or thirty spots at a time move with one common movement is a strong reason for believing that each doesnot go wandering about by itself, in the manner of the plnaets going around the sun. "To begin with, the spots at their first appearance and final disappearnance near the edges of the sun generally seem to have very little breadth, but to have the same length." "In the second place, one must observe the apparent travel of the spots day by day. The spaces passed by the same spot in equal times become always less as the spot is situated nearer the edge of the sun." "A third thing which strongly confirms this conclusion may be deduced from the spaces between one spot and another.... The events are such that they could be met with only in circular motion made by different points on a rotating globe."	Galileo deduces from the observations of the sunspots that they are on the surface of the sun, that the sun rotates as a sphere, and that they follow regular patterns of motion around the sun.
113	"For I seem to have observed that physical bodies have physical inclnation to some motion (as heavy bodies downward), which motion is exercised by them through an intrinsic property and without need of a particular external mover." "And therefore, all external impediments removed, a heavy body on a spherical surface concentric with the earth will be indifferent to rest and to movements toward any part of the horizon. And it will maintain itself in that state in which it has once been placed; that is, if placed in a state of rest, it will conserve that; and if placed in movement toward the west (for example), it will maintain itself in that movement."	Galileo predates Newton with this statement about "inertia" - the property of matter to resist changes in motion. Newton generalizes this into his first of three famous laws of motion.
118	"Now in order that we may harvest some fruit from the unexpected marvels that have remained hidden until this age of ours, it will be well if in the future we once again lend ear to those wise philosophers whose opinion of the celestial substance differed from Aristotle's."	Galileo suggests an important lesson for all scientists - to not trust too strongly to our beliefs in what others have told us - even if they are illustrious. The belief in Aristotle's science held sway for 1500 years, and although much he believed was correct, other things were totallly wrong. Yet no one challenged him, so great was his reputation.
119	"Yet in this respect also we must recognize divine Providence, in that the means to such knowledge are very easy and may be speedily apprehended."	Galileo suggests that science - and observation - is really open to all of us. Good science demands a multiplicity of observers, independently verifying discoveries.
124	"But if what we wish to fix in our minds is the apprehension of some properties of things, then it seems to me that we need not despair of our ability to acquire this respecting distant bodies just as well as those close at hand - and perhaps in some cases even more precisely in the former than in the latter. Who does not understand the periods and movements of the planets better than those of the waters of our various oceans?"	Galileo understands the difficulties present with astronomy, where we cannot visit nor do direct experimentation upon the objects of our study. But he offers some rather optimistic thoughts about how that distance may not necessarily preclude our developing an understanding of the nature of those objects.
128	"But whatever te course of our lives we should receive them as the highest gift from the hand of God, in which equally reposed the power to do nothing whatever for us. 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."	Is Galileo religious? Many believed him not at all religious, but perhaps they haven't really read these words?
140-141	"I believe that there are not a few Peripatetics on this side of the Alps who go about philosophizing without any desire to learn the truth and the causes of things, for they eny these new discoveries or jest about them, saying that they are illusions. It is about time for us to jest right back at these men and say they likewise have become invisible and inaudible." "Still, one may not conclude as these men do from such a generalization; one must come down to the particular things observed in stars and in spots."	Galileo was never one to hold his opinion on those who practiced poor science. He maintained that to be sure, one should make observations of the phenomena in question, and decide upon the actual eveidence.
143-144	"Hence I stopped observing Saturn for more than two years. But in the past few days I returned to it and found it to be solitary, without its customary supporting stars, as as perfectly round and sharply bounded as Jupiter."	Galileo never learned the reason for this observation - that Saturn and its rings are tilted relative to its orbital plane, just like Earth, and consequently it is sometimes seen "edge on" and sometimes seen tilted significantly so that its rings are even more obvious."