Department of History
University of California, Irvine
 Instructor:    Dr. Barbara J. Becker

Lecture 7.  The Starry Messenger.

Galileo Galilei (1564-1642)

  • Was Galileo a necessary ingredient of the Copernican Revolution?
  • Does the intellectual, political, social, economic climate of a time provide a "script" for individuals to play out?
  • If not Galileo, would someone else have stepped in to meet the challenges imposed by Copernicus' strange new heliocentric plan?

Stained glass window in Pisa's Santa Maria cathedral (1063) showing the Earth encircled by the heavenly spheres.

Notable Events in Galileo's Life
born at Pisa
enrolled at University of Pisa as medical student
abandoned medicine for mathematics
took teaching post at Pisa
began writing down his thoughts how things move
took position as professor of mathematics at University of Padua

earned money selling gadgets and serving as military consultant

served as mathematics tutor to Cosimo de' Medici
Cosimo became Grand Duke of Tuscany
published Sidereus Nuncius [Starry Messenger]
  -resigned post at Padua
  -took position of "Chief Mathematician and Philosopher" to Grand Duke Cosimo II

Revealing great, unusual, and remarkable spectacles, opening these to the consideration of every man,
and especially of philosophers and astronomers;

Gentleman of Florence
Professor of Mathematics in the University of Padua,


lately invented by him,

In the surface of the Moon, in innumerable Fixed Stars, in Nebulae,
and above all


swiftly revolving about Jupiter at differing distances and periods, and known to no one before the Author recently perceived them and decided that they should be named


To the Most Serene
Cosimo II de' Medici
Fourth Grand Duke of Tuscany

...scarce have the immortal graces of your spirit begun to shine on earth when in the heavens bright stars appear as tongues to tell and celebrate your exceeding virtues to all time.

Behold, then, four stars reserved to bear your famous name; bodies which belong not to the inconspicuous multitude of fixed stars, but to the bright ranks of the planets....

...Indeed, the Maker of the stars himself has seemed by clear indications to direct that I assign to these new planets Your Highness's famous name....

For just as these stars, like children worthy of their sire, never leave the side of Jupiter ... so clemency, kindness of heart, gentleness of manner, splendor of royal blood, nobility in public affairs, and excellency of authority and rule have all fixed their abode and habitation in Your Highness....

New Instruments as Tools of Persuasion

Galileo's Telescope
Institute and Museum of the History of Science
Florence, Italy
  • Does the telescope help, or hinder our vision?
  • Does it extend, or merely distort our senses?
  • Aren't our eyes adequate to see all we are entitled to see?
  • Why would the natural world include things we can't see with our unaided eyes?
Galileo's Telescopic Views of the Fixed Stars

What was observed by us in the third place is the nature or matter of the Milky Way itself, which, with the aid of the spyglass, may be observed so well that all the disputes that for so many generations have vexed philosophers are destroyed by visible crertainty, and we are liberated from wordy arguments.

For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters.

To whatever region of it you direct your spyglass, an immense number of stars immediately offer themselves to view, of which very many appear rather large and very conspicuous but the multitude of small ones is truly unfathomable....

Galileo's sketch of the stars in Orion's belt and sword compared with a modern day photograph of the same region (right).

Galileo's sketch of the stars in the Pleiades compared with a modern day photograph of the same region (left).

Galileo's Observations of the Medicean Stars
Compared with their Appearances for the same Dates
as Determined using a
Modern Computer Planetarium Program (Redshift)
Galileo's descriptions:
January 7, 1610

[T]wo stars were near [Jupiter] on the east and one on the west....  I was not the least concerned with their distances from Jupiter, for ... at first I believed them to be fixed stars....
January 8

[O]n the eighth ... I found a very different arrangement.  For all three little stars were to the west of Jupiter and closer to each other than the previous night....  I was aroused by the question of how Jupiter could be to the east of all the said fixed stars when the day before he had been to the west of them....
January 10

[O]n the tenth ... [o]nly two stars were near him, both to the east....

January 11

There were only two stars on the east, of which the middle one was three times as far from Jupiter than from the more eastern one....  I therefore arrived at the conclusion, entirely beyond doubt, that in the heavens there are three stars wandering around Jupiter like Venus and Mercury around the Sun....
January 13

[F]or the first time four little stars were seen by me....  They formed a very nearly straight line, but the middle star of the western ones was displaced a little to the north from the straight line....
January 15

[T]he four stars ... were all to the west and arranged very nearly in a straight line, except that the third one from Jupiter was raised a little bit to the north....  They were very brilliant and did not twinkle, as indeed was always the case....
January 17

On the seventeenth, 30 minutes after sunset, the configuration was thus.  There was only one star on the east, 3 minutes from Jupiter.  Likewise, one was 11 minutes from Jupiter to the west.  The eastern one appeared twice as large as the western one.  There were no more than these two....
January 17

...But after 4 hours ... on the eastern side a third began to emerge, which, I suspect, had earlier been united with the first one....
January 18

On the eighteenth ... [t]he eastern star was larger than the western one and 8 minutes distant from Jupiter, while the western one was 10 minutes from Jupiter.
January 19

There were three stars exactly on a straight line through Jupiter....  At this time I was uncertain whether between the eastern star and Jupiter there was a little star, very close to Jupiter, so that it almost touched him.
Kepler's Response to Starry Messenger

[Your] assertions about the body of the moon are made by others on the basis of mutually self-supporting evidence.  Their conclusions agree with the highly illuminating observations which you report on the same subject.  Consequently I have no basis for questioning the rest of your book and the four satellites of Jupiter.  I should rather wish that I now had a telescope at hand, with which I might anticipate you in discovering two satellites of Mars (as the relationship seems to me to require) and six or eight satellites of Saturn, with one each perhaps for Venus and Mercury....

--Kepler's comments on Starry Messenger in a letter to Galileo (April 1610)
New Instruments Change How and What We See

The Moon by William Gilbert, c. 1600

Sketch of the moon based on naked eye observation, by English observer, Thomas Hariot, July 1609.

Sketch of the moon from Sidereus Nuncius; based on telescopic observation, by Galileo, early 1610.

Sketch of the moon based on telescopic observation after reading Sidereus Nuncius, by Thomas Hariot, July 1610.

The Moon by Carl Scheiner, c. 1614

My First Observations of the New Planets (1610) 
by Thomas Hariot (c. 1560-1621)

Go to:
  • A Perfit Description of the Celestiall Orbs... (1576), by Thomas Digges (c.1546-1595)
  • Conversation with the Sidereal Messenger (1610), by Johannes Kepler (1571-1630)
  • Dialogue Concerning the Two Chief World Systems(1632), by Galileo Galilei (1564-1642)
Weekly Readings
Lecture Notes