The Galilean Revolution, 400 years later

By Phil Plait | January 7, 2010 10:00 am

Four hundred years ago tonight, a man from Pisa, Italy took a newly-made telescope with a magnifying power of 33X, pointed it at one of the brighter lights in the sky, and changed mankind forever.

The man, of course, was Galileo, and the light he observed on January 7, 1610 was Jupiter. He spotted "three fixed stars" that were invisible to the eye near the planet, and a fourth a few days later.

Here is how he drew this, 400 years ago:


He noted the stars moved around Jupiter as they followed it across the sky, and so was the first to figure out that other planets had moons like our own. It wasn’t an easy observation; his telescope was still small, the field of view narrow (so not all the moons were visible at the same time), and the moons faint next to Jupiter’s brilliant glare. But Galileo persisted, and figured it out. We call these four the Galilean moons in his honor: Io, Europa, Ganymede, and Callisto.

Here’s how we see them today:


The image above [click to embiggen] is from the New Horizons spacecraft as it shot past Jupiter in early 2007, showing all four moons. Each is scaled to show its true relative size to the others. It’s impossible not to wonder what Galileo would have thought, knowing that just shy of 400 years after he made his first observations, we would fling our robotic proxies out into the solar system and get close up views of the objects he discovered.

Think of it! For all of time before, Jupiter was just a light in the sky. And then, forever after that night forty decades ago, it was a world, surrounded by more worlds.

[See more pictures of Jupiter and its moons in a gallery over at 80 Beats.]

Galileo went on to observe craters on the Moon, spots on the Sun, and the phases of Venus. It was that last that may have been his crowning achievement, because the way Venus showed phases meant it could not possibly orbit the Earth, and that it must orbit the Sun. The geocentric theory had held sway for over a thousand years, but Galileo proved it was wrong almost overnight. Of course, the Church wasn’t thrilled with this, though I suspect they might have rolled with it if Galileo hadn’t been such an arrogant jerk and published a manuscript insulting the Pope, a man who used to be his friend and supporter.

If there is a lesson in there, I leave it to my readers to suss it out.

Now, all these years later, a lot of legends exist over the man. He didn’t invent the telescope, he wasn’t the first to point it at the sky, and he wasn’t even the first to publish his drawings. But he was a merciless self-promoter, and because of that we do remember him now (again, any lessons learned here are up to you). And it’s not entirely unfair to do so; he was a tireless observer, a wonderful artist, a great inventor (he may not have been the first to build a telescope, but he made his far better than its predecessors) and a brilliant scientist who, even if he hadn’t done so much for astronomy, would still be remembered today for his other work.

Tonight, just after sunset, Jupiter will be a glowing white beacon in the southwest. I have a Galileoscope, an inexpensive telescope created as part of the International Year of Astronomy 2009, an effort to get as many people on Earth to look up as possible. I think perhaps it would be fitting if I brave the subzero temperature outside, maybe for just a few minutes, and take a look at the mighty planet. Tonight’s display is better than Galileo himself had it: all four moons will be perfectly arrayed, two on each side of Jupiter’s face.

I’m not a very religious man, nor am I a very spiritual man. But I know there will still be a sense of connection, a sense of wonder that I will have tonight that I will share with a man long dead, but whose life and achievements still echo through time.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

Comments (51)

  1. Cheyenne

    It would be pretty cool if I could look through my telescope in 20 years, see those moons, and know that there were fleets of small robots scouring them for signs of life. They would be looking to solve one of the biggest questions we have about this universe. A rather nice little continuation of what Galileo started.

  2. Matt G.

    If Galileo could see us now his mind would explode out the back of his head.

    In the best way possible.

    I am suddenly very envious of the scientists generations from now that will look back and talk about how impressive it was that we figured out as much as we have about the cosmos with our tiny little observatories and without fusion powered rockets.

  3. SlyEcho

    It would be cool, but seems as it’s snowing again, it’ll probably be overcast again tonight. When will the snow stop?

  4. If only I wasn’t getting buried by snow today… maybe there will be a break in the clouds tonight.

  5. Wait, I thought we were celebrating the International Year of Astronomy in 2009 because the anniversary of this was in 2009? Or was that just the invention of the telescope, and he waited until after Epiphany to look up with it?

  6. I stood at Galileo’s tomb in Florence 7 days ago, on New Year’s Eve. He’s interred at the Basilica of Santa Croce, along with Michelangelo, Machiavelli, and other noted historical figures.

    His tomb features a diagram of Jupiter and the four moons orbiting it.

    I found it heartwarming that a great scientist (who was openly prosecuted by the church at the time, no less) was given such a dignified final resting place (even if almost a century after his death) – you usually only find politicians’, religious leaders’, and artists’ tombs in illustrious churches.

  7. Aerimus


    Galileo’s first telescope was presented in 1609. His first observations of Jupiter was in Jan, 1610.

  8. Jeff

    I hope it’s clear enough here in SF tonight for me to get a 400th anniversary glimpse of the Galilean satellites.

  9. Doc

    What really surprised me the first time I saw Jupiter’s moons through my telescope was how far away they were from Jupiter.

  10. bob

    Not very spiritual? You’re a lot more spiritual than most of the “religious” people I’ve met. Once you’ve rejected superstition, spirituality becomes about contemplation, ability to admire the life you live and the ability to be happy in that.

  11. Chris A.

    “He didn’t invent the telescope, he wasn’t the first to point it at the sky, and he wasn’t even the first to publish his drawings. ”

    Umm, Phil, isn’t the reason that Herriott isn’t well known precisely the fact that he _didn’t_ publish his pre-Galilean drawings of the Moon? (Sent them to friends in letters, but didn’t publish like Galileo.)

  12. Torbjörn Larsson, OM

    More than the telescope I think Galileo changed the world by helping to show how observation triumph dogma. Newton, among others for sure, showed how theory is connecting facts, which is a greater, um, theoretical achievement. Both these merciless self-promotors achievements prevailed despite themselves, which is another proof science works.

    I suspect they might have rolled with it if Galileo hadn’t been such an arrogant jerk and published a manuscript insulting the Pope, a man who used to be his friend and supporter.

    If there is a lesson in there, I leave it to my readers to suss it out.

    Oh, we can suss it out. It means that if you criticize the church on good grounds you are “an arrogant jerk” according the evangelism of accomodationism.

    Historians seems to, um, know better though:

    Pope Urban VIII personally asked Galileo to give arguments for and against heliocentrism in the book, and to be careful not to advocate heliocentrism. He made another request, that his own views on the matter be included in Galileo’s book. Only the latter of those requests was fulfilled by Galileo. Whether unknowingly or deliberately, Simplicio, the defender of the Aristotelian Geocentric view in Dialogue Concerning the Two Chief World Systems, was often caught in his own errors and sometimes came across as a fool. Indeed, although Galileo states in the preface of his book that the character is named after a famous Aristotelian philosopher (Simplicius in Latin, Simplicio in Italian), the name “Simplicio” in Italian also has the connotation of “simpleton.”[97] This portrayal of Simplicio made Dialogue Concerning the Two Chief World Systems appear as an advocacy book: an attack on Aristotelian geocentrism and defense of the Copernican theory. Unfortunately for his relationship with the Pope, Galileo put the words of Urban VIII into the mouth of Simplicio. Most historians agree Galileo did not act out of malice and felt blindsided by the reaction to his book. [Wikipedia, heavily referenced, my bold.]

    So doing as the individual himself requested on his own views and “not act out of malice and felt blindsided” translates to “arrogant jerk”, if you leave the historic evidence behind.

    To be fair to the church, more historic evidence reveals that it knows better than that by now:

    In 2000, Pope John Paul II issued a formal apology for all the mistakes committed by some Catholics in the last 2,000 years of the Catholic Church’s history, including the trial of Galileo among others.

    As always, a mistake among people can have many contributors. But I feel safe to say that the church was “an arrogant jerk” in this matter, more so if they didn’t care that Galileo didn’t act out of malice and was blindsided by their reaction.

    But mostly, the lesson here is that society, even religion, has moved on since then. When we are looking at the last couple of decades, we can observe that accomodationism doesn’t work.

  13. I think that’s his publication, not his original drawing … but he probably had to calligraph the publication too …

  14. John. S

    I always found it funny that Galileo refused to believe that the Moon had anything to do with the tides because that smacked of “magical forces at a distance”. He tried to prove that it was the earth moving and sloshing the water around that caused the tides. Ironically, Pope Urban believed the results of thousands of years of “experimental observations” while Galileo relied on wild hypothesis.

    And yes, Galileo was known to be an arrogant jerk that didn’t suffer fools gladly but, worse yet, he insulted those that he thought were his intellectual inferiors (kind of like Fritz Zwicky). It was Newton that later said that “Tact is the knack of making a point without making an enemy.”

  15. Gus Snarp

    Since I don’t have a telescope to look at Jupiter, can anybody recommend something? I really want bottom end on price, but I want to be able to at least see the large planets as more than stars, and I want easy to use, something to spark an interest in my young son.

    @Atilla – I was so glad I went to Santa Croce and to Galileo’s tomb. I agree with you about it. It was probably the most moving thing to me of my whole trip to Italy. I have a decent if slightly blurred picture with the tomb (dad was still figuring out his new camera, and flashes aren’t allowed in the dimly lit church).

  16. Oded

    @John, 15
    I know very little about Galileo’s behavior, so I cannot comment about that, but I find it VERY ironic to use Newton (!) of all people as a model for tact and politeness… Newton is pretty much legendary in how despicable he was…

  17. John. S

    @Oded, 17

    Hah! You caught me. The quote is a good one, even if the source was a hypocrite. I think that Newton was polite to those that could harm him but vengeful and petty to those that couldn’t. His remark upon hearing of Leibnitz’s death, “I guess that I broke his heart” ranks as one of the most indecent things that I’ve heard coming from a scientist.

  18. Alareth


    When people ask me what scope to buy I usually point them to the Advisor at Click a few multiple choice questions and it will list suitable gear. A good starting point to narrow the type you need down.

  19. Trebuchet

    Gus, number 16: You can easily see the Galilean moons of Jupiter with an ordinary pair of binoculars. Jupiter even appears as a small disk.

  20. Cindy

    Crap, the snow is supposed to come tonight. If it’s still clear, I’ll fire up my Galileoscope and hopefully show my 5 year old and 2 year old.

  21. izzy

    Thirding the Santa Croce comments-I also went there and stood at Galileo’s tomb, back in October. On that same day I visited the Florence science museum (sadly I didn’t see Galileo’s infamous finger! But I saw his telescope! Sure it may not be as good as today’s telescopes but it’s a beautiful object), and later that night I saw a conjunction of the Moon and Jupiter, made all the more beautiful by the fact that it was above the Ponte Vecchio! It was an incredibly moving, haunting experience. I even took a picture (not very good quality pic, but it gives an idea of what I saw):
    and Galileo’s tomb:

  22. George

    Nicely done, Phil! Your views are more accurate than much that I have read. Galileo was not the inventor. Optics had been around too long for others to conduct documented, though scantly, magnification observations.
    Galileo’s slam against the explicit main view that the Pope wanted placed at the end of his book, Dialogue, was clearly insulting to the Pope who had a fair theological viewpoint, at least for a 17th century teleological mindset. [It is surprising, however, that Galileo got approval from two different Church sources (Rome and Florence) to even publish it that way.]

    A few nits, or possible nits….

    “We call these four the Galilean moons in his honor: Io, Europa, Ganymede, and Callisto.”

    Yes, they are known as the Galilean moons, but ironically, it was another person who claimed to have been first to see these moons before Galileo that introduced, as suggested to him by Kepler, the names you state.

    “The geocentric theory had held sway for over a thousand years, but Galileo proved it was wrong almost overnight.”

    Yes, but Galileo was convinced it proved the Copernican model, which is not the case, though it was a resonable argument for it. The Jesuit scholars, surprisingly, seemed to be quick to agree that the Ptolemy/Aristotle/Thomist model was out, so, for the Church, in came the earlier Tychonic model. A modified version still works today, apprently within GR theory, though it makes no sense to claim an absolute Earth center with this model.

    “Of course, the Church wasn’t thrilled with this, though I suspect they might have rolled with it if Galileo hadn’t been such an arrogant jerk and published a manuscript insulting the Pope, a man who used to be his friend and supporter.”

    Indeed, the prior Pope was a pain toward Galileo, so Galileo muffed it by his behavior with his old friend, now the new Pope, who had given Galileo praise and privelge. It is hard to say, though, what really would have happened since there were many others who resented Galileo’s claims that seemed to contradict a simple and literal interpretation of a handful of scripture. [Where have we seen this before? :)] I doubt he would have had to recant or go into permanent house arrest had he been tactful, as another has mentioned.

    I do think Galileo deserves to be called the father of modern science, and am curious if you also think so. His quick publication of Starry Messenger is arguably the first true work of modern science. Claims founded on objective evidence far outweigh subjective ones, especially when testable predictions are included in the claims (ie modern theory). Galileo seemed to recognize the entire package of using objective evidence, measurements, mathematics, testable predictions, etc., whereas others didn’t quite have it all put together.

    It is interesting that some publications today, which show somewhat erroneous Lunar drawings, are likely the result of 17th century pirated publications of his popular book. Galileo’s drawings were beautiful; in his early years, he wanted to be an artist but his father prevented it.

    The Church was the big loser in not being quick to allow some rather simple reinterpretations of those few passages. There are more passages on pride and warnings from people like Augustine that they should have heeded. The 1992 apology was late, but better late than never.

  23. Torbjörn Larsson, OM

    In fact it hit me there is another lesson in this. [And I will forgo any reactions to my previous comment for now, as I would like first to try out the role as “arrogant jerk” that critics of religion often gets labeled with.]

    Suppose that historians who scrutinized Galileo’s correspondence hadn’t found out that he didn’t mean to seem to attack his relation the church leader in person, that this is a folk myth of accommodationists.

    That means that we wouldn’t know if he was doing this or not. Which in turn gives that the church not only mistook religious criticism for personal criticism against evidence (that they either didn’t turn up or rejected), but would do so anyway.

    So it isn’t enough as accommodationists believes against facts that one benefits from claiming that religion is compatible with science. It turns out one can’t criticize religion, at all! By the accommodationists favorite tall tale, no less.

    Not only has the accommodationist project not worked the last couple of decades, it didn’t work 400 years ago. Embrace reality, atheists are the new gay. Welcome to our NOTHING TO HIDE festivals! Get used to it.

  24. Ian

    He also had a problem proving stellar parallax, and don’t forget the times they lived in – The Reformation had just occurred and the Catholic Church was under pressure to prove it too was ‘Biblical’.

  25. Galileo made his first Jupiter observation notes on a sheet of scrap paper (a draft of a letter to the Doge of Venice), which is now in the University of Michigan Special Collections library.
    If you can read it (I can’t, but I take the expert’s word for it), it is particularly interesting to note that his first notes are in Italian, but the last comments are in Latin, indicating he realised he might have something worth publishing.

  26. Matt T

    Dr P, I think I agree with every jot and tittle in this post. I did my Galileoscope observation of the “Medicians” a few months back, on the anniversary of Galileo’s demonstration of the telescope (before my back yard was covered in 2 feet of snow). Somehow it’s even more spine-tingling to see what he saw — little dots of light — rather than the Hubblesque pictures we’re used to now.

    WRT the “arrogant jerk” comments, I think the point is that Galileo had a very … uh … brusque manner. The fact that he insulted the pope and then was surprised at the reaction probably indicates something about his personality and style. He may not have meant to be an arrogant jerk, but I think those two terms are both technically accurate.

  27. Flavio

    I, like others, went to your old post and found the very surprising “arrogant jerk”.

    I think he doesn’t deserve this characterization, though I recognize this is probably what some scholars think. Yes, in the “Dialogo sopra i due massimi sistemi del mondo” he put a “Simpleton” to express the church’s position, but this wasn’t quite for mockery. The Dialogue was addressing the layman too, and this was a pretty groundbreaking choice for the time. If I’m not mistaken, Kepler and Copernicus still wrote in Latin, so just for their peers.

    So coming out with theories that weren’t just speculations, but facts, pissed off the catholic church. Arrogant jerk? That’s what a fundamentalist would say about you and every scientist that doesn’t shut up in front of ignorance.

  28. Brian Too

    The man published his results, which is profoundly scientific, and he paid the price for telling the truth. That’s integrity.

    I object to describing him as an arrogant jerk and self promoting. The mere fact that he wrote his work as a dialogue speaks to the fact that he felt constrained by the Church and needed a fig leaf. The power of the Church in his day was formidable and would have given anyone pause to cross them. He knew all of this and published anyway. That’s courage.

    We remember Galileo because he was right. We admire Galileo because he spoke truth to power and opened doors for all who followed. This is recognition that, whatever his personal failings, he was admirable.

  29. JenniferBurdoo

    I wish I’d built my Galileoscope now. I haven’t gotten around to it, it’s still in its box.

    But then, I don’t have a tripod either. And it’s sadly overcast in South Florida.

    But I’m gonna build the thing now.

  30. I live in the Philippines. If ever it will not rain tonight will I see Jupiter and its moons too?

  31. Jeremy

    Just came in from viewing Jupiter and all four Galilean moons using 20×50 binoculars mounted on a tripod. I thought that it was going to be cloudy here in Arizona but it cleared up beautifully. I have just gotten into astronomy (inspired in part by reading Death from the Skies), I must say tonight was awesome.

  32. comatus

    A beautiful article. Our idea of Galileo has little to do with the Pope’s thinking, and a great deal to do with that of Arthur Miller and Bertolt Brecht.

    Both Newton and Galilei were epic woodworkers, but Newton was prouder of nothing than that he died a virgin; Da G-Man was looked after by his illegitimate daughter. Hmmph. Nerds, then and now…

  33. Grimbold

    Makati Condos,

    You should be able to see Jupiter as a bright yellow-white point in the western sky for a few hours after sunset. Its moons can be seen fairly easily through a decent pair of binoculars.

  34. StevoR

    I too think calling Galileo an “arrogant jerk” is going a bit far.

    I’m sure that – like us all – he had his flaws and maybe rubbed the wrong people up the wrong way.

    But his use of words and arguments and evidence vs him being shown the implements of torture and forced to recant & abjure his (correct) Copernican “heretic” beliefs and then being confined to house arrest for the rest of his life while other Copernican’s such as Giordano Bruno were burnt at the sake at least in part for their scientific beliefs says a lot about who are the “arrogant jerks” here.

    As does the fact the Catholic Church finally apologised many hundreds of years later.

    Galileo might have been tactless or it might have been a misunderstanding but the “arrogant jerkery” was mostly on the other side, IMHON.

    I wonder how Galileo would’ve felt about the spaceprobe named in his honour ( ) exploring the Jovian system for some years in the 1990’s?

  35. Grant Gordon

    Jupiter was the very first thing I pointed my new scope at (aside from the moon) and wow! What a sight, I was treated to all four moons and slight hint of banding and that was from my relatively bright back yard. I can’t wait to be able to take my new telescope to a really dark sky site.

  36. Shoot! Missed it tonight!

    We were working on my son’s car in the driveway (routine maintenance before he heads back to college Sunday) as darkness fell. Remembering this post, I looked up and spotted Jupiter among the high clouds. Made a mental note to grab the G’scope when finished. Forgot about it until after dinner. Now nice and dark…and totally overcast.


    – Jack

  37. This is a great little article from Bad Astronomy on the significance of today is cosmology circles. People could do worse than paying a little more respect to the impact that early pioneers in the stargazing past had on how we view things today. It’s a shame that most of the people I talk to barely know Galileo (“that space guy”) and usually have not heard of Copernicus or Kepler.

  38. tom swift

    We don’t know what telescope G. used in his observations of the Jovian moons. He mentions an instrument of about 33x in Sidereus Nuncius, but there is no claim or evidence that he was using it in Jan. 1610. He does say that in order to see the things described in Sidereus Nuncius a telescope of at least “400 power” (what we would call 20x) is needed, implying that he used such a telescope to see them himself, although such an instrument is NOT explicitly described in S.N..

    Even worse, from Galileo’s scanty description of the instrument he was using for the observations of Jupiter, it’s clear that the objective was astigmatic. None of the three surviving objective lenses in the Florence Museum of Science which are attributed to Galileo are astigmatic, even though one lens is in an 18th century frame, with an inscription claiming it to be the actual lens used during the discovery of the “Mediciean stars”. So THE telescope has, so far as we know, not survived.

    Too bad. Galileo’s realization that these “stars” were actually moons orbiting another “star” was indeed a great moment in intellectual history.

  39. I remember when I first viewed the moon through my new 20X binoculars about 30 years ago and thinking “This must have been what Galileo saw”. Actually, Galileo might have killed for the quality of the coated optics that I was using.

  40. “Of course, the Church wasn’t thrilled with this, though I suspect they might have rolled with it if Galileo hadn’t been such an arrogant jerk and published a manuscript insulting the Pope, a man who used to be his friend and supporter.”

    Boo — you’re blaming the victim here, Dr. Phil. This is not up to your usual standards. You should retract this statement.

  41. #47. I think you meant that Phil should RECANT of that statement :-)

  42. Even if Phil recants, surely he should serve the rest of his life under house arrest as Galileo did. It’s only fair, since in his scientific arrogance Phil has no doubt offended the Church many times.


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