Copernicus and the Star that was bigger than the Universe

I’ve been try­ing to watch Cosmos by Carl Sagan. I’ve never seen it and it’s prov­ing to be a bit of a struggle. He def­in­itely can write. Some of the sequences are fant­astic, but some of it is badly dated. The thing that really grates to me is his dis­missal of Ptolemy and his geo­centric uni­verse. For Sagan at best Ptolemy’s sys­tem held back astro­nomy by 1,500 years. At worst he’s only worth men­tion­ing to say he’s dead wrong, like in the first episode.

It’s not really fair to lay into Sagan for his atti­tude to Ptolemy. His work is a product of its time and it was writ­ten over thirty years ago. But the idea that Ptolemy was clearly wrong seems to the pop­u­lar under­stand­ing of Renaissance astro­nomy. The ques­tion here is Why did some people oppose the helio­centric the­ory of the uni­verse? not Who in their right mind would accept it? It over­looks the power of the Ptolemaic sys­tem. If you fol­lowed Ptolemy’s work you could pre­dict where the plan­ets would be with enough accur­acy for naked-eye astro­nomy. If Copernicus had only used simple circles, then his model might have seemed bet­ter, but he too needed to add epi­cycles and fudges to make his sys­tem match the observ­able sky. It needed fewer epi­cycles, but it was hardly perfect.

Popular belief is that the prob­lem was solved when Galileo picked up his tele­scope and proved the helio­centric the­ory. In fact a recently pub­lished paper by Christopher Graney, The Telescope Against Copernicus: Star Observations by Riccioli Supporting a Geocentric Universe in the Journal for the History of Astronomy shows that the tele­scope could have dealt a ser­i­ous blow to the Copernican model of the uni­verse.

One obvi­ous prob­lem was the lack of stel­lar par­al­lax. If the Earth was going round the sun, then the closest stars would seem to shift pos­i­tion rel­at­ive to the more dis­tant stars on an annual basis. No shift­ing was seen. In real­ity that’s not a sur­prise. Even the closest star has a par­al­lax of around an arc second. This is a tiny angle. Imagine one degree divided into sixty parts. Each of these parts is an arcminute. Now divide one of these arc minutes into sixty parts and one of these parts will be an arc second. There simply wasn’t the equip­ment to meas­ure a star’s pos­i­tion with that kind of accur­acy in the sev­en­teenth century.

The answer, said the Copernicans, is that the stars are a very, very far away. That’s why no shift due to par­al­lax was vis­ible. If that was the only data they had then no one would be able to say if this were true or not. But not only was there an argu­ment about the size of stars.

Tycho Brahe had meas­ured the sizes of stars. He did this by naked eye, meas­ur­ing the size of the appar­ent disc of the star. If you know the angu­lar dia­meter of the star, and how far away it is, you can work out the size of a star. It’s this fact that the Jesuit astro­nomer Riccioli used to debunk the Copernican model. Graney points out a table com­piled by Riccioli that says that Tycho thought the stars were 14,000 Earth radii away, but they could be up to 40,000 Earth radii away accord­ing to some Ptolemaic mod­els. Sirius was meas­ured as 18 arc seconds across. That meant its dia­meter was between 0.6 to 3.5 the size of the Earth’s roughly. He also had his own estim­ate of 210,00 Earth radii for the dis­tance to the stars, but even this only made Sirius 17.5 times the dia­meter of the Earth.

He then made a table of the required dis­tances to the stars pro­posed by some Copernicans, to account for the lack of stel­lar par­al­lax. In the case of the astro­nomer Wendelin, this dis­tance to the stars was over 600,000,000 Earth radii. That made Sirius’s dia­meter over 50,000 Earth radii. Comfortably big­ger than the entire uni­verse accord­ing to Tycho or Ptolemy. Even the Sun was only thought to be 180 times big­ger than the Earth. The stars would have to be com­pletely unlike any­thing else in the uni­verse, or else Copernicus was wrong.

The stand­ard nar­rat­ive in the his­tory of sci­ence is that Galileo solved this by stat­ing the stars appeared at points, and so could not be meas­ured. In his first men­tion of the sub­ject in the Starry Messenger of 1610 he says:“The fixed stars are never seen to be bounded by a cir­cu­lar peri­phery, but have rather the aspect of blazes whose rays vibrate about them and scin­til­late a great deal.”

The constellation Delphinus

Credit: Digitized Sky Survey, ESA/ESO/NASA FITS Liberator

Graney points out this is simply not the case. Through tele­scopes stars appear as discs. This is true with mod­ern equip­ment. Above is the con­stel­la­tion Delphinus com­piled from expos­ures taken by the ESO as part of the Digital Sky Survey. The major stars are vis­ibly discs. This is some­thing that hap­pens with tele­scopes, it’s an effect known as the Airy disc, and it’s due to the dif­frac­tion of light. Graney can also show that his later work, when he was more adept at using a tele­scope, refers to stars being round and not points.

Graney takes this idea for­ward. It’s not simply enough to show that Riccioli was pok­ing holes in the weak parts of Galileo’s idea. Graney goes on to ask if this was a fun­da­ment­ally flawed attack on the Copernican sys­tem. Eventually the true nature of the discs would be dis­covered, and Graney also notes that while Riccioli was meas­ur­ing these discs, Horrocks was watch­ing stars wink out instantly when the Moon passed in front of them. That meant that the stars must have been points, even if they appeared to be discs. There are a few pub­lic­a­tions that say the stars can­not have large discs in the sev­en­teenth cen­tury, but Graney also notes they don’t ref­er­ence each other. It shows that the dis­cov­ery was inev­it­able if it was being made inde­pend­ently in sev­eral places, but he also points out that it must have also been needed to be made again and again for that to hap­pen. Clearly the idea that stars were points wasn’t get­ting traction.

Graney’s paper makes sev­en­teenth cen­tury sci­ence a bit more com­pre­hens­ible. The cari­ca­ture of the period is that Galileo turned up on the Pope’s door­step and said “Guess what your holi­ness! The Earth goes around the Sun!” and Pope Urban VIII stuck his fin­gers in his ears and yelled “LA LA LA I CAN’T HEAR YOU!” Obviously there was a polit­ical dimen­sion to the helio­centri­cism debate, but Graney shows that it wasn’t purely about polit­ics and there were sound sci­entific reas­ons, given the state of instru­ments and know­ledge, for hold­ing to a geo­centric universe.

If you frame the whole debate in terms of who was right and who was wrong then you could argue this elev­ates Ptolemy at the expense of Copernicus. But sci­ence isn’t a zero-sum game. Graney’s work shows that the Ptolemaic sys­tem was sup­por­ted by a lot of observ­able evid­ence. If that’s the case, then the achieve­ments of Copernicus, Galileo and Kepler are even more impress­ive. There’s no dif­fi­culty in dis­prov­ing some­thing that’s obvi­ously wrong. Spotting there’s some­thing not quite write about an answer when every­one else is happy with it is much more dif­fi­cult. Downplaying the argu­ments for a geo­centric uni­verse denies some of the high-powered brain­work that was needed to develop a helio­centric sys­tem. Us and Them his­tor­ies of sci­ence can bring mod­ern dis­putes into a his­tor­ical set­ting and risk los­ing sight of what was unique about a time.

Graney’s paper doesn’t have a DOI, but it is cur­rently (Jan 2011) on Ingenta Connect. If you have a sub­scrip­tion to JHA you can down­load it from there. Graney, C.M. (2010). The Telescope Against Copernicus: Star Observations by Riccioli Supporting a Geocentric Universe Journal for the History of Astronomy, 41 (4), 453–467

And just after fin­ish­ing up this post and won­der­ing when to sched­ule it for I see Christopher Graney is in the news with another Riccioli paper, this time on the lack of evid­ence for a Coriolis force.


When he's not tired, ill or caught in train delays, Alun Salt works part-time for the Annals of Botany weblog. His PhD was in ancient science at the University of Leicester, but he doesn't know Richard III.

4 Responses

  1. Peter L. Griffiths says:

    Further to my com­ment of 15 April 2011, Galileo’s law of fall­ing bod­ies v^2=d can be recon­ciled with Kepler’s dis­tance law v^2=1/r, with L indic­at­ing a small change as fol­lows.
    v^2+Lv^2=d+Ld=1/(r-Ld), this is the usual way of meas­ur­ing velo­city. For the recip­rocal way of meas­ur­ing the same velo­city we have
    v^2+Lv^2=r+Lr=1/(d-Lr). d+r equal the length of the major axis of the ellipt­ical orbit.

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