Category Archives: History of Astronomy

Perpetuating the myths addendum – ‘The Copernican Shock

Frequent Renaissance Mathematicus commentator (comment-writer, commenter, commentor), Phillip Helbig, sent me an interesting email in response to my previous blog post. In skewering the Nadlers’ comic book I didn’t actually comment on every single detail of everything that was wrong with it, one of the things I left out was Galileo saying:

It is not the center of the cosmos it is a planet just like the others and they all orbit the sun.

As Phillip correctly pointed out in the Ptolemaic-Aristotelian geocentric model of the cosmos the Earth was not viewed as the centre of the cosmos but rather as the bottom. I wrote a brief post long ago quoting a wonderful passage by Otto von Guericke, the inventor of the vacuum pump on exactly this topic:

Since, however, almost everyone has been of the conviction that the earth is immobile since it is a heavy body, the dregs, as it were, of the universe and for this reason situated in the middle or the lowest region of the heaven

Otto von Guericke; The New (So-Called) Magdeburg Experiments of Otto von Guericke, trans. with pref. by Margaret Glover Foley Ames. Kluwer Academic Publishers, Dordrecht/Boston/London, 1994, pp. 15 – 16. (my emphasis)

Phillip then asks, “So what was the “shock” of the Copernican Revolution (how many even get that pun?)?  Was it demoting humanity from the centre of the universe, or promoting the Earth to be on par with the other heavenly bodies?”

Before I answer his question I would point out that the idea that Copernicus had demoted the Earth from the centre of the cosmos first emerged much later, sometime in the late eighteenth or early nineteenth century, as an explanation for the supposed irrational rejection of the heliocentric hypothesis. Of course as is now well known, or at least should be, the initial rejection of the heliocentric hypothesis was not irrational but was based on solid common sense and the available empirical scientific evidence nearly all of which spoke against it. For a lot, but by no means all, of the astronomical arguments read Chris Graney’s excellent Setting Aside All Authority.

So back to Phillip’s question, what was the real Copernican shock? The answer is as simple as it is surprising, there wasn’t one. The acknowledgement and acceptance of the heliocentric hypothesis was so gradual and spread out over such a long period of time that it caused almost no waves at all.

First up, there was nothing very new in Copernicus suggesting a heliocentric cosmos. As should be well known it had already been proposed by Aristarchus of Samos in the third century BCE and Ptolemaeus’ Syntaxis Mathematiké (Almagest) contains a long section detailing the counter arguments to it, which were well known to all renaissance and medieval astronomers. Also in the centuries prior to Copernicus various scholars such as Nicholas of Cusa had extensively discussed both geocentric models with diurnal rotation and full heliocentric ones. All that was new with Copernicus was an extensive mathematical model for a heliocentric cosmos.

At first this was greeted with some enthusiasm as a purely hypothetical model with the hope that it would deliver better predictions of the heavenly movements than the geocentric models for use in astrology, cartography, navigation etc. However it soon became apparent that Copernicus was not really any better than the older models, as it was based on the same inaccurate and oft corrupted data as Ptolemaeus, so the interest waned, although it was these inaccuracies in both model that inspired Tycho Brahe to undertake his very extensive programme of new astronomical observations on which Kepler would base his models.

As Robert Westman pointed out, in a now legendary footnote, between the publication of De revolutionibus in 1543 and 1600 there were only ten people in the whole world, who accepted Copernicus’ heliocentric cosmology, not exactly earth shattering. Even after 1600 the acceptance of a heliocentric worldview only increased very slowly and in gradual increments as the evidence for it accumulated.

The first two factors are the work of Kepler and the early telescopic discoveries. Because Kepler couldn’t or rather didn’t deal with the physical problems of a moving earth his work initially fell on deaf ears. The early telescopic discoveries only refuted a pure Ptolemaic geocentric model but were consistent with a Tychonic geo-heliocentric one and as this had a stationary earth, it became the model of choice. Of interest, and I think up till now not adequately explained, a Tychonic model with diurnal rotation, i.e. a spinning earth, became the preferred variation. A partial step in the right direction. Kepler’s publication of the Rudolphine Tables in 1627 led to an acceptance of his elliptical astronomy at least for calculations if not cosmologically. Then Cassini, with the help of Riccioli, demonstrated with a heliometer in the San Petronio Basilica in Bologna that the sun’s orbit around the earth or the earth’s orbit around the sun was indeed a Keplerian ellipse, but couldn’t determine which of the two possibilities was the right one. Another partial step in the right direction.

Both Kepler’s first and third laws, solidly empirical, were now accepted but his second law still caused problems. Around 1670 Nicholas Mercator provided a new solid proof of Kepler’s second law and it is about then that the majority of European astronomers finally accepted heliocentricity, although it was Kepler’s elliptical astronomy and not Copernicus’ model; the two models were regarded as competitors; also there was still a distinct lack of empirical proof for a heliocentric cosmos.

The developments in physics over the seventeenth century combined with the discovery of the physical reality of the atmosphere and Newton’s gravitation law finally solved the problems of why, if the earth is moving various disasters don’t occur: high winds, atmosphere blowing away etc., all of those arguments already listed by Ptolemaeus. The final empirical proofs of the annual orbit, Bradley and stellar aberration in 1727, and diurnal rotation, measuring the shape of the earth, around 1750, were delivered in the eighteenth century.

As can been seen by this very brief outline of the acceptance and confirmation of heliocentrism it was a process that took nearly two hundred years and proceeded in small increments so there was never anything that could possibly be described as a shock. As already stated above the concept that the ‘Copernican Revolution’ caused consternation or was a shock is a myth created sometime in the late eighteenth or early nineteenth century to explain something that never took place. One might even call it fake news!

Addendum: A lot of the themes touched on here are dealt with in greater detail in my The transition to heliocentricity: The Rough Guides series of blog posts

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Filed under History of Astronomy, History of science, Myths of Science

Bringing the heavens down to earth

The Frisian Protestant pastor and amateur astronomer, David Fabricius, was beaten to death by one of his parishioners on 7 May 1617. Because he corresponded with both Tycho Brahe and Johannes Kepler and was quite a significant figure in Early Modern astronomy the Society for the History of Astronomy had a short post on Facebook commemorating his death on last Sunday, which contained the following claim:

David Fabricius was, following Galileo’s lead, one of the early users of the telescope in astronomy[1]

This claim contains two factual errors. The first is that it was Johannes, David’s son, who introduced the telescope into the Fabricius household and not David, although David soon joined his son in his telescopic observations. I’ll explain further later.

The Fabricii, father and son, remain largely unknown to the world at large but a monument to them both was erected in the churchyard in Osteel, where David had been village pastor, in 1895.

The second error is more serious because it indirectly perpetuates a widespread myth concerning the introduction of the telescope into astronomy and Galileo’s role in it. There is a popular perception that Galileo, and only Galileo, had the genius, the wit, the vision to realise that the newly invented telescope could be used as an astronomical instrument and that he singlehandedly pioneered this new discipline, telescopic astronomy. This is of course complete rubbish and seriously distorts the early history of the telescope in astronomy and does a major disservice to all of the others who contributed to that early history. I will admit to having done a small fist pump when I read the following in John Heilbron’s Galileo biography:

The transformation of the Dutch gadget into an instrument powerful to discover novelties in the heavens did not require a Galileo. His unique strength lay in interpreting what he saw.[2]

That the telescope could be used as an astronomical instrument was recognised during its very first public demonstration by its inventor, the German/Dutch spectacle maker Hans Lipperhey, which took place at the court of Prince Maurice of Nassau in Den Haag during the Dutch-Spanish Peace Conference on an unknown day between 25 and 29 September 1608. We have a detailed account of this demonstration from a French flyer or newsletter describing the first visit of the Ambassador of Siam to Europe, the Ambassador being present at the demonstration. Through this flyer the news of the new invention spread rapidly throughout Europe. Amongst the other descriptions of the wonderful abilities of this “…device by means of which all things at a very great distance can be seen as if they were nearby, by looking through glasses…” we can read the following:

The said glasses are very useful at sieges & in similar affairs, because one can distinguish from a mile’s distance & beyond several objects very well, as if they are near & even the stars which normally are not visible for us, because of the scanty proportion and feeble sight of our eyes, can be seen with this instrument. [my emphasis]

The first astronomer to build and use a telescope as an astronomical instrument was Thomas Harriot, who drew a sketch of the moon using a telescope on 26 July 1609 before Galileo even had a telescope.

Thomas Harriot’s 1609 telescopic sketch of the moon

This of course raises the question where Harriot obtained his knowledge of this instrument. In the early phase of the telescopes existence it became a common habit to present heads of state and other worthies telescopes as presents. In England James I (VI of Scotland) was presented with one at the end of an elaborate masque created for the occasion by Ben Jonson, the Renaissance playwright. The telescope was obtained from the United Provinces through the offices of Cornelis Drebbel, the Dutch inventor and scholar, who was employed at James’ court. This telescope was probably Harriot’s, who enjoyed good connections to court circles, introduction to the instrument.

Portrait often claimed to be Thomas Harriot (1602), which hangs in Oriel College, Oxford. Source: Wikimedia Commons

Harriot did not observe alone. In London he observed together with his instrument maker Christopher Tooke in London, whilst Harriot’s pupil the landowner and MP, Sir William Lower observed in Wales, together with his neighbour John Prydderch, with a telescope made by Harriot and Tooke. Each pair took turns in observing comparing their results and then Harriot and Lower compared results by letter. This meant that they could be reasonably certain that what they had observed was real and not some optical artefacts produced by the poor quality of the lenses they were using. So here we have four telescopic astronomical observers independent of Galileo’s activities.

In Franconia Simon Marius also built and used telescopes in 1609, at the time unaware of the similar activities of Galileo in Padua. As I have written in another blog post Marius discovered the four largest moons of Jupiter just one day later and independently of Galileo. Marius also made the first telescopic observations of the Andromeda Nebula, significant because the Andromeda Nebula would later become the first galaxy to be recognised as a galaxy outside of our galaxy.

Simon Marius frontispiece from his Mundus Jovialis

Another telescopic pioneer in Southern Germany was the Jesuit astronomer in Ingolstadt, Christoph Scheiner, who famously became embroiled in a dispute with Galileo over who had first observed sunspots with a telescope and what exactly they were.

Christoph Scheinet (artist unknown)

The dispute was rather pointless, as Harriot had actually observed sunspots earlier than both of them and Johannes Fabricius, to whom we will turn next, had already published a report on his sunspot observations unknown to the two adversaries. Christoph Scheiner and his assistant, another Jesuit astronomer, Johann Baptist Cysat, would go on to make several important contributions to telescopic astronomy.

Johann Baptist Cysat, holding a Jacob’s staff

Johannes Fabricius brought his telescope home from the University of Leiden, where he had almost certainly learnt of this instrument through the lectures of Rudolph Snel van Royan, professor of mathematics and father of the better know Willibrord Snel of Snell’s law of refraction fame. Rudolph Snel van Royan was probably the first university professor to lecture on the telescope as a scientific instrument already in 1610.

Rudolph Snel van Royan
Source: Wikimedia Commons

It is also known that Cort Aslakssøn and Christian Longomontanus acquired lenses and built their own telescopes in the first couple of years of telescopic astronomy in Copenhagen, but unfortunately I haven’t, until now, been able to find any more details of activities in this direction. If any of my readers could direct me to any literature on the subject I would be very grateful.

Christian Severin known as Longomontanus

Turning to Italy we find the astronomers on the Collegio Romano under the watchful eye of Christoph Clavius making telescopic astronomical observations before Galileo published his Sidereus Nuncius in 1610, using a Dutch telescope sent to Odo van Maelcote by one of his earlier students Peter Scholier. Grégoire de Saint-Vincent would later claim that he and Odo van Maelcote were probably the very first astronomers to observe Saturn using a telescope. It was the astronomers of the Collegio Romano, most notably Giovanni Paolo Lembo and Christoph Grienberger, who would then go on to provide the very necessary independent confirmation of the discoveries that Galileo had published in the Sidereus Nuncius.

As can be seen Galileo was anything but the singlehanded pioneer of telescopic astronomy in those early months and years of the discipline. What is interesting is that those working within the discipline were not isolated lone warriors but a linked network, who exchanged letter and publications with each other.

Some of the connections that existed between the early telescopic astronomers are listed here: Harriot had corresponded extensively with Kepler and was very well informed about what Tycho and the other continental astronomers were up to. David Fabricius corresponded with Kepler and Tycho and even visited Tycho in Prague but unfortunately didn’t meet Kepler on his visit. Johannes would later take up correspondence with Kepler. Tycho corresponded with Magini in Bologna who passed on his news to both Galileo and Clavius. Clavius was also very well informed of all that was going on in European astronomy by the Jesuit network. Almost all of the Jesuit astronomers were students of his. Marius corresponded with Kepler, who published many of his astronomical discoveries before he did, and with David Fabricius, whom he had got to know when he visited Tycho in Prague to study astronomy. Longomontanus had earlier been Tycho’s chief assistant and corresponded with Kepler after he left Prague to return to Copenhagen. Interestingly another of Tycho’s assistants, Johannes Eriksen, visited both David Fabricius in Friesland and Thomas Harriot in London on the same journey.

What we have here is not Galileo Galilei as singlehanded pioneer of telescopic astronomy but a loosely knit European community of telescopic astronomers who all recognised and utilised the potential of this new instrument shortly after it appeared. They would soon be joined by others, in this case mostly motivated by Galileo’s Sidereus Nuncius, a few of them even supplied with telescopes out of Galileo’s own workshop. However what is very important to note is that although Galileo was without doubt the best telescopic observer of that first generation and certainly won the publication race, all of the discoveries that he made were also made independently and contemporaneously by others, so nothing would have been lost if he had never taken an interest in the spyglass from Holland.

 

 

 

 

 

[1] Because I pointed out the errors contained in this claim in a comment, it has now been removed from the Facebook post!

[2] J. L. Heilbron, Galileo, OUP, 2010, p. 151

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Filed under History of Astronomy, History of Optics, History of science, Uncategorized

A birthday amongst the stars

Readers will probably be aware that as well as writing this blog I also hold, on a more or less regular basis, semi-popular, public lectures on the history of science. These lectures are as diverse as this blog and have been held in a wide variety of places. However I have, over the years, held more lectures in the Nürnberg Planetarium than anywhere else and last Thursday I was once again under the dome, this time not to hold a lecture but to help celebrate the ninetieth birthday of this august institution.

Before the twentieth century the term planetarium was a synonym for orrery, a mechanical model, which demonstrates the movements of the planets in the solar system. The beginnings of the planetarium in the modern sense was as Walther Bauersfeld, an engineer of the German optics company Zeiss, produced the plans for the construction of a planetarium projector based on earlier concepts. In 1923 the world’s first planetarium projector, the Zeiss Mark I, was demonstrated in the Zeiss factory in Jena and two months later on 21 October in the Deutschen Museum in Munich. Following further developments the first planetarium was opened in the Deutschen Museum on 7 May 1925.

Zeiss Mark I Planetarium Projector

Various German town and cities followed suit and the city council of Nürnberg signed a contract with Zeiss for a planetarium projector on 12 February 1925. The contract called for the city council to pay Zeiss 150, 000 Reichsmark ( a small fortune) in three instalments and 10% of the takings from the public shows. In a building on Rathenauplatz designed by Otto Ernst Schweizer the Nürnberg planetarium opened ninety years ago on 10 April 1927.

Original Nürnberg Planetarium

Fitted out with a new Zeiss Mark II projector the first of the so-called dumbbell design projectors with a sphere at each end for the north and south hemispheres. It was the world’s ninth planetarium.

Zeiss Mark II Planetarium Projector

From the very beginning the planetarium was born under a bad sign as the NSDAP (Nazi) city councillor, Julius Streicher, (notorious as the editor of the anti-Semitic weekly newspaper Der Stürmer) vehemently opposed the plans of the SPD council to build the planetarium. On 30 January 1933 the NSDAP seized power in Germany and the days of the planetarium were numbered. In November the planetarium director was ‘persuaded’ to recommend closing the planetarium and at the beginning of December it was closed. There were discussions about using the building for another purpose but Streicher, now Gauleiter (district commissioner) of Franconia was out for revenge. In March 1934 the planetarium was demolished on Streicher’s orders, with the argument that it looked too much like a synagogue! However the projector, and all the technical equipment, was rescued and put into storage.

Historischer Kunstbunker Entrance: There are guided tours

During the Second World War the projector was stored together with the art treasures of the city in the Historischer Kunstbunker (historical art bunker), a tunnel under the Castle of Nürnberg.

Following the war, in the 1950s, as Nürnberg was being rebuilt the city council decided to rebuild the planetarium and on 11 December 1961 it was reopened on the new site on the Plärrer, with an updated Zeiss Mark III. During the celebrations for the five hundredth anniversary of the death of Nicolaus Copernicus in 1973, whose De revolutionibus was printed and published in Nürnberg, the planetarium became the Nicolaus-Copernicus-Planetarium. In 1977 the Mark III projector was replaced with a Mark V, which is still in service and in 2010 the planetarium entered the twenty-first century with a digital Full-Dome projector.

Nicolaus-Copernicus-Planetarium am Plärrer in Nürnberg (2013)

The Zeiss Mark V Planetarium Projector in Nürnberg

Since the 1990’s the planetarium has been part of the City of Nürnberg’s adult education complex and alongside the planetarium programme it is used extensively for STEM lectures. I shall be holding my next lecture there on 28 November this year about Vannevar Bush, Claude Shannon, Robert H Goddard and William Shockley- Four Americans Who Shaped the Future (in German!) and if you’re in the area you’re welcome to come and throw peanuts.

 

 

 

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Filed under Autobiographical, History of Astronomy, History of Optics, History of science, Uncategorized

On an excursion

If you wish to read the latest words of wisdom, this time on the conception and invention of the reflecting telescope, then you will have to take an excursion to AEON magazine, where you can peruse:

How many great minds does it take to invent a telescope?

Isaac Newton’s reflecting telescope of 1671. Photo ©The Royal Society, London

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The poetic astronomer

 

Regular readers of this blog will know that I can on occasion be a stroppy, belligerent, pedant, who gets rather riled up over people who spread myths of science and who has a tendency to give such people a public kicking on this blog. This tendency earned me the nickname, the HistSci_Hulk in earlier years. The subtitle to a podcast that I stumbled across yesterday on the BBC website provoked my inner Hist_Sci Hulk and has generated this post.

The podcast is a BBC Radio 4 “Radio 4 in Four” four minute documentary on the work of the Indian mathematician and astronomer, Aryabhata: Maths expressed as poetry. The subtitle was: In 5th century India, clever man Aryabhata wrote his definitive mathematical work entirely in verse and long before Galileo, argued the world was round [my emphasis]. It was that final clause that provoked my HistSci_Hulk moment.

I’ve lost count of how many times over the years I have explained patiently and oft not so patiently that educated society in European culture have known and accepted that the world is a sphere since at least the sixth century BCE. This is the most recent account here on the blog. Bizarrely in the podcast no mention is made of Aryabhata’s cosmological or astronomical views, so it is real puzzle as to why it’s mentioned in the subtitle. What is interesting is the fact that as a cosmologist Aryabhata held a fairly rare position, although he was a geocentrist he believed that the earth revolved around its own axis, i.e. geocentrism with diurnal rotation. You can read about the history of this theory here in an earlier blog post.

Statue of Aryabhata on the grounds of IUCAA, Pune. As there is no known information regarding his appearance, any image of Aryabhata originates from an artist’s conception.
Source: Wikimedia Commons

More interesting is the correct fact that Aryabhata wrote his astronomical/mathematical thesis in verse form. As the podcast points out this is because the culture in which he was writing was an oral one and complex facts are easier to remember in verse rather than in prose. What the podcast doesn’t say is that Aryabhata was not the only astronomer/mathematician to express his results in verse and was in this sense by no means unique. In fact he is part of a solid tradition of mathematical Sanskrit poetry.

India was not the only culture to use poetry to express scientific content. Probably the most famous example is the Latin poem De rerum natura by the first century BCE Roman poet Lucretius, which is the most extensive description of the physics of the ancient Greek atomists. The poem played a central role in the revival of atomism in the early modern period; a revival that several historians of science, such as David Lindberg, consider to be a key element in the so-called scientific revolution of the seventeenth century.

In astronomy/ astrology there is a poem from antiquity that played a significant role in the Renaissance. This is the Astronomica probably written by the poet Marcus Manilius in the first century CE; the first printed edition of this was published by Regiomontanus in Nürnberg in 1473.

Many people are not aware of some highly significant scientific poems from the eighteenth century written and published by Charles Darwin’s grandfather, Erasmus.

Joseph Wright of Derby, Erasmus Darwin (1770; Birmingham Museum and Art Gallery).
Source: Wikimedia Commons

Darwin’s The Loves of the Plants was the first work in English to popularise the botanical works of Linnaeus in English. The poem caused something of a scandal because it emphasised the explicit sexual nature of Linnaeus’ system of botanical nomenclature and was thus considered unsuitable for polite society. The Loves of the Plants was published together with another poem, The Economy of Vegetation, a more general poem on scientific progress and technological innovation, of which Darwin as a prominent member of the Lunar Society of Birmingham was very much aware. The Economy of Vegetation expresses an evolutionary view of progress. A footnote to The Loves of the Plants contains the first outlines of Darwin’s theory of biological evolution, which he would then expand upon in his prose work Zoonomia. Erasmus Darwin’s is an adaptive theory of evolution and is thus oft referred to as Lamarckian, although as Erasmus preceded Lamarck, maybe his theory should be referred to as Darwinian! A posthumous poem of Darwin’s, The Temple of Nature, contains a full description of his theory of evolution in verse.

Writing this led me to the thought that maybe editors of modern scientific journals should require their authors to submit their papers in iambic pentameters or in Shakespearean blank verse, with the abstracts written as sonnets. It would certainly make reading scientific papers more interesting.

 

 

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Sorry Caroline but you were not the first, Maria was

Today is the birthday of Caroline Herschel, important member of the Herschel astronomical clan and significant astronomer in her own right, who was born 16 March 1750.

Caroline Herschel
Source: Wikimedia Commons

Throughout the Internet this anniversary is being acknowledged and celebrated, and quite rightly so, but all of those doing so that I have stumbled across, including such august organisations as the BBC, the Royal Society, NASA and ESA amongst other, have all being perpetuating a history of astronomy myth, namely that Caroline Herschel was the first woman to discover a comet. She wasn’t Maria Kirch was!

Caroline Herschel made her first cometary discovery, having been trained to sweep for comets by her brother William and being provided by him with her own comet sweeping telescope, on 1 August 1786, almost sixty-six years after the death of her fellow German female astronomer and the real first woman to discover a comet, Maria Kirch.

Maria Kirch, who I’ve written about briefly in the past, was the wife and working partner of Gottfried Kirch, who was a pupil of Erhard Weigel and who became the first Prussian state astronomer in Berlin in 1700. Maria and Gottfried had married in 1692. On 21 April 1702 Maria discovered the so-called comet of 1702 (C/1702 H1). You will note this is eighty-four years before Caroline Herschel discovered her first comet. Unfortunately for Maria, the sexist eighteenth century attributed the discovery to her husband Gottfried and not to her. Although Gottfried publically attributed the discovery to Maria in 1710 the official attribution has not been changed to this day.

Not only was Maria Kirch robbed of recognition of her discovery in the sexist eighteenth century but people too lazy to check their facts deny her achievement every time they falsely claim that Caroline Herschel was the first woman to discovery a comet.

 

 

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An orb by any other name would circle as smoothly

Alan Stern, the principal investigator of the New Horizons Mission to Pluto is calling for a new definition for planets in order to return Pluto to, what he and other see as its former glory, the status of a planet. The so called demotion of Pluto caused the release of strong emotions amongst the distant planet’s fans and the stunning success of the New Horizons mission added fuel to the flames in the on going debate. Many of those participating seem to be somewhat unaware of the fact that the definition of what is a planet has changed down the centuries and I thought I would write a brief guide to the changing fortunes of the term planet since its inception in antiquity.

It should be made clear that I shall only be talking about European astronomy and not any other traditions such as Chinese, Indian, Mayan astronomies etc. European astronomy/astrology has its roots in ancient Babylon. The Babylonian tradition was most concerned with the Moon and the Sun but the Babylonians were aware of the planets Mercury, Venus, Mars, Jupiter and Saturn, which they like other ancient cultures regarded as divinities. They tracked their orbits over very long periods of time and developed algorithms to determine their appearances and disappearances for omen astrological purposes. They don’t appear to have been interested in the mechanism of the planetary orbits. I’m anything but an expert on Babylonian astronomy/astrology and I don’t know if they had a collective name for them.

The direct inheritors of the Babylonian celestial interests were the ancient Greeks and they were very much interested in orbital mechanics and they also coined the term planet. For the Greeks all illuminated objects in the heavens were stars (aster, astron), as I explained in an earlier post. The stars as we know them were the fixed stars because they appeared to remain in place relative to each other whilst the sphere of the fixed stars rotated about the celestial axis once every twenty-four hours. It was of course the Earth that rotated about its axis and not the stars but the Greeks were not aware of that. The illusion that the stars, visible to the naked-eye, are all equidistant to the Earth is easy to experience. Just go out into the countryside were there is no light pollution and look up at the night sky on a clear night. You will see the ‘sphere of the fixed stars’, as experienced by the ancient Greeks. Comets, much rarer and apparently random, were hairy stars, the word comet derives from the Greek aster kometes, literally long-haired star. The five planets known to the Babylonians and the Moon and the Sun were all present on a regular basis but unlike the fixed stars they appeared to wander around the heavens and so they became asteres planetai that is wandering stars, from planasthai to wander. The Greeks had seven wanderers Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn. The Earth was of course stationary at the middle of the whole system and so was not a planet.

16th-century representation of Ptolemy’s geocentric model in Peter Apian’s Cosmographia, 1524
Source: Wikimedia Commons

Subsequent European cultures and the Islamic Empire inherited the Greek model of the heavens with its seven wanderers and nothing of significance changed down the centuries until the Renaissance and the advent of Copernican heliocentrism in 1543. Copernicus’s new model was of course a major upheaval. The Sun became stationary and the Earth became a planet wandering through the heavens. The Moon acquired a strange new status, no longer orbiting the centre, now the Sun, but orbiting the Earth. Heliocentricity took more than one hundred years to become establish and Copernicus’ upheaval brought no immediate change of terminology.

The heliocentric order of the heavens from Copernicus’ De revolutionibus 1543

The first change came in 1610 with the telescopic discovery of the four largest moons of Jupiter by Galileo and Simon Marius. Here we have four new celestial bodies orbiting a planet, as with the Moon, and not the centre of the cosmos. At first Galileo referred to them as stars or planets, leading Kepler, who was at first not clear what the four new objects were, to panic and fear that Giordano Bruno was right and that all stars had planets. This conflicted with Kepler’s own finite universe cosmology. He was greatly relieved to discover that the new planets were in reality moons and coined the term satellite from the Latin satillitem meaning attendant, companion, courtier, accomplice or assistant. Kepler was very fond of creating new scientific terminology. The term was not adopted immediately but by the end of the seventeenth century astronomers differentiated between planets and satellites, around the same time as heliocentricity became firmly established and the Sun finally ceased to be a planet and the Earth finally became one. Around the same time astronomers became convinced that the Sun was actually one of the ‘fixed’ stars.

We entered the eighteenth century with six planets, Mercury, Venus, Earth, Mars, Jupiter and Saturn and so it remained until the musician and amateur astronomer William Herschel shocked the world with the discovery of a seventh one, Uranus on 13 March 1781. The first new planet discovered in about four thousand years of planetary astronomy.

William Herschel. Portrait by Lemuel Francis Abbott 1785, National Portrait Gallery, London
Source: Wikimedia Commons

In the middle of the eighteenth century Johann Elert Bode published what is now know as the Titus-Bode law in which the distance of the planets from the sun seemed to fit an arithmetical series with a gap in the series between Mars and Jupiter. Herschel’s discovery of Uranus beyond Saturn fit the Titus-Bode series, which led the German astronomer Baron Franz Xaver von Zach to organise a systematic search for that ‘missing planet’ between Mars and Jupiter. In fact the discovery was made by the Italian astronomer Giuseppe Piazzi, who was not part of Zach’s search team but discovered Ceres on 1 January 1801, exactly, where it should be according to the Titus-Bode law and then there were eight. Interestingly Piazzi lost Ceres and Carl Friedrich Gauss developed a new method of determining planetary orbits, which allowed astronomers to find it again. Very soon other astronomers discovered Pallas, Juno and Vesta and there were now eleven planets. It was not long before it became clear that the four new celestial bodies were somehow different to the other planets and Herschel coined the term ἀστεροειδής, or asteroeidēs, meaning ‘star-like, star-shaped’, in English asteroid. These smaller wanderers were also known as minor planets or planetoids although it was first in the later nineteenth century, by which time several more asteroids had been discovered that these terms became established and the number of planets was once again reduced, not to seven but to eight!

Piazzi’s book “Della scoperta del nuovo pianeta Cerere Ferdinandea” outlining the discovery of Ceres, dedicated the new “planet” to Ferdinand I of the Two Sicilies.
Source: Wikimedia Commons

It was eight because in the mean time both the English astronomer John Crouch Adams and the French astronomer Urbain Le Verrier had predicted the existence of an eighth planet based on gravitational anomalies in the orbit of Uranus and on 23 September 1846 the German observational astronomer discovered Neptune, the eighth planet, based on the predictions of Le Verrier.

Urbain Le Verrier
Source: Wikimedia Commons

In the late nineteenth century similar anomalies in the orbit of Neptune led Percival Lowell to predict the existence of a ninth planet and he set up his own observatory to search for it. In 1916 Lowell died without having found his predicted planet. However in 1929/30 the young Clyde Tombaugh discovered Pluto, the ninth planet.

From: O’Hara, Elva R. (2006). Clyde W. Tombaugh: Farm Boy Reached for the Stars. Borderlands 25.
Source: Wikimedia Commons

As with Ceres and the asteroids Pluto’s planetary status was challenged by the discovery of other orbiting objects in the Kuiper belt outside of the orbit of Neptune from the 1990s onward. The discovery of Eris in 2005 led to a serious reconsideration of Pluto’s planetary status and famously in 2006 the International Astronomical Union introduced a new formal definition of the term planet, which removed Pluto’s planetary status and according to Pluto’s fans demoted it to the status of a dwarf planet. At the moment there are five recognised dwarf planets Pluto, Ceres (the largest asteroid), Haumea, Makemake and Eris.

Eris (center) and Dysnomia (left of center), taken by the Hubble Space Telescope
Image NASA
Source: Wikimedia Commons

As I said at the beginning the Pluto fan club has not given up the fight and are now proposing a new definition of the term planet, which would not only return Pluto to its planetary status but also apparently the Moon. I hope I have shown that the term planet has gone through quite a lot of changes over the last two and a half thousand years or so since the ancient Greeks first coined it and we can, I think, assume that it will go through quite a few more in the future in particular with respect to the thousands of exoplanets that astronomers are busy discovering.

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Filed under History of Astrology, History of Astronomy, History of science