By Tom Anderson
A long way to go.
Picture courtesy Wikimedia Commons.
My fellow SLP writer Andy Cooke has been known to opine his annoyance with pub quizzes where the ‘right’ answer is ambiguous. There are a number of ways that this can happen.
My problem is when a pub quiz question crosses one’s specialist subject. Firstly, one is then under pressure by one’s teammates to get it right, and secondly, one basically has to guess whether the quiz setter wants the ‘actual’ answer or the popular misconception simplified one. I still have vivid memories of being put on the spot by the question: “Which chemical element has the highest melting point?” The correct answer could be either tungsten or carbon. Carbon’s is technically slightly higher, but it depends on what allotrope and, under normal conditions, it sublimes directly to a gas rather than melting. I guessed tungsten as it was a more clear-cut answer (it’s also the one Google will tell you) and got it ‘wrong’ as their answer was carbon. Aaargh.
So that was my story, but Andy’s is more relevant for this article series. Andy’s complaint is when a pub quiz question is phrased in a way that basically amounts to: “Guess which year my encyclopaedia was published.” For example: “How many moons does Jupiter have?” Astronomers continue to discover new ones every year. There are the four big Galilean ones that were first discovered in the 17th Century and can be seen even with a small telescope or good binoculars, and some people will just say four based on that. The Galilean moons also make up well over 99% of all the mass orbiting Jupiter, and were the only ones to be discovered for almost 300 years more. However, there are also many smaller moons, the first of which, Amalthea, was discovered in 1892. A total of 13 moons had been found by the time Voyager probes reached Jupiter in 1979, and then the Voyagers discovered three more.
Trajectory of the two Voyager probes.
Picture courtesy Wikimedia Commons.
The figure of 16 moons is the one that appeared in post-Voyager books which I learned from at school (and elsewhere) when I was growing up, and it is still the one I mentally default to. However, subsequent probes and observations have found even more, and today Jupiter is recorded as having 95 moons. We are now reaching a problem of classification and blurry edges, because Jupiter also has rings (albeit far less spectacular than Saturn’s) which are comprised of dust particles, and lots of tiny ‘moonlets’ shed from the moons. At exactly what point does one say an object is small enough that it no longer counts as a moon? Furthermore, do the probes that have orbited Jupiter count as moons? Therefore, then, do all the artificial satellites orbiting Earth count as moons?
This is a reductio ad absurdum, but hopefully it illustrates that, in fact, there can never be a clear answer to the question: “How many moons does Jupiter have?” For much of history, though, this is the sort of question where it was assumed that there was a clear answer. Furthermore, as with the period between 1610 and 1892 when there were four known moons, sometimes we can go decades or centuries with a question seemingly ‘settled’, only for it to be reopened again. That’s science for you. It causes problems for people writing textbooks who end up accidentally perpetuating outdated information, as the expensive books are used for many years to come (many of my astronomy books were from the 1970s or 80s). But it also causes problems for writers writing period pieces.
Kids Today™ will grow up being taught that there are eight planets in the Solar System. They share this distinction with their ancestors who went to school between about 1846 and 1930, almost a full century. That time period describes the time between the discovery of Neptune by Johann Gottfried Galle and Heinrich Louis d’Arrest, and the discovery of Pluto by Clyde Tombaugh. In fact, all these scientists were not simply looking blindly, but were knowingly searching for objects that seemed to be gravitationally perturbing the orbits of the known planets. To explain this, we have to go back to the beginning.
The Pluto Paradox.
Stupid picture editor.
Picture courtesy Wikimedia Commons.
This Pluto.
Picture courtesy Wikimedia Commons.
Since prehistory, humanity has been aware of the existence of seven ‘lights in the sky’ that moved against the background of the (seemingly) fixed stars. The two largest are, of course, the Sun and Moon. I feel we don’t talk enough about how the Moon just happens to be 400 times smaller than the Sun and also 400 times closer, so the two are exactly the same size in the sky. You can talk about the weak and strong anthropic principles till the cows come home (or jump over the Moon) but I feel that one is a way bigger and more obvious factor than anything about liquid water or the Goldilocks Zone or whatever. Even if one thinks it’s pure coincidence, it is a bit troubling when one considers just how much of our cultures around the world take a dualist cue from the idea that the Sun and Moon, gold and silver, are equal and opposite rulers of the day and night. What if there are other intelligent races in our galaxy, but humans are the only one who have a Moon like this, and everyone else thinks we’re weird for the whole dualist culture thing?
That’s a question that is too big for this article. Anyway, after the Sun and Moon, the other five ‘lights in the sky’, the planets or ‘wandering stars’ known in antiquity, are Mercury, Venus, Mars, Jupiter, and Saturn. It’s not quite as simple as that, because it wasn’t always to the ancients that the Morning Star that precedes the sunrise and the Evening Star that follows the sunset were the exact same object – Venus. However, it’s certainly possible that one reason for the veneration of the number seven as mystical in several cultures is related to those seven celestial bodies. The ancients didn’t really know what the planets were, but there were plenty of theories and speculation in many cultures around the world.
It is worth noting that, in those days before light pollution, it was much easier to see more detail in the sky, even for the average person. For example, the Milky Way stands out in such a dark night sky, and was given various mythological interpretations around the world (some of them surprisingly similar). The name ‘galaxy’ comes from the Greek word for milk – gala – and the ancient Greeks related the Milky Way to a stream of milk from a goddess suddenly realising she had been breastfeeding Heracles and then pushing him away. Hindus relate the Milky Way to both a dolphin and a river, the Maori to a canoe, the Babylonians to the trail of the dragon Tiamat, the Cherokee to a stream of stolen cornmeal dropped by a dog, the Chinese and their neighbours to the Silver River of Heaven, and the Finns to a stream of migrating birds. People today, on the other hand, have probably never even seen the Milky Way at all, which is just sad.
The dark sky also made it more obvious that the Moon is a sphere, because the darkened side can be seen outlined against the starry background. The Moon also wobbles around slightly and reveals a bit more than one side at a time. Indeed, the Moon being round was one clue to the Earth being round. The Greeks also showed that the circular shadow of the Earth on the Moon during a lunar eclipse suggested a round Earth, whereas a flat disc Earth would leave an elliptical shadow on the Moon. Of course, these were the days before attention-seeking morons on the Internet were invented.
Anyway, the point is that from antiquity right up until the 18th Century, there were only five planets, or six counting Earth. It is interesting to reflect that all the controversy over geocentrism vs heliocentrism happened in the absence of discovering more planets, which would be a more obvious death knell to the idea that the ancients had known everything. It wasn’t until well after the whole Earth round thing was settled that William Herschel discovered Uranus in 1781. Herschel initially thought Uranus was a comet, but others found that it was a planet and this was accepted by 1783. The discovery of Uranus might have been more shocking or controversial in a different era, but this was the Age of Enlightenment, and the idea that new discoveries might tear down the old order were very much in keeping with the spirit of the age.
What was more controversial was what to name it. Herschel wanted to name it Georgium Sidus after King George III, not merely to suck up but with interestingly forward-looking reasoning: “In the fabulous ages of ancient times the appellations of Mercury, Venus, Mars, Jupiter, and Saturn were given to the Planets, as being the names of their principal heroes and divinities. In the present more philosophical era, it would hardly be allowable to have recourse to the same method and call it Juno, Pallas, Apollo, or Minerva, for a name to our new heavenly body. The first consideration of any particular event, or remarkable incident, seems to be its chronology: if in any future age it should be asked, when this last found Planet was discovered? It would be a very satisfactory answer to say: ‘In the reign of King George the Third’.”
Again, that is the values of the Enlightenment at work, the idea that obviously one wouldn’t use a mythological name from a vanished age. (Of course, those are only the name used for the planets in English, Roman mythological names with Greek roots; the planets have other names in other cultures which named them long ago). Unsurprisingly, other nations were not too keen to name the new planet after George III (though in hindsight, this would have been a great way to annoy the Americans once they started building Voyager probes) and many argued the planet should be named for Herschel himself instead. Others did suggest mythological names; a Swedish astronomer, Erik Prosperin, suggested Astraea, Cybele, and, confusingly, Neptune. Neptune almost caught on thanks to some British astronomers thinking it would do nicely to commemorate the Royal Navy’s victories during the American Revolutionary War (see, they’re thinking along the same lines as me now).
Other mythological names suggested included Minerva and, indeed, Uranus, proposed by the German Johann Elert Bode. Bode’s logic was that, as Saturn was the father of Jupiter, Uranus was the father of Saturn in mythology so there would be a pleasing symmetry to it as one moved farther out in the Solar System. Unfortunately, Bode had confused Latin and Greek; Uranus was the father of Cronus who was the father of Zeus, whereas Caelus was the father of Saturn who was the father of Jupiter. We should really be calling the seventh planet Caelus for consistency, but it was Bode’s name that caught on – thus dooming hundreds of classrooms to dirty jokes. Interestingly, in 1789, a chemist named Martin Klaproth decided to lend his support to Bode’s name suggestion, and used it for a new element he had just discovered – uranium. In the long term, Klaproth’s discovery, initially thought just to be a useful source of yellow pigment, would change the world even more than Herschel’s. Just think, if Bode had thought to look up the mythology first, we would now be building nuclear reactors and weapons that run on caelium.
The name controversy for Uranus actually raged on for as much as 70 years before Bode’s choice won out. I have even found an atlas on the excellent David Rumsey Map Collection from the late 1840s, which includes the eighth planet, yet still refers to Uranus as ‘Herschel’ (and the eighth planet, our Neptune, as ‘Leverrier’ – more on that in a moment). There was also a fight over what astronomical symbol to give Uranus. These symbols, better known from the world of astrology (from the days when the line between astronomy and astrology was blurry) also have related symbolic meanings, such as Mars signifying male and Venus female, or referring to chemical elements in alchemy. The German astronomer Johann Gottfried Koehler suggested associating Uranus with the then recently discovered element platinum (I suppose uranium was too obscure at this point) and invented a new alchemical symbol by combining those for iron and gold (as platinum is known as white gold). There were other proposals, so this could easily have turned out differently.
Uranus has a very long ‘year’ as it orbits so far out from the Sun, taking 84 years to complete an orbit. By the 1840s, almost one whole Uranian year had passed since the planet’s discovery, and observations suggested that the orbit was being gravitationally perturbed by some mysterious extra planet further out. Several astronomers tried to mathematically model and predict where the eighth planet might be, including Urbain Le Verrier in France and John Couch Adams in Britain. Using Le Verrier’s calculations, Galle and d’Arrest were able to pinpoint the new planet. It was a sensational breakthrough and an important piece of evidence for Newtonian gravitation; a fellow astronomer, François Arago, said that Le Verrier had discovered a planet “with the point of his pen”. Interestingly, there were proposals to name the new planet Leverrier rather than after the two men who had used the telescope. Much less controversially than Uranus, however, the mythological name Neptune – fitting for the planet’s blue colour – was soon adopted.
Thus, for almost a century, everyone knew there were eight planets in the Solar System. However, more gravitational perturbations suggested there might be a ninth – referred to as Planet X. In 1930, Clyde Tombaugh finally found another planet and it was named Pluto. This was another suitably mythological name, but also honoured the initials of astronomer Percival Lowell, who had led the search for Planet X before his death. Almost immediately, however, others began to question whether Pluto was really a planet. It was soon found to be too small to be responsible for the gravitational perturbations that Planet X had been meant to explain, and its mass was revised down repeatedly as more accurate observations came in. Finally, after other objects of similar or larger size were discovered in the distant Kuiper Belt beyond Neptune, the International Astronomical Union finally pulled the plug in 2006 and demoted Pluto.
How many are planets?
Picture courtesy Wikimedia Commons.
This is still controversial today, but it’s not widely known that it wasn’t the first such controversy. In 1801, Giuseppe Piazzi had discovered Ceres and it was claimed for an eighth planet after the still fairly recent Uranus. However, Ceres is far smaller than the other planets, and its classification remained contentious. Nationalism came into this, as Ceres had been discovered in the middle of the Napoleonic Wars, and unsurprisingly British astronomers tended to be more dismissive of what was seen as an attempted copycat on Herschel’s discovery. Ceres continued to be variably cited as a planet well into the 19th Century, but was eventually demoted to merely the largest asteroid. However, Piazzi had the last laugh in 2006 when the new intermediate category of dwarf planet was created, including Ceres as well as Pluto. Nationalism probably also played a role in Pluto’s status, with Americans wanting to keep it a planet, and those biased against America having another reason to demote it.
Pity the period writer who mentions astronomy and has to remember how many moons Jupiter or Saturn were known to have had in 1821 or 1946. But pity more the Alternate History writer. If writing a timeline with a POD before the late 18th Century (such as my own Look to the West , then all the classical planets, the four Galilean moons of Jupiter, Titan etc. will have the same names as Our Timeline (OTL) – but anything discovered afterwards will most likely have a different name. After all, look at all the proposed names for Uranus in OTL! In Look to the West, the seventh planet is Dionysus, the main asteroids in the asteroid belt are referred to as Trojans (a name, confusingly, used for a different group of asteroids in OTL), Ceres is named Agamemnon and so on.
Keeping track of all this is a nightmare, naturally. And that’s just our solar system, and I haven’t talked about the moons of Uranus or Neptune. The Star Trek novel Federation (from 1994) includes a segment set in the 2070s which has habitat domes on Jupiter’s moons, but also states that 100 exoplanets (planets orbiting other stars) have been discovered so far. In reality, here at the time of writing in 2023, a whopping 5425 exoplanets have been discovered! This is a good example of failing to foresee a sudden, rapid increase of pace in one field of science. In fairness, in 1994, it was still a slightly controversial question whether exoplanets existed at all, with several claimed ones later being disproven.
So, good luck keeping track of all those new planets. Three hundred years ago, it was possible for writers to invent fantastical islands like Lilliput and set stories on them. One hundred years ago, the world was mapped, but who knew what lurked beneath the clouds of Venus, so Flash Gordon or Dan Dare could find whatever the writer wanted there. Today, we still have works of science fiction that invent whatever planets the author wants orbiting Alpha Centauri or Epsilon Eridani, but pretty soon we’re going to be living in a time where we know all the planets there and what they’re like. We will no more be able to invent planets there than we can invent extra islands off the coast of Virginia today. And then we have to remember that our ancestors still had blank space there they could fill in with their imaginations, lest we forget – just as we have forgotten those who could look up and see the Milky Way in the sky.
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Tom Anderson is the author of several SLP books, including:
The Look to the West series
among others.