Not been on here for a long while but a thought has occurred to me that I can’t find an answer to - now it’s up to you people . . .
As I am a Physics and Astronomy teacher in a secondary school I’m hoping that this question isn’t as stupid as it sounds! Let me try to elaborate. Jupiter is the biggest planet, and Jupiter is the king of the Roman gods, which at first sight makes some sense. However it is my understanding that whenever Jupiter acquired its name we wouldn’t have known it was the biggest planet - we’re talking well pre-telescope and gravitational theory. So why did it get this name and is it’s size just a coincidence?
I can make a case for Venus been named after the god of love as it is the brightest and is associated with sunrise and sunset. Similarly I can see the fastest planet in the night sky been named after Mercury, messenger of the gods. Mars’ red tinge is visible to the naked eye, so the god of war also makes sense. There might also be a reason why the planet Saturn is linked to the god of agriculture, although I can’t think of it. But why is Jupiter Jupiter?
Hope I’m not being too stupid, and many thanks for any help.
Yes, I suppose so, but I’m still not happy (see above). Although, as I say, they had no idea whether any of them was ‘big’. They were just ‘planetos’ - wandering stars as opposed to fixed ones.
Why do you assume that? We can pretty easily demonstrate that something further away appears to move slower at the same actual speed. Today we could actually put it in mathematical terms in terms of degrees subtended per unit time and Kepler’s Laws but in they ancient world they were empiricist and would see a ship on the horizon “moving slower” than a ship near the harbor.
So assuming the planets actual speed are all the same (no Kepler Laws) we know that Jupiter appears to move significantly slower than Venus and so we presume it is much further away - but wait, it is almost as bright as Venus so its apparent size must be the same. I know the further away an object is the smaller it appears so Jupiter must be the biggest planet to have that brightness at a distance.
I betcha most ancient would agree with that line of logic.
Yup, I’m liking your thinking, although there are a few assumptions in it. The idea that distance from the Earth in the Ptolemaic solar system is based on speed works for the planets, but I don’t quite see where the Moon (1st out) and the Sun (4th out) work with this. But given that, its brightness would work for the question.
Does anyone know if there is actual evidence for this line of thought in Greek or Roman texts?
I think bup has it right. Jupiter was named Jupiter because the other planets weren’t.
Mercury, Mars, and Venus had characteristics that made them stand out. It’s the same as giving people in your group nicknames. You focus on the one thing that everybody first notices about them.
Then you get to Jupiter. It’s bright, but not as bright as Venus. Slow, but not as slow as Saturn. It has no color, no marks that could be seen at the time, nothing specially individualistic.
What can you say of it? It’s important, a bit ponderous, maybe large (since people could work out the size and distance and brightness relationship). Why not identify it with Jupiter or Zeus?
You might try to make that case for Saturn, but you can’t have the king of the gods be quite as dim and secondary. How about the father of Jupiter, worthy but a bit out of the picture.
Thinking of the names as nicknames rather than honorifics brings them down to earth. (Ugh. Pun. Must stop now.)
Mercury, Venus, and Mars were naturals. Fastest in the sky for Mercury, brilliant Venus adding beauty to the morning and night skies. Bloody Mars. What’s left is the brightest of the wanderers, must be important so Jupiter it became.
Pre-Kepler our assumption were IIRC*:
Planets move at the same actual speed
Moved in circles
Brightness was associated with size
All or which were reasonable although wrong.
Luna is obviously the closest because it occults everything else including the Sun.
The Mercury/Venus/Sol relationship is interesting. It is obvious that Mercury and Venus are unique in that they always lie within the same area of the sky as the Sun. Later this would be resolved by having Mercury and Venus revolve around the Sun and the Sun revolve around the Earth (which introduces a whole list of philosophical problems). Now I could point out that Mercury and Venus transit the Sun which to the ancient would mean they are between us and the Sun & since Mercury is faster than Venus there you go: Luna - Mercury - Venus - Sol
The problem with that is that it is still an open question as to if the ancients did observe these transits.
tl;dr version, given the unusual relationship of Mercury and Venus position in the sky re: the Sun, their apparent speeds and the fact that the Moon occults all of the other bodies, assuming everything revolves around the Earth the Luna - Mercury - Venus - Sol order is the most reasonable.
*I’d have to dig up an old book that I packed away to properly cite this.
The first Greek cosmology was by Anaximander (610-646 BC). He put the stars closest to the Earth, then the Moon, then the Sun, which is rather the opposite of the assumption that the brightest objects must be closest. I haven’t found a reference to Anaximander’s concept of the planets if any.
I think it’s easy to overthink this because of all that we know today about the planets. The planets were named long before people knew anything more about them than their brightness, color, and apparent speed. I think bup and Exapno probably have it right. Venus is the brightest, but it is only visible for part of the night, and thus can’t be the ruler of the gods. Jupiter is the brightest planet that can be visible all through the night and so is the best choice for the big boss.
Apparently the identification of Jupiter with one of the major gods goes back to the Sumerians. According to this site, the Sumerian names were translated to Babylonian, and from Babylonian to Greek, just as the Greek names were translated to Latin.
I won’t vouch for the accuracy of this site, since it is written by an astrologer, but it states that the ancient Egyptian named Mars, Jupiter, and Saturn for different aspects of the sky-god Horus.
Of course all this pre-supposes that there was a reasoned decision. It could just have been an arbitrary allocation of names.
I can imagine a group of astrologers/astronomers sitting out the night with a couple of bottles to keep the chill off; watching the stars and planets and taking it in turns to name the planets. Later on they would announce their decision to the faithful and the rest is history.
It is pretty common for the naming of things to be lost in time and for clever people to come up with a reason years later.
What reason did the ancients have to assume that planets moved at the same speed, though? They don’t; Jupiter is orbiting at about four tenths Earth’s speed (and in fact all the planets appear to move at speeds that are correlated to the size of their orbits, with the nearest planets moving faster.)