The planets weren’t named for the gods. The planets WERE the gods. You look up in the night sky and you notice that some of the lights are wandering around. Clearly they must be beings who live up in the sky! It would be like seeing campfires off in the distance and deducing there must be another tribe living over there.
How the different lights look and behave tells you what kind of people they are. Saturn is faint and hobbles along slowly like an old man. Jupiter is bright and moves sedately across the entire sky with the dignity befitting a king. Mars is red like blood – he must be a warrior. Venus is beautiful but shy – she only comes out in the morning and evening. And Mercury is fast – he must be running back and forth carrying messages.
If I remember correctly, there are times when the apparent size of Jupiter exceeds that of Venus, so regardless of actual size, Jupiter is often the “largest” planet in the sky after the moon and the sun.
The classical Greeks are far too modern to be reliable sources for the origins of names of the planets. We need to look at reconstructions of stone age beliefs. When They Severed Earth From Sky gives a good accounting of this theory.
Classical planetary names were initially adopted from Babylonian ones (along with numerous other features of Babylonian astronomy), so there’s no point arguing about how or why the ancient Greeks chose to assign certain deities to certain planets. The ancient Greeks were not the ones who came up with that idea.
The deities assigned to the planets in Babylonian astronomy were Ninib, Marduk, Nergal, Ishtar, and Nabu for Saturn, Jupiter, Mars, Venus and Mercury respectively. Greek astronomers adapted those identifications to correspond to the deities that seemed most similar in the Greek pantheon.
E.g., Ishtar was a female deity of fertility and love (among other things), hence Aphrodite/Venus; Nergal was a fiery deity of destruction and war, hence Ares/Mars.
How and why the ancient Mesopotamians originally came up with the particular deity/planet associations that still persist today is by no means clear, partly since the system probably originated at least 4000 years ago. But there are some informative speculations in Jastrow’s Aspects of Religious Belief and Practice in Babylonia and Assyria.
A more up-to-date discussion is Francesca Rochberg’s “What is the nature of the heavenly bodies?” in D. Snell’s Companion to the Ancient Near East:
I am pointing out that, as a group, we are awfully quick to throw out speculative answers that seem reasonable on the surface, when perhaps it would be wiser to look for actual supportable answers based on actual solid information first.
You contend that it is a natural assumption that the planets travel at different speeds. If you did not know about Kepler’s Laws or the inverse-square law of gravity then why is that a natural assumption?
The assumption that all planets travel at the same speed is a much more natural one because you don’t have to answer the question of WHY they travel at different speeds.
Wait, what? The planets as perceived in the sky with the naked eye most certainly do “travel” at different speeds among the fixed stars. That’s no “assumption”, that’s a plain and simple observation.
Look at Venus, for example, from one night to the next over a few days and you’ll notice that it quite detectably changes its position with respect to the fixed stars around it. It takes much longer to see the same kind of position shift for Jupiter or Saturn, say.
That’s what the ancients meant when they spoke of some planets being “fast” and others “slow”.
Now, if you’re trying to put that observed position shift into some kind of kinematic physical cosmology, where you develop a model of orbital motion to geometrically describe and predict the position changes you see in the planets, that’s another matter. In that case, you do have to figure out some kind of hypothesis about how fast the planet’s revolving and how far away it is, and tweak the model so that it actually corresponds to what you see in the sky.
But there’s certainly nothing “natural” about assuming that that intrinsic angular speed would be the same for all planets. Some ancient geocentric models assumed it was and others did not.
I think we’re far too liable to project back onto the pre-Greek astronomers our own concepts of 3-D orbital models where the planets revolve in space at different distances from a common center (or focus if we’re being post-Kepler about it).
There’s no evidence that the ancient Mesopotamians had any geometric models of planetary motion at all. The sky for them was just this surface (not even necessarily a spherical surface) upon which a bunch of bright lights moved in various regular or irregular “paths” that were evidently cyclical and periodic in nature, and thus could be modeled numerically if you accumulated enough data to figure out the patterns.
First of all, I never said same angular velocity. The assumption is the same linear velocity. And as I stated above, the ancients would be familiar with the idea that two objects with the same linear velocity would have different apparent velocities at different distances.
Because it’s the default assumption, and the one consistent with any observations they might have made.
No, you have to answer the question of why they travel at the same speed. Nothing else they might have observed in nature would do that. Waves on the beach, horses, thrown objects, and so on all travel at different speeds.
