# In geocentric models, did the Earth rotate?

In geocentric models, did the Earth rotate around its axis?

I assume that if the Earth rotated around its axis, then the Sun would not have to orbit around the Earth at a very fast rate, whereas if the Earth didn't rotate, then the Sun would have to orbit the Earth every day.

• Since the earth is flat, you would fall off if it rotates. – LDC3 Jan 31 '15 at 15:02
• You got it right! Earth was considered still. – astromath Jan 31 '15 at 16:31
• @LDC3 No one ever thought that Earth is flat. That is a myth. Every sailor can see with his bare eyes that the Earth is round, and everyone sees it when the round shadow of earth eclipses the Moon. Flat Earth was a myth invented by lying propagandists during the Darwin debate in the later half of the 1800th century. – LocalFluff Jan 31 '15 at 17:04
• @LocalFluff Your information is wrong, it goes back before Ancient Greece. Your myth pertains to the belief during the middle ages, not before. Many ancient cultures have had conceptions of a flat Earth, including Greece until the classical period ... most Pre-Socratics retained the flat Earth model. The flat earth belief started to be replaced after 330BC. From en.wikipedia.org/wiki/Flat_Earth – LDC3 Feb 1 '15 at 15:46
• Right the sun has to move across the sky very quickly. That is why he has a chariot drawn by horses. – Tyler Durden Oct 1 '18 at 22:19

In the historic geocentric models, and in particular in the Ptolemaic model, the earth is immobile in the centre of the cosmos, and the sphere of the fixed stars rotates around it once a day, carrying the sun, moon and planets with it. There were astronomers who realised that this apparent motion could also be explained, from a purely mathematical point of view, by assuming that the earth rotates on its axis, but they rejected this option. By the way, neither Ptolemy nor any other serious astronomer believed that the earth was flat.

• From what I read, Ptolemy was aware of Aristarchus of Samos's Earth orbits the Sun model, but he felt that an Earth rotating at close to 1,000 mph at Mediterranean latitude was unworkable as they'd surely feel it. He had a footnote that this was a model and the precise answer wasn't known, but he was in favor of Aristotle's model. People later took his model as gospel though, but he never felt 100% certain. – userLTK Sep 16 '16 at 21:09
• @userLTK. Could you give us a reference for this (page or chapter number in the Almagest)? And what do you mean by "footnote"? (Greek books never have footnotes). – fdb Sep 16 '16 at 22:29
• It's from memory. Will take some looking, but I'll see if I can find. – userLTK Sep 16 '16 at 23:58

The idea of a rotating Earth was the biggest hurdle for the non-geocentric ideas to overcome. If the Earth were rotating, wouldn't we notice it? Riding a horse is a motion which one cannot help but notice. And a rotating Earth would be much more violent, at 465 meters per second, as was estimated already in ancient times. Ten times a hurricane. That is obviously not the case. I can stand up on the ground without falling over, but I can't stand up on a galloping horse's back, people argued quite reasonably.

It requires careful cleverness to apply our gardening-evolved intuition to the reality beyond the fence. I personally find the Oberth effect to be unintuitive, that in empty space a push gives more acceleration to a fast object than to a slow one. With friction from roads, rails and air the opposite is true in our everyday life.

The resolution (certainly with ancient predecessors if documents are preserved) is called Gallileo's ship.

• This reads as conjecture of ancient beliefs rather than anything historical. I suggest editing to include citation that this was a hurdle to geocentrists--maybe even a quote from a historical source. – called2voyage Apr 6 '16 at 15:18
• Eppur si muove. – James K Sep 16 '16 at 18:41
• @called2voyage I don't really know what "reliable" source to invoke, other than Aristoteles and the couple of thousand of years of followers his ideas had. We must assume some basic education of the readers when we are posting here, mustn't we? – LocalFluff Sep 16 '16 at 22:04
• In depth discusion of Plato's model of the universe and in particular the question of whether the Earth rotates can be found in books.google.co.uk/… – James K Sep 17 '16 at 22:56
• The 14th Century scholar Jean Buridan considered the possibility of a rotating Earth, but ultimately rejected it; one of his arguments was that an arrow shot straight up should not fall straight back down, because the Earth would rotate under it during its flight. (Ptolemy may have used a similar argument.) – Peter Erwin Sep 18 '16 at 23:39