# Problem with Tychonian model of geocentric universe?

Just like other proposed models of universe in history, Since Tycho's aren't being used, there MUST be any flaws in compare with widely agreed today understanding

Is there any detailed reference criticizing his concept? Moreover in recent years? I've been searching for awhile by now but couldn't find one

If universe is both isotropic & homogeneous, shouldn't Tycho model be just fine for now? We simply take current model, but using earth as perspective. The rest still maintain the same

Or no?

edit: I think i missed Tycho's whole idea. I thought all the time he was referring to rotating earth, but after @James answer and some reading it doesn't seems so. My central question is on the assumption that his system have rotating earth. If it does, what likely be the problem?

• cross-posted in Space SE I think it belongs here and voted closed or deleted there.
– uhoh
Dec 24, 2020 at 10:41

Tycho was concerned that the Earth must be too "heavy" and "sluggish" to move. So his system solves this problem. It turns out not to be a problem at all. The problem it solves is "we know that the Earth can't move, so how can we describe the motions of the planets". As the Earth can move, this isn't a problem. So the whole purpose of Tycho's system is gone.

It is a kinematic, not a dynamic model. It describes the motion of the planets but does not give any reason for that motion. This is also the case for Kepler's and Copernicus model. But Newton's model, on the other hand, describes planetary motion in terms of forces.

Now you can do Newtonian mechanics in any coordinate system. It is just more convenient to do it in an inertial system. When you do Newtonian mechanics in a barycentre based inertial frame you derive Kepler's orbits. But if you do Newton's mechanics in an Earth based non-inertial coordinate system, you get Tycho's model. (Or something very like it, but with elliptical orbits)

There are other problems:

• It treats the Earth as "special". This treatment is unjustified. So it isn't isotropic and homgenous: The Earth is still, everything else moves about it.
• It requires the stars to orbit the Earth once a day, this would require them to move faster than light.
• It does not predict things like Foucault pendulum, which demonstrates a rotating Earth.
• It does not predict stellar parallax.

If you start from the axiom "The Earth cannot move" then you must get Tycho's system, or something like it. But if that axiom is rejected then you can get a much simpler model with an inertial coordinate system in which the sun is almost stationary.

Some of these can be "fixed", for example by allowing for a rotating central Earth. But remember the whole point of Tycho's model is to have a stationary Earth. If you can have a rotating Earth, then why not a moving Earth too?

• Thank you in advance. I just updated the question, would you able to update yours with similar assumption of tychonian model BUT with rotating central earth? Dec 24, 2020 at 11:52
• Why, what is the point? You can have any coordinate system. Why not the dwarf planet Ceres? Why not Jupiter's moon Io, what is so special about the Earth? The whole reason for Tycho's system is to have a motionless Earth. If you can have a rotating moving Earth, then why not use an inertial frame? Dec 24, 2020 at 11:54
• Ah yes correct. This question can be closed now, it's clear already. Thank you Dec 24, 2020 at 12:05
• Hi @cyanide congratulations on your first question and answer, and welcome to Stack Exchange! I usually wait a few days before accepting an answer because that encourages others to post answers as well, but it's up to you. But accepting an answer is not called "closing" it, you can un-accept at any time and accept an other answer. "Closed" means a question has a problem and answers can not be posted. Also, thanks for deleting the copy in Space SE.
– uhoh
Dec 24, 2020 at 17:38
• For future reference, there is also History of Science and Mathematics SE
– uhoh
Dec 24, 2020 at 17:39