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Still I wouldn´t say that "the appel idea" is a model but rather a mathematical equation of weight, force and motion - which, in your own opinion, "is not the heart of science". But when asked into this, you STILL refers to mathematical equations as the holy grail answer to everything. Well, never mind.
About ad hoc assumptions:
1. Scientists assumed a falling apple to be the result of gravitational attraction from the Earth.
2. Then they assumed this gravitational attraction to work over infinite distances. in spite weightlessness for a spacecraft and persons in it, is reached when leaving the Earth´atmosphere.
3. Then they assumed this gravity idea to work all over in cosmos. When contradicted in this, they assumed "dark matter" was the cause.
Edit
4. In spite that it is generally accepted in cosmological science that the Newtonian/Keplerian "laws of celestial motion" doesn´t count on larger cosmological scales, lots of scientists STILL assumes gravity to rule everything in the entire Universe.
In my opinion there are lots of assumptions right from the beginning and more are added along this path i.e. ad hoc assumptions.
And they put these assumptions into a mathematical model that then worked to *correctly* predict the motion of planets (as well as ordinary things on Earth). It even served to *predict* the existence and location of a previously undetected planet.
Yes, that is exactly how science is supposed to work.
Wow. You really think there is no gravity in space? Because astronauts are weightless? Really?