In physics the word theory is used synonymously with mathematical model or mathematical framework. The theory is a mathematical construction that can be used to describe physical phenomena. A theory should, at least in principle be falsifiable, that is make predictions that can be tested.
People who are not trained in physics take theory to mean either “hypothetical” or loosely an “idea”. One may hear “but it is only a theory”, which takes the physics use of the word theory out of context.
A theory, in the sense of modern physics must by definition be phrased in mathematics. We need something to mathematically manipulate and calculate things that can be tested against observation. Without the mathematical framework it is hard to judge if an “idea” has any merit or not.
Often by theory physicists may have something a little more specific in mind, they often mean a specified action or Lagrangian. Most of physics can be stated in terms of actions and so it usually makes sense to start there. Again the action or equivalently the Lagrangian are mathematical notions.
Even today people confuse astronomy and astrology. It is not hard to see why when almost every newspaper has a horoscope and lots of adverts for astrology phone lines. Lets set the record straight.
Astronomy the scientific study of celestial bodies, for example the Sun, planets, starts, comets etc. The science is based on observation of the celestial bodies and the application of physical laws to such bodies. Mathematics and physics are essential in astronomy.
Astrology the belief that the position of celestial bodies influences the personality and human affairs. It is based on superstition and no physical mechanisms have been established. The superstition does not apply the scientific method and in no way follows modern scientific principles.
In short, astronomy and the closely related astrophysics and cosmology add to the human understanding of nature and our place in the Universe. Astrology is a superstition that people exploit to make money.
It is of course true that the origins of astronomy lie in astrology. Careful observations and recording of data was necessary in order to write astrological charts. One could equally argue that chemistry owes a lot to alchemy. But we have come a long way in our thinking and philosophy. Astronomy and chemistry are sciences.
Please do not confuse the two, it is rather insulting to all astronomers!