I know he explained gravity but I don’t understand how Newton’s work linked physics and astronomy? Thanks in advance
Here’s the answer in Newton’s own words (from the introduction of Book 3 of his "Principia"). Slightly re-written from a published translation of Newton’s original Latin:
Rules of Reasoning in Science:
Rule 1: We are to admit no more causes of natural things, than those that are both relevant and sufficient to explain them.
Rule 2: Therefore, to the same natural effects, we must so far as possible assign the same causes.
Rule 3: The properties of objects which are found to belong to all the objects within the reach of our experiments, are to be assumed to be universal properties of all objects in the universe.
Those ideas may seem "obvious" today, but they were completely different from ancient Greek and medieval science, which for example assumed that the moon, planets, and stars were made something completely different to any material on earth, with different properties, and moved under the direct control of God and Angels, not according to the same "laws of nature" that applied to objects on earth.
Newton was the first person who explicitly said that the whole universe followed the same laws of motion and gravitation as objects on earth, and he used the most up-to-date data that was available from the newly invented use of telescopes in astronomy to demonstrate that assumption was correct – for example by showing that the orbits of the moons of Jupiter could be explained by the same law of gravitation that applied on earth and to earth’s moon.
Of course Newton got some of the details completely wrong. For instance he gave as an example of Rule 2: that "light of the sun has the same cause as the light of fires on earth". But making that wrong assumption is much better science than saying the sun shines because God makes it shine (the Bible ) or the sun shines because it is made of a material that is intrinsically shiny, and which doesn’t exist on earth (Aristotle).