Marty,
Here’s an example I use to explain why some things in the Church can change, while others are unchangeable.
For example, in the house, the parents have the perogative of setting the bedtime. If they say it is 8:00, then the children are bound by the pain of sin to go to bed at 8:00. There is nothing in natural law that says children must go to bed at 8:00. Therefore if the parents so choose they can switch the time to 8:30 or whenever. The parents may even lift the bedtime entirely. As long as the child obeys his parents, the child does not sin. But if the child willingly disobeys, the child sins.
This is different say, from sassing back to a parent. This would always be a sin. Even if the parent did nothing about it, the sassing back would still be a sin. In this case the parent would also be guilty of remiss in their duties.
The respect owed to a parent is grounded in natural law, no man has the authority to change natural law. Therefore sassing back is objectively disrespectful. And so is disobedience. But in order to disobey, you must be given an instruction first. Those instructions are the perogative of earthly authorities.
So, the Church has the authority to tell Christians how to observe the Sabbath. Though the Sabbath was “changed” by God on his resurection. The Spanish speaking world remedied the whole stupid muddle over the Sabbath-day malarky. They start the week with Monday and Sunday is, you guessed it, the seventh day.
As for Galileo, you have to read up on that history. What your relatives are reciting is possibly some distortion of the truth. My understanding is that Galileo wasn’t punished for asserting that the earth revolved around the Sun, but he was punished because he began to make theological assertions based on his scientific evidence.
Here’s an example of the difference between a scientific assertion and a scientific assertion made theological. It is plane scientific assertion to say, “The earth revolves around the sun”. It can be made theological to say, “The earth revolves around the sun, therefore we aren’t at the center of the universe, therefore God doesn’t really give a hoot about us.” I don’t know if this is exactly what Galileo was saying, but this is a general example of how one could cross a line as Galileo did.