This is my request for everyone to use capitalisation for proper nouns. I've seen it misused in many questions and their respective answers on our main Q&A and they often cause problems with understanding the nature of the question. I won't list any examples not wanting to explicitly point out any of those that did, but a short browsing through the list of the questions should prove me right easily.
Why is this so important in astronomy? Because this is the spelling differentiation between a galaxy and the Galaxy, a solar system and the Solar system, a sun and the Sun, a moon and the Moon. There is even a whole Wikipedia page about it, and the Internet is otherwise rather filled with similar posts describing the difference between them.
For example, if you mention in your question "Why does galaxy rotate?", we have no way of knowing if you're asking about a galaxy, i.e. galaxies in general, or the Galaxy, our very own Milky Way (yes, the Galaxy is a perfectly correct term for it). Answers for it might be different for whichever might be you're inquiring about. And same goes for spelling and grammar rules in answers. We need to know what you meant, so please be precise and state so with the use of determiner the and capitalisation of proper nouns and common nouns derived from proper nouns.
Quoting from Curious About Astronomy post on What are the names of the earth, moon, sun, and solar system?:
The name of our planet is the Earth. The name of our moon is the Moon. The name of our solar system is the Solar System.
Notice that I capitalize them, because when used as names, they are proper nouns. This also helps us distinguish between the planet Earth and earth (meaning soil), between the Earth's Moon and moon (meaning the natural satellite of a planet), and between our Solar System and any other solar systems (since any system containing a star and a planet or a planet-forming disk can be called a solar system.)
I would also ask of all of you to edit your own contributions if they suffer this inconsistency, suggest edits to any such user contributions you feel confident you know what their authors meant, or if that is unclear, request from their respective authors to clarify what they're inquiring about in their questions or writing to explain in their answers.
Feel free to chip in with your take on all of this in the answers below. Thanks!