The general rule is that: Whenever you are talking about general things, like children, people, etc. don't use "the". However, when your are talking about specific people, like someones' children, use "the". This is a general rule and there are always exceptions, an example of which being your specific example. The best way is memorization though. Always say "the sun" and "the moon". For more information, I refer you to "English Grammar in Use" by Raymond Murphy. There are 4 complete pages dedicated to the definite article.

We say "the sun and the moon" because we have know which sun and moon we are talking about. I might ask "Does the planet Mars have a moon?" because I am not referring to a particular moon. You might hear the phrase "Too much sun is bad for the skin." In this case, the phrase that is left out is "Too much exposure to the sun is bad for the skin." It can be complicated and the only way to learn most situations like the above is just exposure to the language. Another example would be "water": "Do you have water?" - do you have any water. "I left the water back on the table." - I left the container of water back on the table. In this case, it is assumed that all people in the conversation know which water that is being talked about. There. I hope I haven't confused you too much.

Hi! I'm Ron in New York, USA and I am a communications engineer. I like very much to make new friends all over the world and to help people improve their spoken English. If you want to improve your English, I will be happy to help you! And if you want a new friend, lets be friends! :) My Skype: ron.jenkins86 My QQ: 1990992857