The sun warming the earth is an empirical fact that can be observed. It's a physical reality. This does not pertain to morals and is a different category from moral principles. Moral principles are not material objects and cannot just hang in mid-air like magic - they must have a source - and the only two possible choices are man or God. Name another possibility.
The nature of our existence.
We are here, therefore we had a source. One offered source of our existence is God. It is either true or false that God is the source of our existence.
If there is no God, then there is another source of our existence. That other source may also have absolutes, things that are the source of man, and which man may not be able to change. That is, they may not be "relative".
The absolutes possible due to the source of man's existence may be of the sort that we have come to know as "moral" things. According to our understanding of things, they would be "moral absolutes".
In fact, I contend liberty is just such an absolute. That it is a condition of our existence, it is moral, i.e., good, and it is absolute. You may disagree, but I doubt you'll get very far in making a case for the absence of liberty. I'd be very interested in hearing such a case, though.