I have been reading through the questions and this seems to pop up quiet a bit. The question I have is not "how" but "how do we know its tilting at 7.25 degrees, when actually the sun could be positioned at 0 degrees and it is infact us that is tilting that 7.25 degrees. How can they say the sun is tilting when there is no ground in space to stand on. How do they know which way is up/down?
You are correct that there's no absolute frame of reference. However, in our solar system, the plane of the earth's orbit was chosen (arbitrarily) to be the "zero" reference, and other orbits and revolutions are measured relative to it. So 7.25 degrees refers only to its rotation relative to earth's orbital plane.
Prof. James Gort