Does the Past exist? Can it be changed?

Does the past exist?

Thinking about this carefully, it seems to me that only the present exists.  Certainly, traces of past events exist now, but now is the present moment, not the past.

To consider something we are looking at, what we perceive "now" depends on how far we are from the object we see.  Should we measure "now" as being our subjective sense?  Imagine we are looking at our home through a gigantic telescope situated a light year from Earth.  What is "now"? Our subjective sense, or something to be found only on the surface of the Earth itself?  And, I cannot see how we can link two places far apart with any one time moment, since we cannot find any way to coordinate the time between the two places.

So the whole idea of "now" is problematic.  I have not researched current philosophical notions relating to "now",obviously there might be some simple explanation.

There is another matter which interests me in this field of time: can the "past" be changed?  If Vadim Zeland's notion of the Space of Variations has any truth in it, it would seem possible.  It may also be useful to look at Richard Bartlett's "Matrix Energetics."  If the "past" does not exist, it's hard to see just what would be meant by changing it; but if it exists in some other universe, perhaps it is possible.  Changing the past is a pretty exciting idea.