|MadSci Network: Physics|
In order to describe an object's placement in the Universe, you need not only three spatial dimensions, but another dimension -- time. As you pointed out, this is because objects in the Universe are not static. They move. The only way this change can be described is if the Universe is described by the 4-dimensional continuum called "spacetime," which was described by Einstein's Theory of General Relativity in 1915. Space and time are intimately related, though in our daily experience, they seem to be separate. This is because effects like time dilation (where time seems to slow in a moving frame compared to the reference frame of an observer) only become apparent at high spacial velocities. However, careful measurements have proven repeatedly that Einstein's Theories of Special and General Relativity are successful at describing the Universe.
Another way to look at it is that all scientific theories are based on observation. Scientists make measurements and propose a theory which is testable and can explain the observations. Time is a quantity which can be measured, and theories can describe our observations, therefore time is real in that sense. Hawking discusses spacetime and other issues in "A Brief History of Time" or you can read his lectures which are on his official web site at http://www.hawking.org.uk/home/hindex.html
There is an interesting article about time in The Internet Encyclopedia of Philosophy at http://www.utm.edu/research/iep/t/time.htm
On the issue of now vs then, I would like to point out that in your frame of reference, you are always "here" "now" but I don't think anybody would deny that space exists, even though you always describe your place in space as "here." The wording involved is an issue of philosophy, but does not change the fundamental quantity of spacetime that is needed for theories which explain phenomena in the Universe.
I hope my explanation has helped!
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