Answer:
You must fully describe how its position changes ACCORDING TO A SPECIFIC OBSERVER. The problem is that this “change of position” may be different for different observers, and this is why it is said that space and time are relative to any specific observer.
Space relativity is some how intuitive at the speeds we are used to. For example, if the object is moving away from you at one meter each second then you will see it moving away from you at 1 m/s, but if the object is not moving according to you as the observer then its speed will be 0 m/s. Certainly, there is always a very small difference in space length, so space will be a tiny bit smaller if measured from the moving objects point of view, but the difference is so small that it can actually be neglected if the object is moving at speeds a lot slower than “c” (“c” being the vacuum speed of light).
Time relativity shouldn’t be trickier but it is and I guess this is because we are used to synchronize. For example, if you synchronize watches with a friend and then your friend starts running then you would expect both of your watches to be synchronized when you get together once again, and this is what you will actually observe, but what will actually have happened is that your friends watch would have ticked a little bit slower than yours, but once again, difference will be so tiny that in order for you to notice it you will both need to have watches being able to display a lot of decimals of a second.
Bottom line; you can fully describe the motion of any object according to any specific observer by describing how its position changes from that observers point of view, but it will always differ from the description of the same motion from another observers point of view, unless both observers have identical cinematic conditions with respect to the observed object, so there is no way to fully describe the motion of an object according to all of the different possible observers points of view.
