If a tree grows in a dry place, you can tell its age better than when it grows in a wet place.
Switching in between bad and good conditions can result in several rings forming in one year.
Formally, c is a conversion factor for changing the unit of time to the unit of space.
This makes it the only speed which does not depend either on the motion of an observer or a source of light and/or gravity.
This, of course, includes the extended parts of the field.
However, this "change" in the apparent behavior of the field source, along with its distant field, does not represent any sort of propagation that is faster than light.
For example, when an observer begins to move with respect to a static field that already extends over light years, it appears as though "immediately" the entire field, along with its source, has begun moving at the speed of the observer.Motion of an observer does not cause the direction of such a field to change, and by symmetrical considerations, changing the observer frame so that the charge appears to be moving at a constant rate, also does not cause the direction of its field to change, but requires that it continue to "point" in the direction of the charge, at all distances from the charge.The consequence of this is that static fields (either electric or gravitational) always point directly to the actual position of the bodies that they are connected to, without any delay that is due to any "signal" traveling (or propagating) from the charge, over a distance to an observer.It was not until the 19th century that an anomaly in astronomical observations which could not be reconciled with the Newtonian gravitational model of instantaneous action was noted: the French astronomer Urbain Le Verrier determined in 1859 that the elliptical orbit of Mercury precesses at a significantly different rate from that predicted by Newtonian theory.The first attempt to combine a finite gravitational speed with Newton's theory was made by Laplace in 1805.Therefore, the theory assumes the speed of gravity to be infinite.