Fixed points actually move relative to each other. This is measurable even locally if you are doing high-precision localization e.g. with LIDAR. The geometry of

Fixed points actually move relative to each other. This is measurable even locally if you are doing high-precision localization e.g. with LIDAR. The geometry of relationships between objects is in constant motion but below the threshold of what a human can sense. There are many identifiable causes of this motion that vary with locality (tidal, thermal, hydrodynamic, tectonic, geophysical, et al). Additionally, there are local time dilation effects, both static and transient, that influence measurement but aren’t actually motion. This comes up concretely when doing long-baseline interferometry. Lasers are used to precisely measure the distance between receivers in adjacent structures for use in time-of-flight calculations. Over the course of a day, the distance between those structures as measured may vary by multiple centimeters, which is why they measure it.