  The distinctions between “absolute, true, and mathematical” and “relative, apparent, and common” time and space are the conceptual basis Newton employs in laying out the corresponding distinction for motion. He says, “relative, apparent, and common time is any sensible and external measure (precise or imprecise) of duration by motion,” adding a parallel point about absolute space. He points out that the distinction between absolute and relative time has long been part of astronomy insofar as astronomers have long introduced corrections (via the equation of time) to the natural day “in order to measure celestial motions on the basis of a truer time,” and he raises the possibility of there being “no uniform motion by which time may have an exact measure.” Absolute motion is defined as change from one place in absolute space to another. “But since these parts of space cannot be seen and cannot be distinguished from one another by our senses, we use sensible measures in their stead,” adding “it is possible that there is no body truly at rest to which places and motions may be referred” [P, 410]. In short, both absolute time and absolute location are quantities that cannot themselves be observed, but instead have to be inferred from measures of relative time and location, and these measures are always only provisional; that is, they are always open to the possibility of being replaced by some new (still relative) measure that is deemed to be better behaved across a variety of phenomena in parallel with the way in which sidereal time was deemed to be preferable to solar time.