Jochen
"I think that in a sense I want to do away with any reference, and leave as a foundation to an ontology only those kinds of facts I have termed 'relative'---those which, if there were a reference, would be definite"
First you cannot do this, there is always a reference. Second, you are maintaining a reference anyway, and re-labelling it as facts which have a certain attribute and are therefore definite (ie facts). But that is the whole point of science. So the real question is, in the context of our physical existence what constitutes a fact?
Judging the validity of that statement has nothing particularly to do with time. Although none of the statement is that specific, it involves the concepts of rain, in a spatial position, at a time. The validity of that statement therefore depends on that event having occurred as depicted, ie what constitutes 'rain', where was 'here', and when was 'is'.
Your point in respect of the rainbow is incorrect. The actual spatial location of the rainbow does not alter. Its relative spatial location does, obviously, because its actual spatial location is being expressed with respect to a different spatial location.
The point is that everything can only be identified by comparison and the identification of difference. Which necessitates a reference. And in order to ensure comparability of outcomes, consistency of reference must be maintained.
Probably the simplest way to respond to your next paragraphs is to point out that A, B, C are not A, B, C subsequently, they are something else. Because you said "then". The only way A, B, C could still be A, B, C, then, ie at a different time, is if a) nothing about them ever alters, b) nothing has altered in the duration being considered, c) the physical attribute being considered has not altered (although this is not really a condition because the entity is therefore different).
To illuminate this point here are three introductory paras to another paper of mine:
1 Distance is an artefact of physically existent entities, it being a difference between them in terms of spatial position. Existence necessitates physical space, but that can only be assigned via entities. So distance can only involve entities which exist at the same time. And they can only exist in one physically existent state at a time.
2 Therefore, any given distance is always unique, since it reflects a definitive physically existent circumstance at a given time. The notion which presumes there could be varied results when quantifying it, either in terms of space or duration, is a fallacy. Whatever the measuring methodology, there can only be one outcome.
3 Unless this is understood, a problem arises when distance is expressed conceptually in terms of duration. The concept being that it can be measured as the duration which would have been incurred had any given entity been able to travel along it, either way. But this is not possible, because there is no duration available during which that can actually happen, so it must be understood that there is no duration, as such. That is, the result is just an alternative expression to, and the equivalent of, a specific spatial measure. Misunderstanding this leads to the flawed application of the equation x = vt.
Another general response to those paragraphs would be to point out that a) what we observe (ie receive) is a physically existent representation of the reality (eg light), not the reality, b) observation/measurement can have no effect on physical existence, because it occurs after that.
Paul