The historical index of the image doesn't simply say that they belong to a certain time, it says above all that they only enter into legibility at a certain time. And indeed, this 'entering into legibility' constitutes a critical point of the movement inside them. Every Now is determined by those images that are synchronic with it: every Now is the Now of a specific recognisability.. an image is that in which the Then and the Now come together into a constellation like a lightening flash. (W.B. Theory Of Knowledge)
There are facets of the historical object which await the arrival of definite conditions in order to be seen, to be read; definite, i.e., as opposed to the passing of time per se. It is not the sheer empty elapse of a chronologocal span that enables the past to be read, automatically, but a historical situation that is related to the earlier by dint of similarity. Similarity, not anticipation, not causation. The relation between the Then and Now is like that between two halves of a metaphor, tenor and vehicle.
Benjamin speaks of 'legibility': the text of the present and that of the past align, and the one glosses the other. Or, certain historical phenmenon can only be observed at certain moments just as certain astrological phenomenon can. This is why Habermas' image of Benjamin's thought is so apposite:
History revolves like a dead star upon which, now and then, lightning flashes.
for both bodies are in motion, past and present. Only when their orbits are configured in a certain way does illumination take place.
It is not, then, only the object of interpretation which is historical, in the sense of 'belonging' to a certain period, embodying that period's style, concerns etc, the interpreting subject is equally historical. Nor is it only the past that is suddenly illuminated. To paraphrase Jameson, It is not only us who judge the past, but the past which in its recalcitrant difference, interrogates us.
For example, Benjamin's own early analysis of German Baroque drama glosses and is glossed by the Germany of expressionism, of catasrophe and inflation.