makes up the natural clock that is “always already discovered with the factical thrownness of Dasein” (GA 2: 547/SZ 413, em). Being-thrown means that Dasein is delivered over to a clock that has (always) already been given in nature: “The disclosedness of the natural clock belongs to Dasein which exists as thrown and falling” (GA 2: 548/SZ 415, tm).
Since Dasein’s understanding of itself in terms of its daily work relies upon a notion of day, and since the concern over that work requires light, mechanical clocks “must be ‘adjusted’ to the ‘natural’ one if they are to make the time primarily discovered in the natural clock accessible in its turn” (GA 2: 547/SZ 414). Even clocks that are not based upon astronomical observation (cesium clocks, for instance) end up ever more accurately measuring ever smaller parts of the day. This is the priority of day-time. Again, Heidegger traces this back to Dasein’s temporality, “because the clock . . . must be regulated by the ‘natural’ clock, even the use of clocks is grounded in the temporality of Dasein that, with the disclosedness of the there, first makes possible a dating of the time with which we concern ourselves” (GA 2: 549/SZ 415, tm). There are two moments here. First, Heidegger claims that clocks must be regulated by the day; much earlier he had pointed out that “when we look at the clock, we tacitly use the ‘position of the sun’” (GA 2: 95– 96/SZ 71). Secondly, and as a consequence of the first moment, the use of clocks is based upon Dasein’s temporality, because clocks must be regulated by the day. And what is this day? A factically necessary part of Dasein’s inescapably fallen temporality. Dasein’s own temporality is indissociably linked to this day. Heidegger was moving toward this thought as early as 1923, in the lecture course Introduction to Phenomenological Research. In the opening pages Heidegger discusses the etymological meaning of phenomenology, showing its root in the Greek φα (pha) and the connection of this with φῶς (phôs) “light,” or, as Heidegger will understand it, “brightness” (Helle). Brightness is both the sun as present (Gegenwart) and the presence (Anwesenheit) of the sun, he says, in a brief interpretation of Aristotle’s De Anima B. 7 (GA 17: 9/6). Brightness, then, is “the way of the authentic being of the sky, the letting-be-seen of things, the being of day [Tagsein]” (GA 17: 8/5, tm). Once again, however, in this 1923 course the sun likewise belongs to the being of Dasein: “To Dasein in the world there belongs the being-present-at-hand [Vorhandensein] of the sun, precisely that which we mean when we ascertain: it is day” (GA 17: 9/6, tm).
And corporeally so, since Heidegger points out that due to the presence of the sun, everyone brings a shadow with them: “In the shadow that constantly accompanies everyone, we encounter the sun with respect to its changing presence at different places” (GA 2: 549/SZ 416). Heidegger spends a whole paragraph with the intricacies of shadow measurement—