“[The sun’s] breadth is (that) of a human foot.” (R3; Cp. S, D 3, K 47)
Given our common experience of depth perception, it would have been apparent to Heraclitus that the sun is larger than it seems. Many a schoolchild has it brought to her attention that someone in the distance seems to measure the size of the tip of her pinky finger. Yet, as the individual approaches, the schoolchild comes to see the real size.
However, heavenly bodies, like the sun, confront us with a difficulty: we cannot approach them closely enough to apply our standards of measure the way the schoolchild approaches the person in the distance and then is able to correct the initial miscalculations of her size. How, then, do we know the measure of the sun?
As Heraclitus wrote this fragment, the Greeks were only beginning to try to calculate how we would ever know. Anaximander and his students were the first to develop a geometric model for measuring the heavens. Anaxagoras offered the earliest projection of the size of the sun, estimating it to be the size of the Greek peninsula Peloponnesus. By the time of Aristotle it was generally accepted that it was larger than the earth. (See Kahn, 163f.) But we were far from a time where we could apply our own systems of measurement to it in any remotely accurate way.
Though various historical sources take the fragment to be a literal statement of Heraclitus’ view, in accord with Diogenes Laertes, who noted Heraclitus maintained the sun “is the size that it appears to be” (LM, R45), it appears more promising to interpret the statement, rather, to be an ironic way of underlining the relatively and potentially misleading nature of sense perception and of highlighting the problem of correctly measuring the distant sun. The statement is a quip, nearly fitting Diogenes the Cynic.