The great Johannes Hevelius published in 1687 a catalog of 1564 stars visible from Gdansk (latitude 54.4 north), with each star including a report of his observed magnitude. Weirdly, his stated magnitudes for stars approaching the southern horizon have near-zero systematic deviation from modern magnitudes as calibrated by stars passing near his zenith. Stars culminating near the southern horizon must appear substantially dimmed, for example stars culminating at 10° altitude will be dimmed by 1.4–2.8 mag, while the few reported stars culminating at 3° altitude will be dimmed by 3.5–7.0 mag. But Hevelius is reporting extinction-corrected magnitudes. Formally, with his magnitudes, I derive that Hevelius’ catalog displays an extinction coefficient of +0.006±0.006 mag/airmass. He has no way of measuring extinction-corrected magnitudes, as all he can see is the star’s brightness at culmination or lower, so to get his reported values, Hevelius must have somehow corrected for the atmospheric extinction. (Critically, I have a variety of evidential proofs that Hevelius could at most have copied only a modest fraction of magnitudes from Ptolemy or Tycho.) This is all startling for historians of astronomy, because there is zero mention of atmospheric extinction or any resultant effects by any known source anywhere in the world before 1723. Theorists can concoct many explanations (I have tabulated 14 such tries), but all can be confidently rejected, other than that Hevelius intentionally made some sort of quantitative correction for extinction when he was compiling his magnitudes. Such corrections would be entirely empirical, with no need for any physical understanding, and Hevelius did such with roughly 3% accuracy. The phenomenon of atmospheric dimming is readily observable and the rules of correction can be simply measured with just the naked eye and some bookkeeping. Hevelius was an intense observer of the stars, immersed in a culture of quantitative observations with quantitative corrections for various effects, so it is more than plausible that he would know about extinction and decide to correct for it.