Tiny glass wins. Against a Moon that feels infinitely far, a backyard mirror only needs to win an angle, not a distance. Optics cares about separation in direction, and that contest is set by diffraction at the edge of the aperture, not by how long the beam flies through space.
Counter to intuition, the plate‑sized mirror is not drawing a picture of the whole Moon; it is measuring how sharply it can discriminate two incoming directions of light. Physics encodes that in a simple limit, the Rayleigh criterion, which says the minimum angular separation scales as 1.22 times the wavelength divided by the aperture diameter. Make the mirror twice as wide and the smallest resolvable angle is cut in half, regardless of how remote the target is.
Here diffraction is the gatekeeper. Light passing through a finite circular opening does not form a perfect point but an Airy disk, a bright core with rings set by wave interference. Two lunar craters project two overlapping Airy patterns onto the focal plane. They snap into distinct features once their angular spacing just exceeds that diffraction‑set limit. The telescope, barely wider than a plate, becomes a precise angular ruler, and empty space adds no extra blur.
