http://facstaff.uww.edu/mccreadd/intro4.html Oculomotor micropsia/macropsia seems to be a truly fundamental angular size illusion. It shows up in many different kinds of visual spatial illusions (McCready, 1965, 1983, 1985, 1986, 1994a, 1994b, 1995). The present theory simply proposes that the moon illusion also is an example of the ubiquitous illusion of oculomotor micropsia/macropsia Indeed, two independent researchers, J. T. Enright (1975, 1987b, 1989a, 1989b) ) and Stanley N. Roscoe (1979 1984, 1985, 1989) have demonstrated that oculomotor micropsia occurs during viewing of the "zenith" moon, and oculomotor macropsia occurs during viewing of the "horizon" moon: In a nutshell, that can explain why the horizon moon looks angularly larger than the zenith moon. A demonstration of oculomotor micropsia can be conducted by most readers as follows. A Simple Demonstration. The next time you look at the horizon moon, deliberately create oculomotor micropsia by strongly converging ("crossing") your eyes, say by looking at the bridge of your nose, but pay attention to the moon. That over-convergence of the eyes will create double vision of the moon and some blurring, but notice that the moon's angular size momentarily looks smaller than it did. At the same time, the moon will look either farther away than it did, or its linear size will look smaller, or else both of those secondary illusions will occur. That illusion imitates what occurs during viewing of the zenith moon. However, in this demonstration the apparent decrease in angular size undoubtedly is greater than the decrease found during natural viewing of the zenith moon. [Indeed, this demonstration of micropsia even works for the zenith moon, which already looks angularly smaller than the horizon moon.] When you then return both eyes to being aimed straight ahead (their "far," divergence position) the moon will look single again and momentarily will look angularly larger than it just did (relative macropsia). Hence it also will look either closer than it just did, or its linear size will look larger, or else both of those secondary illusions will occur. Try the experiment.