The same afterimages appeared small, however, when projected at the zenith. Thus, much to my satisfaction, the Moon Illusion seems to have been explained. Over the years, many visitors to the l. S.U. Planetarium have asked me why the constellations appear so much larger when viewed outdoors than they do in the planetarium sky. I think that I now have the answer to that question also.
Constellations appear very large when they are viewed near the horizon.
For example, Orion appears larger when it is rising or setting than at any other time. When bright constellations are found near the horizon they are more likely to be noticed by the casual observer. They can thus appear larger for the same reason the moon does when it is viewed near the horizon. As a result, the constellations, and the planetarium sun and moon, do not exhibit this illusory effect. Perhaps future planetariums could be constructed in such a way as to include the audience's perceptions of form and structure in the nighttime sky.