  The moon illusion, in which the moon appears larger at the horizon than at
higher altitudes, has been investigated since antiquity, yet it remains not
fully explained.
  Our method of investigating the phenomenon is based on that attributed to
Martin Folkes. Folkes suggests investigators identify landmarks on the ground
and request human subjects indicate a place in the sky that they perceive to be
above that location by raising their arm and pointing. The intersection of a
vertical line rising out of the landmark and the ray extending from the
subject's finger then identifies a point on the subject's model of the
celestial vault. When repeated for landmarks covering a range of distances, the
subject's entire celestial vault can be traced. We asked 30 subjects to perform
such an identification on a series of points between a horizontal distance of 3
and 12600 meters across featureless open water in Verplanck and Ithaca, New
York. The resulting data are in disagreement with the widely presumed flattened
dome model, and, in particular, the Size-Distance Invariance Hypothesis.
  In addition, we find that the vault does not intersect the ground and leaves
an indeterminate space of about 20 degrees altitude above the horizon with no
correspondence to the ground. It is in this region that the Moon Illusion is
most pronounced.
