The moon tilt illusion

Besides the familiar moon illusion [eg Hershenson, 1989 The Moon illusion ( Hillsdale, NJ: Lawrence Erlbaum Associates)], wherein the moon appears bigg er when it is close to the horizon, there is a less known illusion which ca uses the moon's illuminated side to appear turned away from the direction o f the sun. An experiment documenting the effect is described, and a possibl e explanation is put forward.