Discovering the solar system: projects for the keen–eyed, camera–toting, and binocular wielding observer
The “Big Moon” Illusion
a naked eye and camera activity
Casual skywatchers since the time of the ancient Greeks have seen the rising moon as appearing much larger than after it has climbed higher in the sky. The moon is often portrayed in film and television as being very large and bright when it is near the horizon. All this flies in the face of the fact that the actual apparent size of the moon, whether it is rising or it is at its highest point in the sky is quite small. Fully extend your arm and outstretch your hand. The moon’s apparent diameter is only about 1/4 the width of your index finger.
The common explanation of the “Big Moon” illusion is that when the moon’s apparent size is compared to familiar landscape objects, such as distant houses and trees, our mind interprets the moon as being quite large. Then, when it moves higher in the sky, there are no nearby comparison objects. The moon’s apparent size then appears to shrink, and it seems to lie much farther away. While sounding plausible, this reasoning does not explain why the same effect occurs at the beach when the moon is seen hovering just above a flat, featureless ocean horizon, or in the desert when the moon is cast against sweeping sand formations. Studies have sought a deeper psychological explanation.