The Supermoon and the Moon Illusion

Last night was the night of the “supermoon” which is when the perigee moon is full (when the moon is full and is at the closest point it can get to Earth). Photographers and astronomy buffs were out in force capturing images and measurements of the moon during the night. During the weeks before, though, most scientists had been doing their best to dispel the “supermoon myths” that seem to crop up every year at this time. Some people think that the “supermoon” means that the moon will be so close to the Earth that the tides will become catastrophic. Others think it means that the moon will appear significantly larger than it normally does.

Part of the reason for these myths is the media hype behind the “supermoon.” Another reason, however, could most likely be the optical illusion called “the Moon Illusion.” The moon illusion is also known as the tendency for people to believe that the moon is larger the closer it is to the horizon. The debate behind the exact cause of this issue has raged on for a while — over 2000 years, to be accurate — and it’s not going to be settled any time soon. However, regardless of exact measurements, we do perceive the moon to be larger when it is rising over the horizon than when it is high in the sky even though the disc never changes in size over the course of a single night. Aristotle believed that the moon was made to appear larger due to a magnifying effect of Earth’s atmosphere. Ptolemy believed the illusion was caused by refraction. Modern thinkers sometimes adhere to the “angular size” theory which holds that an object’s position at different angles or arcs can impact the observed size.

However, the most popular and widely accepted explanation currently is the “relative size hypothesis.” This hypothesis suggests that the reason we perceive the moon to be larger at the horizon than at its zenith in the sky is because, when the moon is on the horizon, it is “closer” to objects that we can measure (houses, trees, etc). So, we increase the moon’s perceived size since the objects it appears to be resting near are small, making the moon appear larger than it really is.