Western Slope Skies - The Moon Illusion

Apr 3, 2020

Credit Art Trevena/BCAS

Whether motivated by romance or astronomy, you have probably watched a moonrise. If so, you likely noticed how it looms large against the horizon; yet later, it appears much smaller.

In fact, the Moon’s optical diameter remains nearly constant throughout the night, as demonstrated by time-lapse photography. This Moon Illusion (which also similarly occurs with the rising Sun) is well documented, yet it defies conclusive explanation.

The difficulty lies in nailing down the illusion’s nature. Early theories proposed physical explanations, such as atmospheric refraction of moonlight. However, today the Moon Illusion is generally regarded as a perceptual phenomenon. Among popular ideas are the apparent distance and relative size hypotheses.

The apparent distance hypothesis relates to the brain’s judgment of distance by visual cues. It suggests that the brain perceives a low Moon as large and distant against foreground landscape features (e.g., buildings and trees). Conversely, a high Moon in a relatively featureless sky is perceived as smaller and closer. The relative size hypothesis is based on the brain’s judgement of the size of neighboring objects. It similarly posits a large low Moon perceived against narrow, close landscape features; in contrast, the high Moon appears small against broad swaths of open sky.

Both hypotheses have some experimental basis, but it turns out that a small minority of people have contrary experiences viewing the Moon. A few even do not experience the Moon Illusion at all. Also, sailors and pilots both have reported the Moon Illusion, despite having had no detailed landscape in their fields of view. At best, the hypotheses offer a partial explanation. But rest assured, there is no shortage of alternative hypotheses out there!

You can test the Moon Illusion yourself, using a rolled-up sheet of paper and a strip of adhesive tape. At the next moonrise in your location, find a clear view of the eastern horizon. View the rising Moon through the paper tube, adjusting the tube diameter to just fit the Moon’s disk. Secure the tube’s shape with the tape. A few hours later, view the Moon again through the tube. Does the Moon seem larger, smaller, or the same? Whatever its appearance, don’t forget to simply appreciate the beauty of our closest celestial neighbor.

You’ve been listening to Western Slopes Skies, produced by the Black Canyon Astronomical Society. This episode was written by Michael T. Williams and recorded by Jeff Reynolds.