I Didn’t Know That...

Q: You recently wrote that the moon appears larger on the horizon because of perceptions created by its apparent proximity to buildings and other objects. You are wrong.

A: I Didn’t Know That got a lot of letters like that. Most contended that the correct theory is that the elevation of objects higher in the sky makes them appear smaller. Fortunately, a father-son team from IBM and New York University has come to the rescue.

Lloyd and James H. Kaufman used mirrors and other optics to project moons onto the horizon and higher in the sky for volunteer observers, using sophisticated techniques to allow the observers to estimate their sizes and distances. They reported in Tuesday’s Proceedings of the National Academy of Sciences that it is, indeed, the presence of nearby objects on the horizon that makes the moon look larger. Interestingly, when volunteers closed one eye, the moon appeared the same size on the horizon as it did higher up. Simulations of their experiments and further details can be viewed at https://www.pnas.org.