Earth to Scientific American….I noticed something weird about the Philip Yam post on the Harvest moon. The term, as Paul Raeburn pointed out earlier in the week, refers to the full moon closet to the fall equinox. That's all fine but does this paragraph make sense?
One enduring belief is that the harvest moon is bigger and brighter than any other full moon. That myth is probably the result of the well-known illusion in which the moon looks bigger on the horizon than it does overhead.
Why oh why would the myth about the harvest moon have anything to do with an illusion that pertains to the moon when it’s close to the horizon? Don’t full moons always appear to rise up from the horizon?
If the moon is indeed somehow coming up at a shallower angle and therefore spending more time close to the horizon during this time of year, then this post is missing an explanation – an astronomical one rather than the long-winded video about optical illusions that Yam presents. If not, then the second sentence doesn’t follow at all from the first, the piece is illogical and the video is irrelevant.
In search of help I turned to Space.com. In this piece, Elizabeth Howell seems to suggest there is a real connection between the harvest moon and the horizon illusion:
This trick of the mind is true for the moon all year round, but it's particularly pronounced with the Harvest Moon because this full moon's path around Earth creates a particularly narrow angle with the horizon. As a result, the moon rises only 30 minutes later every day around the fall equinox, far below the average of 50 minutes.
That’s not very helpful. This says something about timing of the moonrises from evening to evening. But what does that have to do with the horizon illusion?
This piece from EarthSky by Deborah Byrd explains why successive moonrises are only 30 minutes later each subsequent evening. Her explanation for why the full moon seems bigger this time of year: Mystique.
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