Hello Moon. What do I call you?

April 14, 2014 5:00 PM

What's this illusion called? The full moon is rising, but has not yet cleared the horizon. The bottom of the moon, however, appears concave, its outer edges bending up from the horizon. (Image link inside illustrating this concept).

I'm looking for the scientific term to describe the illusion you see in this image.

The full moon is rising, but has not yet cleared the horizon. The bottom of the moon, however, appears concave, its outer edges bending up from the horizon.

Google-fu fails me on this, mostly because the term "moon illusion" brings in the more widely known illusion of the moon looking larger at the horizon.

posted by jrchaplin to Science & Nature (11 answers total)

Refraction is what's going on there. That article describe it happening with the sun, but the principle is the same. More here on atmospheric refraction.
posted by jquinby at 5:05 PM on April 14, 2014

The bottom of the moon, however, appears concave, its outer edges bending up from the horizon.

I think that may be the standard shape of a gibbous moon.

That particular gibbous moon is looking a bit squashed because of greater refraction near the horizon, however.
posted by jamjam at 5:57 PM on April 14, 2014

Best answer: Looming.
posted by nixt at 6:27 PM on April 14, 2014 [1 favorite]

Response by poster: Thanks, all. Just FYI, jamjam, this was a rising full moon. Photos in the moments following this one show a full moon above the horizon.
posted by jrchaplin at 6:40 PM on April 14, 2014

That is truly amazing, jrchaplin, and it makes for a very striking photograph.
posted by jamjam at 6:55 PM on April 14, 2014

I don't know if illusion is the best google term for this: it's an actual optical distortion, not a trick of the mind.
posted by Dr Dracator at 9:39 PM on April 14, 2014 [1 favorite]

Jquinby has it. more here
posted by bitdamaged at 10:04 PM on April 14, 2014

The pedant in me would like to note that the shape of the moon in your particular image is not concave, but convex.
posted by fancyoats at 8:05 AM on April 15, 2014

I would call it a refraction effect, a type of mirage, perhaps a false horizon or a mock mirage.

The distinction between mirage and looming is new to me. Interesting.
posted by fantabulous timewaster at 8:32 AM on April 15, 2014

Lots of info on this phenomenon and other atmospheric optics here.
posted by TedW at 2:10 PM on April 15, 2014 [1 favorite]

It looks like the scientific term is differential refraction.
posted by TedW at 2:13 PM on April 15, 2014