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Treebeard's Stumper Answer
21 January 2000

More Lunacy

Right or wrong, the moon is often associated with lunacy (luna-see!) and illusion. Here are two moon illusions to explain. I've long noticed that many kids at school are surprised to see the moon in daytime. But in fact, the moon is out during the day just as much as it's out at night. It must be, since the moon orbits the turning earth once a month! Why do so many people think otherwise? The full moon looks extra large when it rises over the mountains as the sun sets. It's not so impressive high in the sky, but it's really just the same size. Explain!

Full moon rising over Isla Vista as viewed from Devereux Point on the night of the winter solstice on December 22, 1999, when the moon is closest to the earth (near perigee) and the earth is closest to the sun (near perihelion). This combination of circumstances was said to make the moon appear larger and brighter than it's been for 133 years. We didn't notice any difference, but it was a beautiful moonrise and the -1.7 low tide was spectacular.

The full moon rises as the sun sets and rules the night. But it rises and sets an hour later each day, which brings it into daylight hours. It shrinks to a crescent, and we lose it in the sun's glare until it comes back. The moon is out as much in day as night, but it's pale instead of brilliant. Look for it, and you'll see it. The moon looks larger on the horizon where we can compare it with familiar things, but the image is the same size high or low. All sight involves interpretation. Psychologists debate the details, but I'm glad the moon still has its mysteries!


The moon rises and sets every day because the earth turns on its axis. But the moon also orbits the earth once a month. It's that motion that creates phases and brings the moon into the daylight as much as night. Graybear gave this answer:

there are two reasons that work together but the main reason is that the brightness of the sun diminishes our ability to notice the moon, after all, the stars and other planets are out during the day also, but we don't see them. The other reason is that when the moon is out during the day, we see a smaller portion of the illuminated side so there is less light being reflected to the earth.
It's interesting to track the moon at the same time every day after the full moon. You'll see it higher in the sky and closer to the sun and a bit more of a crescent every day until you finally lose it in the sun's glare. Then it returns before the sun. Warning: Don't look AT the sun! The daytime moon is always a crescent and the new moon is too close to the sun to see, so I suppose the daytime moon is less visible than the nighttime moon. But it's definately there.

Watching the daytime moon about a week after the full moon, I noticed another stumper. At 10:00 a.m. the sun was about half way up in the southeast, and the half moon was about half way up in the southwest. But the lit half of the moon was pointing upwards, well above the sun. I think of the crescent moon as a drawn bow, so shouldn't the moon's arrow be aimed at the sun?

I think I'll save this one for a future stumper!

The moon really is in the sky during daytime, and to think otherwise is just a mistake. The classic moon illusion that the rising full moon looks larger than the same moon high in the sky is different. In fact, the moon is the same size to our eye or a camera lens, whether it is high or low. The larger appearance is a true optical illusion. It's not a physical effect due to refraction.

My friend Lester Hunt put this to me as a stumper.

The problem was this: "Why does the moon appear larger when it is on the horizon than when it is high in the sky? It is relatively easy to say why the atmosphere can change the apparent color the moon when it is in that position, and why it might even change its shape to some extent, but what could cause it to be magnified?"

The solution, published in the Proceedings of the National Academy of Sciences is, in effect, that this phenomenon is an optical illusion. When the moon is close to the horizon, the mind has distance-cues, like houses and trees, with which to compare the moon, and judges it to be relatively far away. Of course, a relatively far-away object making the same-sized blob on the retina would be relatively large, so it looks big.

Graybear agrees:

That is truly an illusion! I think it's because when we see the moon is low, we compare it with things on the horizon that are a mile or so away, and compared to them, it looks large. When it is overhead, we compare it with nearby tree branches that are only a hundred feet or so away, so it looks smaller.

The illusion is undeniable. The rising full moon does look huge. But it doesn't look so big in photos! I took the picture at the top of this page just as the winter solstice full moon cleared the Santa Ynez Mountains as viewed from Devereux Point in Isla Vista. It was impressive at the time, but it looks pretty ordinary in the photo. Photos with huge rising moons are usually taken with long telephoto lenses or are composites of separate images. The camera isn't tricked, but we are! In fact, the image of the full moon on camera film or our eye's retinas is always about 1/2 degree of arc.

I took pictures of the winter solstice full moon with my Olympus 320 digital camera as it rose, and later when it was high in the sky. My camera has automatic exposure and a slightly wide-angle lens.

Full moon rising above the Santa Ynez Mountains. This is a 64x64 pixel piece of the picture at the top of this page.     The same full moon high in the sky at about midnight, taken with the same camera and lens.

In these pictures, the high moon is definately larger. That's probably due to over-exposure from the bright full moon, though I think we're actually a bit closer to the moon when it's above us (by the amount of earth's radius, about 4000 miles) , so the difference may be real. The eye may have optics like a camera, but seeing is not like looking at pixels. We see objects, not retinal images. All sight involves insight.

I read the recent PNAS paper "Explaining the Moon Illusion" by Lloyd and James Kaufman. This paper was widely reported on CNN and Science News and other popular sources, but it's difficult reading and controversy continues! Is the horizon moon larger, or is the zenith moon smaller? Which appears closer? Does perceived distance effect size or vica-versa? Like I said, I'm glad the moon still has its mysteries!

For some reason, the moon illusion brings out strong feelings and real lunacy! Here are some links into the controversy:

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