Moon Illusion and Tonight’s Flower Moon

Hayden Pearson, Weather Observer & Research Specialist

Going out to look at a full moon has always inspired an awe for me, especially when I’m able to catch it cresting over the horizon and it looms large and a deep firey red or orange, silhouetting the skyline.

The Harvest Moon in the fall is typically my favorite because it signals the changing of seasons, cooler air, and starts getting everyone in the spooky spirit for Halloween. There is also a full moon in spring, which occurs tonight, known as the Flower Moon. This one, like the Harvest Moon, signals the changing of seasons but is a mark of spring, flowers blooming, and trees budding.

While here on the summit of Mount Washington spring is typically delayed several weeks from when it is experienced in the valley, we have started to enjoy a taste of it. Instead of budding trees, our spring is marked by the clearing out of the Mt. Washington Auto Road (as soon as the weather cooperates!) and the transition from riding in our snow tractor to being able to finally use our new passenger van!


The Moon behind the Observation tower in winter.

Going back to Earth’s orbiting celestial body, the Moon, many of you may have similar memories of large full moons rising and appearing seemingly huge when compared to the mountains, barn, town, or cityscape in the foreground. That being said, have you ever noticed that later on in the night, once the Moon is higher, it no longer appears as large or as red? What you are experiencing is in fact called the Moon Illusion.

This is an optical illusion, and some aspects are sill fairly poorly understood, however there are some fun and wacky ways that we can try. First, let’s look at the coloring of the Moon. When the Moon first rises and appears red or orange, it is because the light that is reflecting off of it is traveling through the densest part of the atmosphere, the troposphere. Since red light has some of the longest light waves within the visible spectrum, it is able to travel the farthest, thus making the Moon look red or orange. Red light is much harder to scatter than, say, blue light.

As the Moon rises higher into the sky, it becomes less obscured by the atmosphere, allowing it to transition more into the yellow spectrum of light and eventually appear a brilliant white as it is reflecting back sunlight. Sunlight is in fact white since it contains all of the color spectrum, however during the day you shouldn’t look directly at the sun, so it is hard to tell.

The Moon Illusion can be simulated in pictures using a long lens when the Moon is low to the horizon and there are buildings, mountains, or other objects in the foreground. Photo from The Conversation.

Now for the second, and less understood part of the Illusion. Why does the Moon appear bigger? Is it actually closer? This part is when the real optical illusion starts. While the Moon does have an elliptical orbit and does get closer to the Earth (these are called Super Moons), it is only about 12% to 15% closer than when it is at its farthest point. This is too small a difference to visually account for the Moon Illusion, and unless you’re very familiar with star gazing, it can even be hard to discern with the naked eye.

The reason the Moon looks bigger is all in your head! So why do humans experience this? While a bit unsatisfying to some, there is no concrete answer. The illusion has to do with how our brain perceives objects as people mainly notice the Moon looking bigger when it is near the horizon. We naturally compare objects to judge how big or small they are. Our brains interpret the size of the Moon based on how near or far it is from the horizon and how big we expect an object to be when we see it.

It appears that the human brain has trouble understanding that the Moon’s distance doesn’t change that much no matter where in the sky it is located. Thus, when it is closer to the horizon, we expect it to be close, and when it is high in the sky, we expect it to be far away. Additionally, having objects in the foreground like trees, houses, or mountains tricks our brains into thinking the Moon is closer. This isn’t a perfect explanation though because astronauts in space have also experienced the Moon Illusion and have no foreground objects to act as distance cues.

Our brains naturally perceive the size of objects depending on what is around them. The two orange circles have the same diameter, however the one on the right appears larger because the objects around it are smaller.

So how can we test this? Well there are a couple ways. The first is to go outside with a camera and focus it at the Moon as it rises, then take a picture. Now, keeping all the camera settings exactly the same (don’t use auto focus), go back out when the Moon has risen and take a picture of it again. You will notice that the Moon in both pictures is the same size if you measure it.

Another way is to hold out your hand with a straight arm. Then close one eye and see which finger tip just barely covers the Moon. Once the Moon has risen, go out and do the same experiment again. You should see that while the Moon looks smaller, it is the same finger that barely covers it.

Lastly, and slightly more goofy, if you see the Moon looking abnormally large, look at it upside down. The easiest way is to just look down through your legs at the Moon. I am not that good at handstands but I would think those would work as well! This appears to have the effect of resetting the perspective of your brain making the Moon appear normal sized.

With the Flower Moon happening tonight, try some of these on your own and test the Moon Illusion for yourself!

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