Why is the Moon Bigger at the Horizon?

We’ve all been there, done it and got the t-shirt; glanced out the window and noticed a stunning, slightly orange-colored yet huge moon peeping over the horizon.

Maybe you’ve even tried to take a photograph of it only to be disappointed when confronted with a tiny image of the moon’s disk. That’s not how you remembered it… right?

You my friend have been duped, not by the strange effects of the Earth’s atmosphere but by one of nature’s very own optical illusions.

It’s fair to say at this point that the refractive properties (ability of the gas in the atmosphere to bend light) do slightly change the shape of the moon but the effects aren’t huge and it’s not this that is responsible for the “Moon Illusion.”

In fact, you can prove yourself that it’s just an illusion: take a long stick plus a small coin and when the moon is low, hold the piece of wood and attach the coin to it so the coin just covers the moon in the sky. Then wait until the moon is a lot higher later that night and check. The moon’s size hasn’t changed at all.

Lots of people have asked me over the years why the Moon is bigger when it’s nearer the horizon than when it’s high up in the sky. The answer is really quite simple and it lies in the way that the eye/brain combination interprets distance clues.

For example, take a look at this picture:

 As you look at it, your brain interprets the path and brick wall heading off into the distance. Now turn your attention to the elephants. The one at the back looks bigger than the one at the front, right? Wrong, they are all identical.

The actual image that forms on the back of your eye shows each elephant is the same size. That is until your brain gets in on the act. Because you perceive the path heading off into the distance, you also assume the elephants are at different distances.

However, because the image in your eye of each elephant is identical, your brain decides that the elephant at the ‘back’ must be bigger than the elephant at the ‘front’ to allow it to form the same sized image. This illusion is named after its Italian discoverer Mario Ponzo who first demonstrated it in 1913.

Ponzo Illusion: 1 – You: 0.

So how does this apply to the moon? There are plenty of clues to distance on the horizon, other than the fact that we know the horizon is a long way away. Even clouds or flocks of birds are typically further away when near the horizon so we assume anything near the horizon is at a great distance, even the moon is perceived to be further away than when higher in the sky (due to the lack of distance clues).

Just like the elephants in the picture, the image of the moon in our eyes are identical regardless of whether it is high up or low down but the brain does the same thing again. It assumes that if both images of the moon are the same size because one is down by the horizon and thus further away, it must actually be a lot bigger to form the same size image! So our brains interpret the moon as looking bigger.

Apparently if you turn with your back to the moon bend forward and look at it through your legs, because you see everything upside down and different to usual, your distance clues are all gone and the moon looks its normal size. I’ve never tried that myself though, so I’ll leave that up to you. Source: Discovery News


Spying On Our Warped Moon

Is it real, or is it an illusion?

Sixteen times a day, astronauts aboard the International Space Station (ISS) get to see the moon drop below the horizon. And in this stunning orbital photograph, European Space Agency astronaut André Kuipers managed to snap the Earth’s natural satellite just as it was dipping from view.

But why is the moon’s disk slightly warped? No, it’s not your eyes playing tricks on you, it’s actually an effect of the moon’s reflected light passing through the Earth’s atmosphere causing refraction.

As light travels from one transparent medium to another — in this case, from the vacuum of space to the Earth’s thick atmosphere — an observer (like Kuipers) will see a distortion in the shape of the moon. Light from the lower part of the moon’s disk is traveling through the thick atmosphere and will become distorted, whereas light from the upper part of the disk is traveling through vacuum and won’t experience any distortion.

As the photo shows, the lower part of the moon has been distorted, creating a “squashed” appearance.

Time-lapse video of the Moon setting behind Earth’s atmosphere, seen from the International Space Station. This video was taken by the crew of Expedition 30 on board the International Space Station. The sequence of shots was taken January 9, 2012 from 11:45:52 to 11:55:44 GMT, on a pass over the North Atlantic Ocean.

This sequence of shots was taken as the ISS was traveling northeast over the Atlantic Ocean. Beginning northeast of the Caribbean islands, the crew pointed the camera towards the full Moon to show the moon setting over the horizon. The pass ends west of Western Europe, still over the ocean. Source: Discovery News

The Moon wobbles

 Even though the Moon shows the same basic face to the Earth, it wobbles slightly because its orbit is not perfectly circular and its axis is not aligned exactly with Earth’s. This movie shows the portion of the Moon visible to the Earth over several months around the time when radar data of the south polar region of the Moon was collected (Courtesy of NASA)

It’s what you get if you were to take a photo of the Moon every night for a month, then make a movie out of those pictures. The Moon’s phases aren’t surprising, but the Moon also appears to grow and shrink as it orbits the Earth. This happens because the Moon’s orbit is slightly elliptical; its distance to the Earth varies by about 10%.

Also, the Moon appears to “wobble” from left to right. That’s because the Earth’s gravity pulls harder on the Moon the closer it is to the Earth, so the Moon travels faster in its orbit when it’s closer to the Earth. The Moon’s rotation rate matches its average orbital speed (which is why we only see one face of the Moon), but its orbital speed varies during the orbit while its rotation rate remains fixed, so the Moon appears to wobble from left to right.

Many websites claim that “moon wobbles” are responsible for explosions, mass-murder, earthquakes, terrorism, etc. Sadly, I need to emphasize that the wobbling we’re describing can’t possibly result in these kinds of ludicrous effects. 

* Also, if you look carefully, you’ll notice a slight up-down wobble as well. The origin of this wobble is totally different: it’s caused by the fact that the Moon’s axis of rotation isn’t quite perpendicular to the line that connects the Earth and the Moon. In other words, the Moon wobbles up and down for the same reason that the Earth experiences seasons.



John says:

Can we say that this is something that pertains to the eye nerve when looking straight or upwards?

Dave says:

I think it’s more a ‘perspective’ issue Jon although it’s damn difficult NOT to believe the thing has grown in size!!! heh heh

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