From the ABC 7 Weather team

Watch out for the Super Full Moon (in more than one way)

March 18, 2011 - 12:54 PM
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Here's how the Super Full Moon could get you.

Get ready – it’s coming and I think we will see it. The Super Full Moon.

The blogosphere is giving it a lot of attention, even though it has happened thousands of times in the past. Like many things in the realm of weather and astronomy, it's all about alignment. A Super Full Moon is just the coincidence of the Full Moon and the perigee of the moon.

That last term warrants some explanation. The moon’s orbit around us – and our orbit around the sun – is no perfect circle, but rather an ellipse or flattened circle. Accordingly, the difference between the moon’s greatest distance from the earth (apogee) and closest (perigee) is about 30 thousand miles, or about one tenth of the distance from earth to the moon. 

Stargazers will delight in that proximity tomorrow night. But be careful – here's why this Full Moon might get you.

Moonrise is at 7:39 p.m. tomorrow night, and this thing isn't likely to max out until around midnight. However, if you stay up that late, you won’t think so and you probably won’t believe it. Your brain probably won’t let you believe it.

Again, a little explanation here.

Suppose it's a nice day with a deep blue sky and some fluffy clouds. Now look up at the sky. The sky and clouds above you appear to be closer than the clouds far away. And they are. Those fluffy clouds right overhead are about a mile away and the clouds on the horizon are about three miles away. We see the sky sort of flattened like a saucer, like this:


The clouds in the distance are not any closer to the ground than the clouds right over us, nor are they any smaller or larger.

So what happens when that huge Super Moon pokes its way onto the horizon on Saturday evening? Well, it will be the closest to us in about 20 years, but we will see it in relation to how we see the sky and in relation to smaller objects on the horizon. To us it will appear huge, much larger than when it is directly overhead. No one knows for sure why our brain plays this trick on us, whether it is the apparent distance and how we see the sky or the relative size of the moon in relation to objects on the horizon, as illustrated here.



This is the Ebbinghaus illusion, in which the two orange disks are the same size, but appear to be much different because of the relative size of the objects around them.

Ready to try a fun experiment on Saturday night? Go out with family and friends and watch the Super Full Moon rising. Listen to all the oohs and aahs. 

But bring along an empty paper towel tube or even a toilet paper tube. Preferably also empty. Look at that huge Super Full Moon with your eye as it’s on the horizon or low in the sky. Then look at it through the tube. Surprised? How does that happen?

Better yet, take a picture of that huge moon at moonrise and then 4 or 5 hours later when it is high in the sky. Guess what – the moon is now about 4,000 miles closer to us (the radius of the earth) and the photo will prove the moon is larger when it is high in the sky than when it is on the horizon! We only think it is larger on the horizon when it rises. Don’t believe it? Look at the timelapse of moonrise over Seattle.


(Credit: Shay Stephens)

But it is also more beautiful when it rises – especially on a Super Full Moon night like Saturday – than when it is overhead surrounded by the dark void of space. 

Be careful how you look at this Super Moon, though. I once heard a story of a fellow who was intrigued by the Super Full Moon illusion and decided to bend over and look at the rising Full Moon between his legs and make the sky and horizon upside down. Sure enough, the huge Full Moon illusion wasn’t the same. But his brain maybe did a flip and when he looked standing up the illusion was gone. The story is that he never saw the rising Full Moon as large again.


And if you want to lear a lot more, here is a really complete discussion of the many other explanations (including equations) and neat summaries.

Enjoy the Super Full Moon, and if you’re still interested in illusions, here is a great site.

Finally if you really don't see the huge Full Moon near the horizon, don't feel bad.  I've read that not everyone sees it.

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