Moonrise is at 5:44 p.m. this evening. A perfect time of the year (not too early . . .not too late) to see one of the great tricks there is. How big do you think the rising Full Moon is? Why is it so huge when it rises tonight but then looks smaller high in the sky around midnight? After all, the Full Moon is really 4000 miles closer to us (the earth's radius) at midnight than at moonrise. Shouldn't it look larger overhead when it is closer than on the horizon? Yes it is but no it doesn't.
OK enough of trickery by me. 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 Full Moon pokes its way onto the horizon at 5:44 tonight? Well, 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 tonight? Go out with family and friends and watch the 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 Full Snow 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 Full Moon night like tonight – 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.
By the way if you don't see this illusion, don't feel bad. I've heard that not everyone sees this huge Full Moon at moonrise or the huge sun at sunrise or sunset. But be sure to let me know.
Enjoy the Full Snow Moon, and if you’re still interested in illusions, here is a great site.