You've probably heard we're in for an especially super supermoon. The moon will be full, and it will be super close to the Earth—just 216,000 miles away. You can expect slightly higher tides, and to our eyes, the moon will appear about 8 percent larger in the sky. Which, if you live by water and/or spend a lot of time measuring the moon, you might be able to discern.
If that sounds underwhelming, that's because it will be. Although people will make a big deal about this SUPERMOON on social media and other places, don't get all excited. If you didn't know this was happening, you would probably never notice it.
Astronomically speaking, this might be the dumbest thing—ever. In case you are fortunate enough to miss the whole hashtag-supermoon thing, here is the deal:
- The moon orbits the Earth—but the orbit isn't exactly circular.
- The orbital distance varies from 362,600 km to 405,400 km.
- When the moon is closer, it has a larger angular size and also a higher apparent brightness. If this larger angular size occurs during a full moon phase—this is the SUPERMOON! (I would like to promote my phases of the moon if the moon were a cube post)
But it's not a big deal. First, the moon has been doing this for billions of years. Humans probably noticed this change in apparent size around the time of the Greek philosophers (or maybe during the Renaissance)—but I am just guessing about the time. The point is that this is nothing new.
Second, the angular size of the moon is fairly small. A small increase in the angular size means that the moon is still small. Suppose you could do a "blind seeing test" in which you presented normal humans with either a moon or a "supermoon." Do you think humans (mere mortals) could correctly identify the moon as either super or not super? No, neither do I.
In order to emphasize my point, I have decide to create a simple demonstration. What if I take a tennis ball and hold it out in front of a human? How far would it have to be to appear like the moon (with its changing angular size)? First, I need a tennis ball. That's pretty easy. The diameter of the tennis ball is about 6.6 cm. For the moon, I can write the following expression for the angular size.
Since I want the tennis ball to have the same angular size, I can find the distance the ball should be from a human (I am calling this distance r).
Now it's just simple math. If I put in the diameter of the moon and tennis ball along with the two distances from the Earth to the moon (I'm ignoring the radius of the Earth because in the end it won't really matter) then I can get the distances for the tennis ball. Here they are:
- Closest value = 6.89 meters.
- Farthest value = 7.70 meters.
This isn't really what I wanted. The tennis ball is too far away for an effective conversation about the dumbness of the supermoon. However, I did it anyway. Here is a picture of a tennis ball at both the closest and farthest distances. I have added this to a real picture of the moon—just for comparison.
I know it's hard to see the two tennis balls, but they are right there on the ground. It's difficult to see them because of the small angular size—and that's the point. Also I should emphasize that this is the change in size from the smallest moon to the largest moon. Why isn't there a term for the case when the moon is the farthest away? That's not really fair—how about we call it a puny moon (hashtag punymoon)?
But how about a better demonstration? What if I use a smaller object for the moon? In that case I could hold the moon model in front of a person at much closer distance—conversational distance. Let's use a penny as the moon (yes, it's not spherical, but it will work). A penny has a diameter of about 1.9 cm (and should be easy to get a hold of). Using the same calculations as with the tennis ball, it would have to be just 198 cm to 221 cm from a human to have the same angular size as the moon. That's not too bad (a little over 6 feet).
So here's how to do the demo (with rough measurements so you don't actually need to measure).
- Take a penny. If you can't find one, look in your pocket or on the ground.
- Get about 1 human length away from a human (6 feet approximately).
- Now you want to move the penny back and forth by about 10 cm—or around the width of your palm.
- Be sure to say "supermoon" and "moon" over and over as you move the penny back and forth.
- Have the participant close his/her eyes and then put the penny in either supermoon or normal position. With eyes now open, have the participant estimate whether it is a supermoon or moon.
- Keep the penny. You might need it later.
Here's what this should look like.
Be sure and make an annoying face when you should the "supermoon" version of the penny—just for emphasis. But hopefully this will help you show that the supermoon isn't really a big deal. The apparent size of the moon depends much more on the optical illusion based on the things around it—as I pointed out in my last rant about the supermoon.
Just for fun, let me give you a few homework problems.
- How far away would my head have to be in order to have the same angular size as the moon?
- As an approximation, your thumb at arms length is about the same angular size as the moon. How much closer to your eye should you move it to make it cover up the Supermoon?
- Calculate the percent change in apparent diameter of the moon as it goes from punymoon to moon to supermoon.
- Calculate the percent change in apparent area of the moon as it goes from punymoon to moon to supermoon.
- In my calculations, I ignored both the radius of the Earth and the radius of the moon (in terms of distances for angular size). How much would this change my calculation?
- When the moon gets closer, it looks bigger. It also looks brighter because it's closer. Estimate the percent change in brightness of the moon going from moon to supermoon. You can assume the moon is a point light source such that you can use the one over r-squared expression for brightness.
Now a note for my fellow scientists and promoters of science. Think about what we are selling with this "supermoon" idea. We are telling people to go outside and look at the moon (which they should) because tonight "it's SUPER"—but everyone can see that it just looks ordinary. They will say "oh, those silly scientists they think the dumbest things are super."
But really, there are some pretty super things out there. Even the moon is super. How about this? Take a look at the moon every night for a week. Maybe you will notice that the same side of the moon is always facing the Earth even though the moon is orbiting the Earth. Think about it—that's crazy awesome. So why "sell" the supermoon when science has so much more to offer?
