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How to Think About the Size of the Earth

Do you have a good feel for the size of the Earth? If you’re like most people, you know that it’s big but you don’t have a sense for exactly how big. Today we’re going to change that.

By
Jason Marshall, PhD,
October 29, 2016
Episode #292

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EarthYou’ve no doubt noticed that the Earth is a big place, but do you have a good feeling for exactly how big? If you’re like most people, you probably don’t have an intuition for this kind of thing … because we humans just aren’t very good at thinking about huge numbers and the huge objects described by them. But that doesn’t mean we can’t develop this kind of intuition—we absolutely can, but it takes a bit of thinking. Which is exactly what we’re going to do today.

By the way, when I talk about knowing how big the Earth is, I’m not talking about being able to recite its radius or circumference in some arbitrary human units like miles or kilometers—because deep down that really doesn’t help you understand its size, right? I’m talking about developing intuition about its size—the kind of thing you can only obtain when you figure out a way to think about things in terms that you can personally identify with.

So let’s get to it … let’s start developing our intuition for just how big the Earth really is.

How Big Is the Earth’s Radius?

Since our goal for today is ultimately to figure out how to think about the size of the Earth, we’d best begin with its size. So here goes: the radius of the Earth is about 6,370 kilometers or 3,960 miles. And we’re done! Except as I mentioned earlier if you’re like me, you don’t actually know how to think about these numbers and what they mean. So instead of worrying about the numbers, I want you to instead think about something that you’re more familiar with that’s roughly this size.

The radius of the Earth is about 1.5 times the distance across the United States.

Here’s how I think about this. At various points in my life, I’ve lived in both Los Angeles and New York … and I’ve driven back and forth between the two sides of the United States several times. As such, I have a good intuitive feel for just how far this is. Namely, it’s really far—it takes roughly four very long days in the car to complete the journey. I’ve also spent a fair bit of time staring at maps and globes, so I have a decent big-picture idea about how the distance across the US compares to the total size of the Earth.

And that’s all I really need to understand the size of the giant ball of a planet that we inhabit. Because it turns out that the distance from New York City to Los Angeles is around 4,000 kilometers or 2,500 miles. Which means that the radius of the Earth is about 3,960/2,500 or roughly 1.5 times the distance across the United States. For me, this is a great way to think about the size of the Earth. I just imagine getting in my car and beginning a journey across the country … except in this case, I’m not traveling east from Los Angeles, I’m traveling directly down towards the center of the Earth. If I could somehow do this, it’d take about six long days of driving to complete the journey!

How Big Is the Earth’s Circumference?

Once you know that the radius of Earth is roughly 1.5 times the distance across the United States, you can use this new point of reference to think about the size of the Earth in another way. In particular, if you remember the basics of the geometry of circles, you’ll know that the circumference of a circle (that’s the distance around it) is equal to 2π times the radius of the circle. This geometric relationship is true for any circle—including the circle formed as part of what’s called a “great circle” around the Earth. (Well, if we’re being super-precise, the Earth isn’t a perfect sphere … so this geometry isn’t exactly exact—but it’s close enough.)

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