# Shape of the Earth: Density, Stress & Gravity

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• 0:02 Oblate Spheroid
• 0:41 Gravity
• 1:39 Density
• 2:45 Stress
• 3:50 Lesson Summary
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Lesson Transcript
Instructor: Jessica Whittemore

Jessica has taught junior high history and college seminar courses. She has a master's degree in education.

This lesson will seek to explain geodesy and the Earth's shape. In doing so, it will highlight the concepts of gravity, density, and stress, as well as the role they play in the field of geodesy.

## Oblate Spheroid

If I told you that the shape of the earth is a sphere, there'd be no surprise there. However, I'd be telling you something that isn't entirely true since Earth isn't exactly a true sphere. On the contrary, Earth is actually an oblate spheroid meaning simply that it is almost a sphere but slightly oblong. With this little technicality out of the way, let's start today's lesson on the shape of the earth.

For starters, geodesy is the science of studying Earth's shape and size. With this definition in mind, geodesy usually discusses gravity, density, and stress when speaking of Earth's shape.

## Gravity

We'll start with gravity. Stated simply, gravity is the force that attracts a body toward another physical body that has mass. For our purposes, we'll cater this definition to geodesy, saying that gravity is the force that attracts a body toward the earth's center or toward any other physical body having mass.

The force of gravity is what causes planets to take on the form of spheres or, more technically speaking, oblate spheroids. To explain, when speaking of Earth, gravity causes its mass to pull toward its center of gravity, which just so happens to be its core. Lucky for us, gravity also keeps you and I pulled toward the core rather than floating off into space.

When you combine the force of gravity, along with the rotation of our Earth, the poles of Earth tend to get a bit squished while the areas along the equator get a bit bulgy. With this, Earth earns itself the label of oblate spheroid, rather than a perfect sphere.

## Density

Next we come to density. Getting this topic started, density is simply the degree of compactness of an object or substance. Interestingly, Earth is the most dense planet in our solar system.

Traveling a bit backwards in our discussion, Earth's density is greatly affected by gravity. Now remember, gravity pulls everything toward Earth's core. As gravity exerts its extreme pressure, it causes Earth to become very compact, therefore making it very dense. To really oversimplify this, think about stuffing your suitcase full of clothes. If you're like me, you've found yourself pushing down on the clothes or even sitting on the suitcase to get it to zip. In other words, you're putting force on the clothes in order to get them to become more compact so you can stuff in as much as possible.

In a similar fashion, gravity pushes on the physical substance of the earth causing it to be extremely compact or dense. Of course, just like your over-packed suitcase will often bulge in the middle, gravity, combined with Earth's density, lends to its oblate spheroid shape.

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