# Equatorial Bulge: Causes & Explanation

• Sheneron
In summary: So the Earth would bulge out at the equator.In summary, the equatorial bulge is the result of the Earth's spinning and the forces that act on it. These forces cause the Earth to flatten out a bit so that it has the minimal amount of energy possible. Energy is conserved because the shape that minimizes the energy is no longer a perfect sphere, it is instead an oblate ellipsoid.
Sheneron
What causes of the equatorial bulge? I have tried looking it up online but I am having some trouble finding a good explanation.

The Earth is spinning at one revolution per day.

So then is it because the tangential velocity at the equator is greater than of that at the poles?

Think about how people make a big flat pizza starting from a ball of dough.

So its just because its spinning?

I have two questions I would like you to answer honestly before I answer any further: (1) Is this homework, and (2) how much physics/math have you taken in school?

1. No its not homework
2. I am just starting physics

however, if you would like it to be a homework problem i can certainly make it one

OK. I'll keep this simple. The simple answer is that the Earth is spinning and is not perfectly solid. Like the pizza dough, the Earth tends to flatten out a bit because of this spinning.

What the Earth does (and what the spinning pizza does) is to change shape in a manner that minimizes the total energy of the Earth (or pizza). If the Earth were not spinning, the only source of energy would be the Earth's gravity. Gravity radiates spherically, so this minimum energy configuration would be a sphere. Any deviations from purely spherical would create an Earth with more potential energy than the purely spherical form. The spinning adds another energy source, kinetic energy. This extra source of energy makes it so the shape that minimizes the energy and conserves angular momentum is no longer a sphere. It is instead an oblate ellipsoid.

This represents two very key concepts in physics. The concept of a system moving toward the minimum energy configuration is called Hamilton's principle. Conservation of angular momentum, along with conservation of linear momentum and conservation of energy, are cornerstones of a lot of physics and engineering.

D H said:
What the Earth does (and what the spinning pizza does) is to change shape in a manner that minimizes the total energy of the Earth (or pizza).

DH, just trying to get my head around this quote of yours. If the spinning results in a minimizing of the total energy, then there must have been more energy at an earlier time. If so, where does this energy go?
Secondly, isn't any change in the shape of a spinning object just the result of the centripetal, or opposite, forces? And thus energy is conserved?

Part of an answer to this question is in another related question that my mother recently asked me. Why are so many heavenly bodies so close to being perfect spheres? It is important to realize that when you are talking about objects as large as planets, the pressure exerted on subsurface rock causes it to behave in a ductile manner. Not to say that everything beneath the Earth's crust is liquid magma, the Earth is internally very rigid on human timescales, but over millions of years the Earth (and other planets)behaves like a huge drop of liquid. It doesn't take much rotation to cause distortion.

Another interesting tidbit! Say, for example, the Earth WAS a perfect sphere. Forces on the surface would be asymmertrical and free objects like parked cars (without the E brake on) would role towards the equator.

## 1. What is the equatorial bulge and why does it occur?

The equatorial bulge is the observed phenomenon of the Earth's shape being slightly wider at the equator compared to the poles. This occurs due to the centrifugal force caused by the Earth's rotation, pushing outwards at the equator and causing the Earth's shape to bulge.

## 2. How much does the equatorial bulge affect the Earth's overall shape?

The Earth's equatorial diameter is approximately 12,756 km, while its polar diameter is only 12,714 km. This means that the equatorial bulge causes the Earth to be about 42 km wider at the equator compared to the poles, which is a small difference in relation to the Earth's overall size.

## 3. Are there any other factors that contribute to the equatorial bulge?

In addition to the centrifugal force, the Earth's rotation also causes the planet to flatten at the poles. This is known as the oblate spheroid shape. The Earth's rotation rate, density, and composition can also affect the degree of equatorial bulge.

## 4. Is the equatorial bulge unique to Earth?

No, the equatorial bulge is a common feature of rotating celestial bodies. For example, other planets such as Saturn and Jupiter also have an equatorial bulge, although their rotational speeds and compositions may differ from Earth's.

## 5. How is the equatorial bulge measured and studied?

The equatorial bulge can be measured using various methods, such as satellite missions and gravitational measurements. Scientists also use models and simulations to study the Earth's shape and understand the factors contributing to the equatorial bulge.

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