All those bodies could more accurately be described as "oblate spheroids. To borrow an analogy from the astronomer Phil Plait, they look like a basketball that someone is sitting on. Put more technically, in a celestial body with an oblate spheroid shape, the polar circumference will be smaller than the equatorial one.
So here on Earth, if you were to travel from the North Pole to the South Pole and back, you'd have walked a grand total of 24, miles 39, kilometers. On the other hand, a complete trip around the equator would be a bit longer.
That's because the circumference of Earth's equator is 24, miles 40, kilometers. As such, when you stand at sea level on the equator, you're farther away from the center of our planet than you would be at either pole.
On some other planets, this bulge is even more pronounced. Just look at Jupiter. Earth is only 0. But Jupiter's measurements showcase a much bigger disparity. Indeed, astronomers have found that this plus-sized planet is a full 7 percent wider at its equator than it is between the poles.
The oblate spheroid shape is the result of two main factors: gravity and rotation. Troy Carpenter, director of Washington State's Goldendale Observatory, recently discussed the matter with us in an email exchange. That's because all objects experience self-gravity, a force which pulls their atoms toward a common center.
As the mass of an object increases, so too does its self-gravitational pull. After it exceeds a certain mass, the pull gets overpowering to the point where the object collapses onto itself and becomes spherical. Little items — like, say, a banana or a lug wrench — can resist this fate because their self-gravity is relatively weak, allowing them to retain non-spheroid shapes.
However, in planets, suns and other truly massive bodies, the force is so strong that they can't avoid being distorted into spheroids. While gravity conspires to render the planets spherical, the speed of their rotations is simultaneously trying to flatten them.
The faster a celestial body spins, the more disproportionate its equatorial bulge gets. A significant percentage of stars in the sky rotate much faster and bulge noticeably at their equators. One such star is Altair. Located just Saturn bulges the most of all the planets in our solar system. If you compare the diameter from pole to pole to the diameter along the equator, it's not the same.
Saturn is Jupiter is 6. Instead of being perfectly round like marbles, they are like basketballs squished down while someone sits on them. Earth and Mars are small and don't spin around as fast as the gas giants. They aren't perfect spheres, but they are rounder than Saturn and Jupiter. Earth is 0. Since they're not even one whole percentage point thicker in the middle, it's safe to say they're very round. As for Uranus and Neptune, they're in between.
Uranus is 2. Neptune is 1. They're not perfectly round, but they're pretty close. Do you want to know what it's like to be a spinning planet? You can feel it when you spin around in place. First, make sure there are no obstacles around that you might bump into.
Then either while standing, or in a spinner chair, spin around in circles. Hold your arms close to your body, then extend your arms out. Support Scroll. When we look out at the Solar System, we see objects of all sizes — from tiny grains of dust, to giant planets and the Sun. A common theme among those objects is the big ones are more or less round, while the small ones are irregular.
But why? The answer to why the bigger objects are round boils down to the influence of gravity. The bigger something is, the more massive it is, and the larger its gravitational pull. For solid objects, that force is opposed by the strength of the object itself.
That is because the ground pushes back up at you; it has too much strength to let you sink through it. As Everest gets taller, its weight increases to the point at which it begins to sink. Areas where the water was unusually low would be filled up by water displaced from elsewhere, with the result that this imaginary ocean Earth would become perfectly spherical.
But the thing is, gravity is actually surprisingly weak. An object must be really big before it can exert a strong enough gravitational pull to overcome the strength of the material from which it is made.
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