Try standing on a basketball. Not easy, right? Now try standing on something bigger, like one of those big blue exercise balls. Not only is it harder to do, but you have farther to fall. Okay, I think that's enough standing on round things. Oh, wait. You are standing on a round thing! Earth! Round things are not easy to stand on, but this feels okay, right? Hmm. Why on Earth is the Earth round?
First of all, what keeps your feet stuck to the Earth but not to a basketball? The answer is size. Or maybe I should say, SIZE! The bigger something is, the more pull it has.
Gravity is the force that pulls together things that have mass. Believe it or not, everything has a little gravity--you, a mountain, your pencil--but it's only really, really BIG things that have enough to pull everything into them. The Earth is almost 25,000 miles around so it has a lot of pull. Do you not believe me? Try jumping off of it.
Even with a trampoline I can't escape gravity.
Mountains, rocks, people, houses, dogs--they all stick to the Earth. Sure, you may try to jump into the clouds to get away from the Earth's pull, but it will pull you right back. It takes a lot of energy to get away from a planet's pull, like a rocket. A
planet is a large body of rock or gas that moves in circles around a star. Like Earth, all of them have so much stuff that they pull in more stuff from space. Be glad you do not have the same pull as a planet. If you did, everything would fly into you as you walked down the street. Good things could happen, like ice cream landing on your face. But then a train might follow.
That was tasty!
Earth may be big, but it has nothing on our sun. The sun is 432,450 miles around. And that is only considered to be medium-sized star for a star. A
star is a bright glowing ball of gas in space. Stars are so big and have so much pull that they draw in all of the planets around them. Our star, the sun, gives us heat and light. It also keeps us from sailing off into the universe like a ball in a game of pool. In that game, falling into a black hole would not be good.
So there's only one question left. That's our first question: WHY IS THE EARTH ROUND? And how does it become so round? A
sphere is a round thing, like a ball. Think of a pile of garbage. The more stuff you pile on top, the more it will round out and look smooth. If Earth's gravity wasn't pulling it down anymore, all the stuff would pile and round out on all sides. Things would fill in the spaces as everything pulled toward the middle . . .
A little bit more and we'll have us a sphere.
All right, I lied. One final question. Why isn't the Earth bumpy? Actually, it kind of is! Look at the mountains! The lakes! But our Earth is so big, it's like a sphere, a giant blue marble floating in space. When planets are being made, the small parts that make them up are super hot and are able to move around a lot. So our Earth was once a big ball of liquid. It was easy for it to be smoothed out and pulled into a ball. That's what gives it a mostly flat surface. I don't think it would bounce very well though.
So there you have it. Earth is a big ball that's pulled in by the gravity of the sun. They are both round because when a large body with a lot of stuff it will pull in anything that's around. When our Earth was being made it was super hot, so it pulled in stuff to fill in the spaces. Then it melted this stuff, to make a perfect sphere. And a big one at that. And that's why the Earth is the only ball you should try standing on. The others just do not have the same gravity to hold you up.
References:
Explorations. "Why Is the Earth Round?" Explorations, 2012. <http://explorations.ucsd.edu/for-kids/voyager/2011/voyager-why-is-the-earth-round/>
Space.com. "How Big Is the Sun?" Space, 2009. <http://www.space.com/17001-how-big-is-the-sun-size-of-the-sun.html>