Why is the Earth round? (Tokyo, 2nd grade)

When we say "the Earth is round," most people think of a perfect ball, like a basketball. A perfect ball means that, if you measure from the very middle of the ball to any spot on its outside, the distance is always the same. When you look at a globe, it looks like a perfect ball, doesn't it?

But actually, the Earth isn't a perfect ball. It's shaped more like something called an "oblate spheroid." What's an oblate spheroid? Imagine an oval shape, then make it 3D! If you cut an orange or a tangerine from top to bottom, it often looks like an oval, right? That's because an orange is a bit wider around the middle and a little flatter at the top and bottom. The Earth is a bit like that, too!

Scientists called "geodesists" have proven that the Earth is this "oblate spheroid" shape. Geodesists study the Earth's shape, size, and even how gravity pulls on things, to learn all about our planet. For example, the distance from the North Pole to the South Pole is about 12,714 kilometers. But if you measure across the middle of the Earth (the equator) as if it is a circle, the diameter is about 12,756 kilometers! See? The Earth is a tiny bit wider around the middle than it is from top to bottom.

So, the Earth isn't a perfect ball after all. That's why, to be accurate, instead of saying "the Earth is round," we can say "the Earth is an oblate spheroid!"

Original post:

なんで地球はまるいのか（東京都、小２）

「地球は丸い」というと、球体を想像する人が多いと思う。球体とは、バスケットボールみたいに、ボール(球)の中心から表面への距離が全て同じの形をしたもののことだ。地球儀を見ると地球は球体に見えるよね。だけど地球は正確には球体ではなく「楕円体」という形をしている。「楕円体」とは、楕円を立体にした形のことだ。みかんを上から切ると、よく楕円形になるね。それは、みかんは横が長くて縦が短い、楕円を立体にした形をしているからだ。地球が楕円体であることは測地学によって証明されている。測地学とは、地球の形や大きさ、重力などを測ることで地球の姿をとらえようとする学問分野のことだ。例えば北極と南極の間の距離は約1万2714㎞だけど、赤道を円だと考えた時、その円の直径は約1万2756㎞だ。赤道の直径の方が北極と南極を結んだ距離が長い。つまり地球は正確には球体ではない。だから「地球が丸い」のもっと正確な言い方は「地球は楕円体」だ。