Purum keeps walking with his eyes glued to the ground. "The ground is flat, so why does everyone say it's round?"
Like little detectives, Didi and Purum gather clue after clue and crack the secret: Earth really is round, and it keeps spinning round and round.
Along the way, we'll find out why we have day and night, and why the stars change with the seasons.
Why Purum walks staring at the ground
"Strange… it's really strange…"
Purum had been walking with his eyes glued to the ground for a while when — bump! — he ran right into Popo's back.
When I asked what was worrying him, Purum stuck out his lip in a pout.
"My sister says Earth is round… but no matter how hard I look, the ground is flat. And it doesn't even move!"
Now that he says it — true enough. Even I find it hard to believe that this ground I step on every day is a round ball.
Saerom, who said she'd looked it up, started with "Well… that's… that is…" before trailing off and shouting, "Popo, tell us!"
Up into the sky to find the evidence
Popo grinned. "Hearing it a hundred times still doesn't convince you, does it? Then let's go find the evidence ourselves."
"Come on out, Popoca!" The spaceship whooshed into view, and up into the sky we went.
Our detective game — seeing with our own eyes that the ground is round — was about to begin.
Clue one — a ship appears top first
Far out at sea, a ship comes sailing in.
But isn't that odd? You don't see the whole ship at once — the top part (the mast) peeks up first.
"The ship isn't rising up out of the seabed, so why does the top show first?" Purum narrowed his eyes, deep in thought.
If the ground were flat, the whole ship should appear small and then just grow bigger.
The fact that the top shows first is a clue that the sea curves outward, bulging like a ball. Oh — clue one, found!
Clue two — the North Star sits at a different height in different places
Popo pointed up at the night sky. "The North Star stays in the north all four seasons long. But its height looks different depending on where you are."
"Huh? If the ground were flat, wouldn't it look the same wherever you stood?" Purum caught on first. He's right — the fact that it looks different is yet another clue.
Clue three — the satellite photo, plain as day!
And now the clinching clue. As we climbed even higher —
Wow, the Earth looks round!
In the photo a satellite took from space, the Earth is a blue ball — anyone can see it.
Long ago people could only guess, but now we have the photo. Purum threw both hands up. "It really IS round!"
Beside him, Saerom puffed up proudly. "See? My search was right after all."
But wait — it moves? The secret of day and night
So it's round, all right. But, just as Purum wondered, does it really move?
Popo gave the Earth a little tap, and it began spinning round and round all by itself, like a top.
This is called rotation.
The side the sunlight reaches is day, and the side turned away from it is night.
"Aha — so it's day and then night because the Earth keeps turning!" Purum slapped his knee. He looked nothing like the boy who'd been staring at the ground a moment ago.
