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Will spun in place, holding a basketball in one hand and a baseball in the other.

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"If the basketball is the sun and the baseball is Earth, the baseball should go around the basketball," he told himself.

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"But why can't we see the sun all the time?"

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Dizzy, he lay down on his bed and stared at his homework.

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"All I know is that it gets dark at night—not why!"

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Will tried to focus on the worksheet, but he soon dozed off.

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When he opened his eyes, he was amazed to see Earth and the moon before him.

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"Cool!" he exclaimed, pulling his phone, QT, out of his pocket. "Am I on a spaceship?"

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"Yes," QT replied. "Earth is beautiful, isn't it?"

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"It sure is. But why can I see only one side of it?" Will asked. "And of the moon too?"

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"That's because the sunlight's coming from the left, so it can't shine on the opposite side.

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On the side that's lit right now, it is day. On the dark side, it is night.

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If you look carefully, you can see the lights of big cities."

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"I see them! But when the sun rises, the sides will switch, right?"

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"What you call the sun rising is not exactly correct.

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We don't experience day and night because the sun moves, but because Earth spins on its axis."

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Will was confused. "Its axis? What's that?"

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QT explained, "An axis is an imaginary line that goes through Earth's poles. I'll show you."

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 QT shot a beam of light at the ship's controls, and a screen lit up one wall.

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"Earth rotates on its axis, spinning like a top, once every 24 hours.

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Although the rotation is too slow for us to see in real time, here's a video of the last 24 hours, sped up," QT said.

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"The planet is spinning!" Will observed.

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"But to us on Earth, it looks like the sun is rising in the east, moving across the sky, and setting in the west."

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"Exactly," QT replied. "The view from Earth is quite different from the view from space.

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Watch the moon in this next video, and you'll see what I mean," QT went on.

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"The moon's shape appears to change—what we call the phases of the moon—but it actually stays the same.

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What changes is how much of it we can see.

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Half the moon, the side facing the sun, is always lit.

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But because the moon orbits around Earth, we can usually only see part of the lit side."

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"Oh . . . ," Will said, watching closely.

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After the video finished, he asked, "So where the moon is in its orbit determines how much of it we can see?

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That creates the phases of the moon?"

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"You've got it," QT answered. "But I want you to see something else.

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Take another look at Earth out there. Doesn't it look like it's tipping a little?"

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Staring at the planet, Will exclaimed, "Hey! It does."

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"Earth is tilted by about 23.5 degrees; that means it's not straight up and down.

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If we move farther out, you'll see what I mean," QT said.

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As they zoomed through the solar system, Earth got smaller and smaller, and the sun came into view.

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Will gasped. "It's enormous! And bright!"

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"It sure is. It looks small from Earth because it's very far away, about 150,000,000 kilometers.

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So the orbit is a long trip. It takes about 365 days to orbit the sun once—"

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"That's a year," Will interrupted.

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"Yes, it is. Earth's tilt causes the seasons. I'll show you a diagram," QT said.

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The screen lit up, showing Earth's orbit of the sun.

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"The planet is always tilted the same way,

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so the northern hemisphere is pointed toward the sun for part of the year, and the southern hemisphere for another part."

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"The seasons in the northern and southern hemispheres are always opposite?" Will asked.

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"Right. The hemisphere that's tilted away from the sun experiences winter, and the one that's tilted toward the sun experiences summer.

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Right now it's December—winter in the northern hemisphere, where you live.

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See how little direct light it's getting compared to the southern hemisphere?

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There it's summer because it's getting a lot of direct light.

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When the northern hemisphere is tilted toward the sun, it gets much more sunlight, so it's summer.

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For spring and fall, neither hemisphere is tilted toward the sun, so the weather is the same."

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Will thought for a moment. "Is that why the days are longer during the summer?

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Because the northern hemisphere is tilted toward the sun?"

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"Absolutely. That's what causes the 24-hour days in the Arctic.

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And the Antarctic doesn't see the sun at all, causing 24-hour nights," QT said.

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"But I'd better turn the ship around now.

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We don't want to be floating out in space forever, do we?"

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"No." Will grinned. "I've got basketball practice tomorrow!"

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The spaceship started moving back toward Earth, but as they flew past the sun, the ship swerved.

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"What's going on?" Will asked. Red lights on the controls were blinking, and a warning bell was ringing.

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"We might have gotten too close to the sun," QT said, sounding worried.

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"Its gravity is pulling us in!" The ship was going faster and faster, straight into the sun.

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Will covered his eyes with his hands to protect them from the blinding rays.

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When he lowered his hands, squinting at the bright light, his dad was standing by the bedroom door with his hand on the light switch.

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"Why was the light off?" he asked Will.

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"I was just napping," Will replied, crawling out of bed.

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"Thanks for waking me up. I have to finish something."

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He picked up his homework and quickly answered the questions.

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"That wasn't so hard," he thought, putting away the basketball and baseball.