Our Cosmic Home: Earth as a Starting Point

Close your eyes for ten seconds. What do you hear? Maybe the hum of your refrigerator, the chirp of sparrows outside your window, or the sound of your sibling playing in the next room. Now open your eyes and look out the window. What do you see? The sun might be beaming through the clouds, or the moon might be hanging low over the rooftops, or if it’s late enough and you live far from city lights, you might spot a few twinkling stars scattered across the sky. All of these things—your home, the oak tree on your street, the sun, the moon, the stars—are part of your cosmic home: planet Earth.

Have you ever wondered why the sun rises in the east every morning and sets in the west every evening? Why summer afternoons are perfect for running through the sprinkler, while winter mornings are so cold your breath fogs up in the air? Why you can only see a handful of stars on a clear night in the middle of a city, but hundreds when you go camping in the mountains? All of these questions have answers, and they all start with the planet you live on right now.

Earth is a rocky planet that orbits a medium-sized star we call the Sun. To give you a sense of scale: if the Sun were a basketball sitting on the 50-yard line of a football field, Earth would be a tiny pea sitting 150 yards away. It’s not the biggest planet in our solar system, and it’s not the smallest, but it’s the only one we know that has liquid water on its surface, air to breathe, and the perfect temperature for plants, animals, and people to thrive. For us, it’s the most important planet in the entire universe—at least for now!

The first big secret of Earth is that it’s always moving, even when it feels like you’re sitting completely still. The first movement we’re going to talk about is rotation. Rotation means spinning around an imaginary line that runs through the center of the planet, from the North Pole to the South Pole. We call this line the axis. Think of it like the stick that goes through a spinning top: the top spins around the stick, and Earth spins around its axis.

Earth spins all the way around its axis once every 23 hours, 56 minutes, and 4 seconds. We round that up to 24 hours, and we call that one day. But here’s the cool part: only half of Earth can face the Sun at any time. The half that faces the Sun gets sunlight, so it’s daytime. The half that faces away from the Sun is in shadow, so it’s nighttime. As Earth spins, the part of the planet you’re standing