LESSON 04 · SPACE
Why do day and night alternate?
The short answer
Earth spins on an imaginary line called its axis. The side facing the Sun has daytime, and the side facing away has night. One full turn takes about 24 hours.
Earth is always spinning
Earth rotates on its axis, an imaginary line through the North and South Poles. It turns from west to east, which is why the Sun appears to rise in the east and set in the west. The Sun is not racing across our sky; we are turning past it.
One turn is one day
Measured from one noon to the next, a rotation of Earth takes about 24 hours. At any moment, roughly half of the planet is in sunlight and half is in darkness. The line between them, where people see sunrise or sunset, is always moving.
Why the time differs around the world
Because different places face the Sun at different times, it can be morning in one country and night in another. Time zones divide the world so that clocks roughly match the daylight in each region.
Why summer days are longer
Earth's axis is tilted by about 23.4 degrees. For part of the year, your half of Earth leans toward the Sun, so days are longer and sunlight arrives more directly. Half a year later, it leans away and days are shorter. The seasons come from this tilt, not from Earth being closer to the Sun.
Try it at home
Make day and night with a flashlight
You will need
- A flashlight, or a lamp without its shade
- A ball or an orange
- A small sticker or a pen mark
- A dark room
What to do
- Mark a spot on the ball to stand for your home.
- Shine the flashlight at the ball from about one meter away.
- Slowly turn the ball counterclockwise, as seen from above.
- Watch your spot move into the light (sunrise), across it (noon), and back into the dark (sunset).
- Tilt the top of the ball slightly toward the light and turn it again. Notice how long your spot stays in the light.
SafetyDo not touch a hot lamp bulb, and never look straight into a bright light.
What's happening?
The flashlight lights only the side of the ball that faces it, just as the Sun lights only half of Earth. Turning the ball creates a cycle of day and night. Tilting it changes how much of each turn your spot spends in the light, which is why day length changes with the seasons.
Key words
- Rotation
- Spinning around an axis. One rotation of Earth is about one day.
- Axis
- The imaginary line through the North and South Poles that Earth spins around.
- Tilt
- The lean of Earth's axis, about 23.4 degrees.
- Time zone
- A region that uses the same clock time.
Check your understanding
Does the Sun travel around Earth every day?
No. Earth's rotation makes the Sun appear to move across the sky.
In which direction does the Sun appear to rise?
In the east, because Earth spins from west to east.
What causes the seasons?
The tilt of Earth's axis, which changes how directly sunlight reaches each half of Earth during the year.