Rocket spins a ball on the tip of a pencil. "If Earth turns, it needs a line to turn around."
"That line is the axis," Raven says. "It goes through the North Pole and the South Pole."
Nova brightens a thin beam onto the ball. "Pretend my light is the Sun. What do you notice about the lit side?"
"Half is lit, half is dark," Rocket says. "And the dark half is night." He turns the ball. "Now the other half is night."
Raven writes it down. "One full turn, one day."
"How long is one turn?" Rocket asks. Nova hums. "A page from NASA has that number. Go and read it."
OpenStax Astronomy says we can put an imaginary stick through Earth's North and South Poles to represent the axis.
Earth turns on this axis every 24 hours. That is why we see the Sun, Moon and stars rise and set with clockwork regularity.
The axis defines the North and South Poles and the equator, which is halfway between them.
Two other directions come from Earth's motion. East is the direction toward which Earth rotates, and west is its opposite.
NASA puts it simply: Earth spins around this pole, making one complete turn each day. That is why we have day and night.
The NASA Earth fact sheet lists Earth's rotation period as 23.9 hours and its length of day as 24.0 hours.
Why two numbers? As Earth turns, it also moves along its orbit. The Sun seems to slip about 1 degree east among the stars each day.
So Earth must turn a little more than one full rotation to bring the Sun up again. That small extra turn makes the 24-hour day.
Every part of Earth's surface gets some day and some night, because the whole planet turns past the Sun.
| Statement | True or false? |
|---|---|
| The axis passes through the North and South Poles. | ? |
| West is the direction Earth turns toward. | ? |
| The equator is halfway between the poles. | ? |
| Earth's rotation period is listed as 23.9 hours. | ? |
Clear thinking. Tomorrow is Sky Deck Lab: a lamp, a ball and a sticker for home.