Rocket arrives at the Sky Deck with a long string tied to a key. "Nova, I read about a pendulum in Paris."
"In 1851, a scientist hung a 60-meter pendulum from a dome," Raven says. "He set it swinging and watched."
"The swing seemed to turn," Rocket says. "But it was not the pendulum turning. It was the floor, the whole Earth, turning beneath it."
Nova hovers over the dangling key. "What do you notice about the proof?" she asks.
Raven thinks. "You cannot feel Earth turning. But you can catch it moving something that does not turn with it."
Rocket swings the key. "Mine is too short to show it." Nova hums. "True. But your porch shadows have shown it all week."
For a long time people argued whether the sky turned around a still Earth or Earth itself turned.
OpenStax Astronomy says that since the seventeenth century it has been generally accepted that Earth turns.
In 1851 the French physicist Jean Foucault hung a 60-meter pendulum from a dome in Paris. It weighed about 25 kilograms.
After a few minutes the pendulum's plane of swing was turning. Foucault explained that Earth was turning beneath it.
You can find such pendulums in many science centers and planetariums around the world today.
Every sunrise is your part of Earth turning into the light. Every sunset is your part turning out of it.
The Moon can often be seen in the daylight too. It rises and sets each day as Earth turns, just like the Sun.
Because Earth turns toward the east, places east of you turn into the sunlight before you do. They see sunrise first.
Next week you follow the Moon. It also rises and sets daily, but its shape changes over about a month.
| What you see | What is really happening |
|---|---|
| The Sun appears in the east in the morning | Your spot on Earth turns toward the Sun |
| The Sun is highest around midday | Your spot faces the Sun most directly |
| The Sun sinks in the west | Your spot turns away from the Sun |
| Stars seem to wheel across the sky at night | Earth keeps turning; the stars stay put |
| Week review | True or false? |
|---|---|
| Earth turns on its axis about once every 24 hours. | ? |
| The Sun travels around Earth once a day. | ? |
| East is the direction Earth turns toward. | ? |
| The ground feels still because everything near you turns with you. | ? |
| A day on Jupiter is longer than a day on Earth. | ? |
Strong week, sky scientist. Tomorrow is Sky Night: the family checks where the Sun sets from the Sky Deck.