Three objects. One invisible force. An elegant balance that has held for billions of years — and makes life on Earth possible.
Before we explain anything — just think. You've seen the first thing happen thousands of times. The second one has been happening for 4.5 billion years.
Gravity pulls the Moon toward Earth — just like it pulled that ball. So why hasn't the Moon crashed into Earth in 4.5 billion years?
Three objects — a star, a planet, and a moon — held in perfect balance by gravity and motion. Here is how it all works.
Look up on a clear night and the Sun and Moon appear almost exactly the same size in the sky. But the Sun is 400 times larger than the Moon. The reason they look identical is that the Sun is also almost exactly 400 times farther away. This remarkable coincidence is what makes total solar eclipses possible — the Moon fits almost perfectly over the Sun's face. Understanding how three objects at such wildly different scales can interact so precisely starts with understanding the force that holds them together: gravity.
A system is a group of parts that interact with and affect each other. The Sun, Earth, and Moon form one system — connected by gravity and motion. Change one part of the system and the rest responds.
Gravity is an invisible pulling force between any two objects that have mass. It always pulls toward the center of a massive object, which is why every person on Earth — no matter where they are standing — feels pulled straight down toward Earth's core.
Earth travels in a nearly circular path around the Sun called an orbit. One full trip around the Sun takes about 365.25 days — that 0.25 is why we add a Leap Day every four years. Earth's speed along this path is roughly 67,000 mph.
The Moon is Earth's only natural satellite. It orbits Earth in about 27–29 days — roughly one month. Earth's gravity holds the Moon in orbit, in exactly the same way the Sun's gravity holds Earth in orbit.
Two words, two completely different kinds of motion — and one of the most common mix-ups in middle school science. Here is how to keep them straight.
| Feature | Rotation | Revolution |
|---|---|---|
| Definition | Spinning on an axis | Traveling around another object |
| Earth's period | ~24 hours | ~365.25 days |
| What it produces | Day and night | The year |
| Direction | Counterclockwise | Counterclockwise |
| Key words | Spin · Axis · Day | Orbit · Year · Path |
Three very different objects, bound into one system. Click any card to read the key facts — then explore the Motion Explorer to see rotation and revolution in action.
Every term below appeared in this lesson. Click any pill to jump to its explanation.
👆 Click any term to jump to its explanation in the lesson
Three questions — no grade, no pressure. Just think it through before the quiz.
Earth completes one full spin every 24 hours and one full orbit around the Sun every 365 days. Which of these is its rotation?
If Earth's gravity suddenly switched off, what would happen to the Moon?
From Earth, we always see the same face of the Moon — the far side was never photographed until a spacecraft flew around it. What does this tell us about the Moon?
10 questions on gravity, orbits, and the motions of our solar system's closest neighbors. Fill in your info below — your score will be sent to your teacher when you submit.
The lesson is just the beginning — go deeper, see the next lesson in the unit, or explore NASA's real data.