When you toss an apple up in the air, it falls back to the ground. Anything that goes up comes back down. But many questions come to mind when we look at the moon in the sky. Why doesn’t the moon fall off the sky? Scientists estimate that our moon is approximately 4.53 billion years old. It means it was only 60 million years after the solar system.
So what has been keeping the moon from falling on Earth for billions of years? The answer is gravity and the shape of the Earth. Everything in our solar system is in its place due to gravity. Gravity attracts masses to the earth’s center, which translates to force. That pulls an apple to the ground. It is the same force that makes the moon orbit Earth. Gravity is a force and a geometric property that describes space-time Einstein found that space and time are one thing in space-time. The warping of space-time is what we feel is gravity.
Why doesn’t Moon fall on Earth?
The Moon does not fall onto Earth because it is in a constant freefall while simultaneously moving in a curved path around Earth. This motion is due to the balance between the Moon’s gravitational attraction toward Earth and its forward momentum or velocity. Here’s a more detailed explanation:
Gravitational Force: The Moon and Earth exert gravitational forces on each other. According to Newton’s law of universal gravitation, every object with mass attracts another object with mass. The force of gravity between two objects depends on their masses and distance. Earth’s gravity pulls the Moon toward it, trying to bring it closer.
Centripetal Force: The Moon also moves tangentially with a certain velocity (speed and direction) around Earth. This velocity gives the Moon a forward momentum. According to Newton’s first law of motion, an object in motion will continue in a straight line at a constant speed unless acted upon by an external force.
Balance of Forces: The Moon’s gravitational attraction toward Earth and its forward momentum create a balance of forces. The gravitational force acts as a centripetal force, continuously changing the direction of the Moon’s velocity, causing it to move in a curved path instead of a straight line. This curved path is an elliptical orbit around Earth.
Constant Freefall: The Moon’s motion can be likened to a satellite in orbit around a planet. It is in a state of constant freefall. The gravitational force continuously pulls the Moon toward Earth, but because of its velocity, the Moon keeps missing Earth and remains in orbit. Combining the Moon’s velocity and Earth’s gravity results in a stable orbital motion.
All the planets, the sun, and the distant stars in galaxies located billions of light-years away all have an attractive force due to their mass. So these large bodies attract other smaller and larger bodies towards them. Sun is around 1.3 million times bigger than Earth and has powerful gravity. It attracts every other celestial body in our solar system toward it.
Similarly, Earth attracts the moon towards it. Planet Earth is almost a perfectly round body. As gravity pulls the moon towards it, it gets pulled toward its center. So actually, the moon is constantly falling on Earth. But it is moving so fast that it never gets to hit Earth. To explain this, let’s take the example of a tennis ball thrown in the air.
When you throw a tennis ball with little force aimed at an angle of 45 degrees, the ball falls slightly farther than you. But when you throw it faster, it travels much farther, eventually hitting the ground. The more the ball’s speed is, the more distance it will travel before hitting the ground.
The moon is located at three hundred and eighty-four thousand four hundred kilometers. So the force of gravity on the moon is much less than on a tennis ball or any other object here on Earth. Also, the moon is traveling horizontally at a very fast speed.
As the Earth pulls it down, it keeps moving horizontally, revolving around Earth in orbit instead of making it fall. The same is the case with the satellites that we send into space. Scientists launch them into space after carefully calculating the speed required at a particular height above Earth to keep the satellite rotating around Earth. Then that satellite is moved at that speed, and the rest is done by gravity. As gravity pulls the satellite down, its horizontal motion and Earth’s round shape allow it to move in an orbit instead of falling.
A name for these forces keeps a body revolving around another body in a circular motion. These forces are known as centripetal and centrifugal forces. Centripetal force is the force that is directed inward toward the center of rotation. Centrifugal force is an apparent force felt by an object moving in a curved path.
Centrifugal force acts outwardly away from the center of rotation. So when the centripetal force, gravity, pulls the moon towards Earth. Also, the Centrifugal force moves it away from Earth and gives it its horizontal motion.
The moon has been revolving around Earth for billions of years, and the chances are that it will continue for billions more years unless something unusual happens. It causes the moon to move away from Earth or fall on Earth. It can be due to being hit by a massive asteroid or some other large celestial body coming close due to mysterious space events. But that is unlikely to happen in the foreseeable future, and we do not need to worry! So the moon isn’t falling on Earth.
Gravity or curved space doesn’t always make things fall in. Sometimes objects are in orbit, like the Moon around the Earth. The Moon is like a giant marble. Einstein’s concept: that matter curves space around it also determines how objects move around massive objects, like the Moon or the Earth around the Sun.
Why does the Moon orbit the Earth? Because space is curved around Earth, too. How fast objects move through space due to gravity and how much space is curved at any point depends on two things.
- How much mass the central object has?
- Distance from the center of that massive object.
Close to the stars, space is curved more than out here, where space is curved less.
Lang, Kenneth. The Cambridge Guide to the Solar System (2nd ed.). Cambridge University Press.
Morais, M. H. M.; Morbidelli. “The Population of Near-Earth Asteroids in Coorbital Motion with the Earth.”
Williams, David. “Moon Fact Sheet.” NASA/National Space Science Data Center.
Smith, David E.; Zuber, Maria. “Topography of the Moon from the Clementine lidar.”