Why do you think the earth and the moon do not bump with each other as they travel around the sun

We’re always on the move! Even when you’re standing still, you’re moving! You are moving because the Earth and everything in our solar system is constantly moving.

Our solar system includes the Sun, nine planets and their Moons, comets and asteroids. These objects are sometimes called celestial bodies, and they are constantly moving, too.

At the center of it all is the Sun. It takes the Sun 25 days to spin, or rotate, completely around.

The Earth, which is the third planet from the Sun,

The Earth’s path around the Sun is called its orbit. It takes the Earth one year, or 365 1/4 days, to completely orbit the Sun.

As the Earth orbits the Sun, the Moon orbits the Earth. The Moon’s orbit lasts 27 1/2 days, but because the Earth keeps moving, it takes the Moon two extra days, 29 1/2, to come back to the same place in our sky.

What is an eclipse?

This site has been designed to give your class an overview of astronomy and help them understand why a Total Solar Eclipse can happen. Each section leads with a simple question which is answered through the use of words, pictures, animations and activities. Each section requires approximately 5 minutes, each activity varies.

Dolores Peterson was a New York City School Teacher in Community School District 3 for nine years when she produced this lesson plan for Kidseclipse. She has a Masters in Education from the City University of New York.

What is an eclipse?

Lesson 2 of Kidseclipse TEACH

The moon does not fall to Earth because it is in an orbit.

One of the most difficult things to learn about physics is the concept of force. Just because there is a force on something does not mean it will be moving in the direction of the force. Instead, the force influences the motion to be a bit more in the direction of the force than it was before.

For example, if you roll a bowling ball straight down a lane, then run up beside it and kick it towards the gutter, you apply a force towards the gutter, but the ball doesn't go straight into the gutter. Instead it keeps going down the lane, but picks up a little bit of diagonal motion as well.

Imagine you're standing at the edge of a cliff 100m tall. If you drop a rock off, it will fall straight down because it had no velocity to begin with, so the only velocity it picks up is downward from the downward force.

If you throw the rock out horizontally, it will still fall, but it will keep moving out horizontally as it does so, and falls at an angle. (The angle isn't constant - the shape is a curve called a parabola, but that's relatively unimportant here.) The the force is straight down, but that force doesn't stop the rock from moving horizontally.

If you throw the rock harder, it goes further, and falls at a shallower angle. The force on it from gravity is the same, but the original velocity was much bigger and so the deflection is less.

Now imagine throwing the rock so hard it travels one kilometer horizontally before it hits the ground. If you do that, something slightly new happens. The rock still falls, but it has to fall more than just 100m before it hits the ground. The reason is that the Earth is curved, and so as the rock traveled out that kilometer, the Earth was actually curving away underneath of it. In one kilometer, it turns out the Earth curves away by about 10 centimeters - a small difference, but a real one.

As you throw the rock even harder than that, the curving away of the Earth underneath becomes more significant. If you could throw the rock 10 kilometers, the Earth would now curve away by 10 meters, and for a 100 km throw the Earth curves away by an entire kilometer. Now the stone has to fall a very long way down compared to the 100m cliff it was dropped from.

Check out the following drawing. It was made by Isaac Newton, the first person to understand orbits. IMHO it is one of the greatest diagrams ever made.

What it shows is that if you could throw the rock hard enough, the Earth would curve away from underneath the rock so much that the rock actually never gets any closer to the ground. It goes all the way around in the circle and might hit you in the back of the head!

This is an orbit. It's what satellites and the moon are doing. We can't actually do it here close to the surface of the Earth due to wind resistance, but on the surface of the moon, where there's no atmosphere, you could indeed have a very low orbit.

This is the mechanism by which things "stay up" in space.

Gravity gets weaker as you go further out. The Earth's gravity is much weaker at the moon than at a low-earth orbit satellite. Because gravity is so much weaker at the moon, the moon orbits much more slowly than the International Space Station, for example. The moon takes one month to go around. The ISS takes a few hours. An interesting consequence is that if you go out just the right amount in between, about six Earth radii, you reach a point where gravity is weakened enough that an orbit around the Earth takes 24 hours. There, you could have a "geosynchronous orbit", a satellite that orbits so that it stays above the same spot on Earth's equator as Earth spins.

Although gravity gets weaker as you go further out, there is no cut-off distance. In theory, gravity extends forever. However, if you went towards the sun, eventually the sun's gravity would be stronger than the Earth's, and then you wouldn't fall back to Earth any more, even lacking the speed to orbit. That would happen if you went about .1% of the distance to the sun, or about 250,000 km, or 40 Earth radii. (This is actually less than the distance to the moon, but the moon doesn't fall into the Sun because it's orbiting the sun, just like the Earth itself is.)

So the moon "falls" toward Earth due to gravity, but doesn't get any closer to Earth because its motion is an orbit, and the dynamics of the orbit are determined by the strength of gravity at that distance and by Newton's laws of motion.

note: adapted from an answer I wrote to a similar question on quora

Force pushing perpendicular to the velocity of the object. Let me call this a "sideways" force.

If it is just a sideways force, the object doesn't speed up and it doesn't slow down. It just turns. Of course, in order to exert a continuous sideways force, the force would have to point in a different direction as the object turns or it wouldn't still be "sideways". Here is an example. Take a ball at the end of a string - or maybe a yoyo since the string is already attached. Swing the ball around in a circle. Why does it move this way? The string pulls on the ball. But since the string can only pull in the direction of the string (you can't push with a string), the ball has a sideways force on it and changes direction.

Can a force be sideways and in the direction of the velocity at the same time? Yes. In this case, the object would both speed up AND change directions.

Back to the Moon

I am sure you have noticed that I still didn't answer the question. Why doesn't the moon fall into the Earth? Maybe I have given you enough information about forces such that you can answer the question yourself. Or maybe I haven't. Here is a diagram of the Earth-moon system.

Oh. You don't like that diagram. I know why - because it is drawn to scale. Yes, the moon is really that far away from the Earth. You never see it this way in textbooks because it is too hard to see. Here is the Earth-moon with the moon only 1/5th the distance it is suppose to be (but the correct relative size).

Here you can see the red arrow represents the gravitational force on the moon. If the moon were moving in a perfect circle, the gravitational force would always be "sideways" and just cause it to change its direction.

But wait! There's more. Guess what? The moon pulls on the Earth with the exact same magnitude of force that the Earth pulls on the moon since it is the same interaction. But wouldn't this also make the Earth move in a circle? Yup. Essentially, it does. The only thing is that Earth's mass is 81 times greater than the mass of the moon. This means that although it moves in a circle, it moves in a much smaller circle. The circle that the Earth moves around is so small that the center of this circle is inside the Earth. Cool, isn't it?

The Real Moon

I said the gravitational force on the moon would be "sideways" if the moon moved in a perfect circle - but it doesn't. Let me draw an exaggerated diagram of the Earth moon system with a non-circular orbit.

Maybe it is difficult to see, but in this case the gravitational force on the moon is NOT perpendicular to the velocity. What happens in this case? Well, part of the gravitational force is in the same direction as the velocity, the moon will increase in speed. Also, since part of the force is a sideways force, the moon will change direction. This is what happens with most orbits. The moon moves closer to the Earth and speeds up as it does so. As the moon moves away from the Earth, the opposite happens. This is part of the reason behind the Super Moon that was popular a while ago.