The moon also moves sideways in accelerating speed, thus making the distance between earth and the moon greater. Doesn’t it makes more sense that we would say that the moon falls out of orbit or falls from the earth? This question is keeping me busy for over 5 years. I really like to hear from scientists about this subject. Thanks in advance!
In: Physics
I’ve never heard of a single person say “the moon is falling towards the Earth” or anything like it. Where did you here this?
[https://www.youtube.com/watch?v=ymC6fuX03ns](https://www.youtube.com/watch?v=ymC6fuX03ns)
Here basically what orbit is. You have a sideway velocity and gravity. Gravity is pulling the object toward the earth, aka it’s falling toward the earth, but the if the velocity is high enough the object will constantly missing the earth. That’s how all orbit work.
Like i said there is two variable, gravity and velocity. If the gravity is higher, the orbit will become smaller and smaller until the object fall back on earth. If the velocity of the object is higher than the gravity, the orbit will become bigger and bigger until the object reach espace velocity and espace the gravity. If the velocity (well the force behind the velocity, but I want to keep thing simple) and the gravity are equal, then the object is in a stable orbit.
Since gravity decrease over distance, so is the velocity of object in stable orbit. Mercury have an average orbital speed of 47.632 km/s, but it’s 13.07 km/s for Jupiter.
The moon is slowing down over time, because of the effect of tides. The moon pull on the water forming tides, but that water is also pulling on the moon, from one side. This slow down the moon, which mean that it’s orbit is slighy destabilized and need to get bigger to regain stability.
An idealistic depiction of the moon would be at a static distance from the Earth. Still, it would accelerate toward the Earth at all times, due to the nature of the [centripetal model](https://en.wikipedia.org/wiki/Centripetal_force) of its acceleration.
It is exactly as a yo-yo you swing in a horizontal circle: the force that keeps the yo-yo moving in a circle is the string directly toward the center (you). The yo-yo is ideally **not** accelerated “sideways” at any point. There’s no string between us and the Moon, but what takes its place and direction is gravity.
However, the Moon must necessarily have an imperfect orbit of an ellipsis, which causes its distance from us to wax and wane. [Tidal effects](https://en.wikipedia.org/wiki/Moon#Tidal_effects) and the larger schemes of gravities of the Solar System and will cause it to slowly move even further away over time.
When an object is in a circular orbit of another object, there is only a single force acting between the two and the direction of that force is pulling the object in orbit *directly* towards the other thing. Not any other angle, only directly towards the centre of the circle. For the earth and the moon that force is gravity. For an object on a string that you are spinning in circles over your head using your hands that force comes from the string. For an object moving around a circular track that force is the track itself which is angled so that the normal (“straight up” relative to the track’s angle) points towards the centre of the circle, etc.
So if the moon is experiencing a constant gravity pull towards the earth, then you can argue it is always falling towards earth, but since it also has sideways motion it is constantly missing the earth as it falls. Same applies to anything in general orbit like the space station… but since they’re a lot closer to the earth their horizontal speed needs to be much higher to “keep missing” the earth as they fall.
Of course, the moon’s orbit is not a perfect circle and it’s not moving at quite the exact speed required to maintain a constant orbit, etc so it does drift a bit.
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The orbit = falling is a bit of a simplification of the terminology. What it means is that an object in orbit is in *free fall* which means it’s only under the influence of gravity (there are no other forces). This doesn’t necesarilly mean the object is falling in the sense of getting closer to Earth, for instance an object that has reached the escape velocity will never fall again and yet technically it’s still in *free fall* as long as it is within the gravitational influence of the Earth.
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