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I understand that Earth has a quite big speed such as it ends up around the orbit. The sun can't really attract it till the end due to Earth's speed and earth really can't escape due to gravity, hence, it ends up in an orbit.

I'm wondering about 2 questions.

  1. Why does the earth have the speed at all ? I understand everything as long as I presume that earth has big speed, but where does this speed come from and why ?

  2. Why doesn't the speed decrease ? Since gravity of sun tries to pull it, each time, it should be causing the earth to slow down, eventually stop it and pull it, but maybe earth always has this speed. If I had a car with an engine, I'd understand as engine gives car a speed and it can always give the same speed without decreasing, but why does earth keep having the same speed(well, not maybe the same speed, but it really doesn't decrease that much).

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    $\begingroup$ The fact that the Sun accounts for 99.8% of the solar system's mass should be a hint that a lot of things didn't have the right speed. Everything we see today is a statistical outlier. $\endgroup$ Apr 28, 2023 at 20:42
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    $\begingroup$ On the other hand most of the angular momentum is in the planets. $\endgroup$
    – James K
    Apr 28, 2023 at 20:45
  • $\begingroup$ @GregMiller So the fact that all stars form planets... is an outlier...?? $\endgroup$ Apr 28, 2023 at 21:47
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    $\begingroup$ @AtmosphericPrisonEscape no, what he is clearly saying is that most of the things that previously went towards the sun, are now inside the sun. It is rare for stuff to end up in orbit by chance. $\endgroup$
    – fabspro
    Apr 29, 2023 at 16:29
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    $\begingroup$ @AtmosphericPrisonEscape he is referring to the solar system's mass, not the fact that most solar systems will have some 'outliers' and therefore have satellites. $\endgroup$
    – fabspro
    Apr 30, 2023 at 2:16

3 Answers 3

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Just by random chance, every giant cloud of gas and dust has some average rotation. Before the sun was even there, the solar system was just a large cloud of dust and gas like this. When it collapses under gravity, particles going in different directions collide, and rotation speeds up just like a spinning figure skater pulling in their arms, so you end up with a rotating disk of gas and dust. Some parts of that disk happen to be denser to others, and their gravity makes them even more dense, until planets form. So the motion is a remnant of the initial random motion of trillions of atoms.

Another way to think about it is that all the stuff that made up the earth was originally far away from where the sun was going to form. As everything fell towards the center, it picked up speed because of gravity. Because of that initial random motion, nothing quite hit the center, and all the collisions in the disk kept stuff from just zooming past the center and flying back out again.

Basically the speed comes from gravity as the stuff in the cloud fell towards the center, and the fact that the speed (properly, velocity) is going past the sun instead of straight towards it comes from all the initial small random motions very far away.

Also, this similar question has a really helpful answer, and this other question has an answer that I wrote very similar to this one, but with a little more detail.

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  • $\begingroup$ "just by random chance, every giant cloud of gas and dust has some average rotation" - they rotated around what ? $\endgroup$
    – Matt
    Apr 28, 2023 at 21:12
  • $\begingroup$ So, there was dust and gas around the sun at first. Question 1: why did they have rotation at the beginning ? Question 2: this gas and dust due to gravity with each other, caused the collapse near the center of the disk of gas and dust, but why would this cause the increase of the rotation speed ? $\endgroup$
    – Matt
    Apr 28, 2023 at 21:17
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    $\begingroup$ If you pick any line, and call that the axis, by chance some more particles will be moving in a clockwise direction than counterclockwise or vice versa. If you look at lines passing through the place where the cloud is most likely to collapse to, there will be some line where the average angular momentum is greatest. The axis of the solar system is likely to end up similar to that line. $\endgroup$ Apr 28, 2023 at 21:17
  • $\begingroup$ Why don't you spell it out, it's the center of mass towards which the entire mass of the cloud will collapse to and start rotating around it, due to its initial spatial asymmetries at the start of the collapse. $\endgroup$ Apr 28, 2023 at 21:50
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    $\begingroup$ Something that rotates rotates about (around) its centre of rotation. $\endgroup$ Apr 29, 2023 at 18:14
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The Earth and the other planets formed in a disc of dust that was in orbit about the sun. The sun and planets formed from a nebula, the matter in this nebula had, by random effects, a small amount of rotation. As the matter fell towards the centre by gravity, that rotation became concentrated into a disc and then into the planets formed from that disc.

The sun pulls at the Earth in a direction perpendicular to its direction of motion. This means that the sun doesn't change the speed of the Earth, it only changes its direction.

To make something slow down you must apply a force in a direction opposite to the direction of motion. If something is moving in a circle, there is no force opposite to the direction of motion.

(Actually, since the orbit of the Earth is an ellipse, sometimes the sun is pulling the Earth forward, and making it speed up, and sometimes it is slowing it down slightly, but the speeding up and slowing down cancel out after a full orbit, so the Earth doesn't lose any speed on average)

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  • $\begingroup$ So, there was dust and gas around the sun at first. Question 1: why did they have rotation at the beginning ? Question 2: this gas and dust due to gravity with each other, caused the collapse near the center of the disk of gas and dust, but why would this cause the increase of the rotation speed ? $\endgroup$
    – Matt
    Apr 28, 2023 at 21:16
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    $\begingroup$ If you have a random cloud of dust and gas, some of it will (by random motions) have some rotation. Any dust that didn't have rotation fell into the sun at the beginning. $\endgroup$
    – James K
    Apr 28, 2023 at 21:25
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    $\begingroup$ By conservation of angular momentum, If you move the mass closer to the centre it must move faster. This is the "ice skater drawing in her arms" effect. As she draws her body into a narrow line, her rate of spinning must increase $\endgroup$
    – James K
    Apr 28, 2023 at 21:27
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Most answers have already went into 1 so for that I will just say:

The starting conditions for creating astronomical objects are messy (the particles in the gas clouds are not perfectly symmetrically distributed) and that messiness inevitably leads to rotation and revolution.

I mainly wanted to focus on 2:

Why doesn't the speed decrease ?

Because of Newton's first law: An object at rest remains at rest, or if in motion, remains in motion at a constant velocity unless acted on by a net external force.

Since gravity of sun tries to pull it, each time, it should be causing the earth to slow down

Imagine holding a string with a ball at the end. It doesn't matter that the ball is constantly being tugged by earth's gravity, it's going to stay still because the gravity is perfectly balanced by the tension of the string. The net forces involved aren't changing, therefor there can be no change in acceleration because F=MA (and thus no change in motion).

And that's what is going on with the Earth-Sun system. For simplicity sake, let's assume the orbit is circular. Then even though sun is constantly pulling, there is no NET change in the forces involved. For Earth's orbit to decay, by Newton's first law there must be a net change in the force on earth (otherwise the object in motion will stay in motion).

If I had a car with an engine

A car constantly needs to apply positive force (that is, force in the direction of the motion of travel) because it is constantly assaulted by both wind resistance and friction which is applying a negative force against the motion of the car. To maintain constant speed the forces must be balanced or the car will either slow down or speed up.

Now, for many orbits similar things are happening and that is why many orbits do decay. Objects in low earth orbit encounter friction from particles which slow them down and crash them back to earth. The moon and earth interact in non-trivial ways through tides and other things and those interactions result in substantial changes over time. Orbits around the moon decay because the moon's mass is not evenly distributed and thus the moon cannot be treated as a point particle by objects in orbit around it.

But for things like the sun and the planets those issues are completely negligible. Yes, the earth also encounters friction from particles while in orbit but the forces involved are for all intents and purposes 0. The sun is far enough away that it can be treated as a point-like mass and there are no significant tidal forces. So the Earth continues at the same speed because there's nothing there to slow it down.

On a final note, there is something that can decay orbits of large massive objects: gravitational waves. This is why massive black holes in orbit around each other eventually merge -- their orbits lose energy in the form of the waves and that loss of energy translates to them getting closer and closer together. But like the other things in the Earth-Sun system, the net force from gravitational waves on the scale of objects the size of the sun or earth is so small it is essentially unmeasurable.

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    $\begingroup$ The Earth is accelerating and it is acted on by a net external force. $\endgroup$
    – ProfRob
    Apr 29, 2023 at 16:19

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