# Sun-path on the sky [duplicate]

We are standing on the Earth and we see Sun on the sky. We see it walking in a certain direction, with predictable moving. Earth is a spheroid, as we all know. It is circling around the Sun, and also it is circling around its own axis which is in the certain plane.

Why Earth is orbiting in the plane, and not in randomly like a ball? So, if it orbits randomly, then a Sun path on the sky would be a continuous random path and we could have days and nights in the same day a lot of times. Why is that not happening? Which force is acting on the Earth so that Earth is rotating around its own axis in that certain plane, and not randomly?

Also, Earth is moving with a certain speed and never has earthquake of higher degree so that we could see Sun on the sky as making Electrogardiographic curve. Why is that not happening and the speed of our Galaxy is so huge?

## marked as duplicate by Qmechanic♦Mar 26 '18 at 16:54

• Why question are ill defined, because we may answer with "because the experiment or the physical laws say so". If you want to understand the mechanism you can look for the laws of dynamics, like Newton's law. For rotating objects they predict the objects would keep spinning unless a torque is applied. Also called angular momentum conservation. – Mauricio Mar 26 '18 at 15:33
• To reopen this post (v1) consider to only ask 1 subquestion per post. – Qmechanic Mar 26 '18 at 16:55

If the Earth were to leave its orbital plane, its angular velocity vector would change; in order for this to happen, there would have to be some torque on the Earth. Gravity, being a force that acts along the line joining two masses, cannot provide any torque. Therefore, the Earth does not leave its orbital plane (neglecting tiny gravitational perturbations from other planets/the Moon).

Earthquakes do slightly shake the Earth, but these motions are so tiny, and the Sun is so far away, that the change in its apparent position is negligible. The reason these motions are so tiny is because earthquakes happen in the Earth's crust, and the crust makes up only a small fraction (less than 1 percent) of the Earth's considerable mass.

The "speed of our Galaxy" is not a well-defined concept. You must specify what you're measuring this speed relative to.

Which force is acting on the Earth so that Earth is rotating around its own axis in that certain plane, and notw randomly?

Your question shows basically a pre-Galileo Galilei understanding of how things work. In those days people thought that a force was always needed in order to make things move in a certain way.

However we now know that in fact, things keep on moving in the same way as they did unless a force is acting on them. In more technical language physicists call this the conservation of momentum or of angular momentum.

Or in other words, constant motion is the default, so the Earth rotating around a constant axis is normal and does not require any force.