What makes Earth...

chani10in

This question has been a mystery for me since years and I asked this to my teacher and she had no answer. So, answer me if you can:

Q. What makes Earth rotate on its axis? Answer is not gravitational force or magnetic field inside the earth as the two are caused by Earth's rotation!
Q. What makes Earth revolve around sun? And how was it initiated?

vanesch

Not many people know this, but there's a small electric motor attached to the earth somewhere

No, seriously, the earth is rotating because it started out rotating, and inertia keeps it turning until something stops it, which didn't happen.
How did it get turning ? The gas cloud from which our solar system was formed, was rotating. Why ? Because it is more difficult NOT to have it rotate when stuff gets together than to get it rotating. Indeed, for not to have it rotating, there would have to be a precise balance between things moving "to the left" and things "moving to the right" ; the imbalance would make the overall cloud turn on average in one way or another. So if arbitary matter gets lumped together, most of the time it will rotate somehow.

Shooting Star

You have said that the gravitational force of the earth is caused by the earth's rotation, which is not correct. The gravitational of the earth will be there even if the earth is not rotating.

Notice that all the planets revolve around the Sun in the same direction, indicating the common origin of all the planets from a rotating gas cloud, as has been pointed out in the previous post.

minorwork

There may be a case for the right hand rule of magnetic field direction. Grasp a wire with your right hand and when the current flows in the direction of your extended thumb the field estends in the direction of the curved fingers. A Faraday homopolar motor turns by the exploitation of this principle. All it would take in a primordial plasma cloud would be the preponderance of direction of current flow to start the rotation. Now where the voltage generator would be is a very good question.

rcgldr

So why is the moon only rotating at the same speed as it's orbital rate (with some oscillation)? What slowed it down to this speed?

pixel01

Earth and other planets in the solar system were created from the protoplanetary disk or accretion disk which revolved around the sun. So when the planets formed, they still keep the kinetic energy by revolving around the sun. You can see all the planets revolve in almost the same plane which is the plane of the once exist accretion disk. All the planets also revolve in the same direction.
The rotation is explained similarly, but rotation can be affected by impacts so each planet has very different axis and angular velocity.

vanesch

Tidal effect. A body like the moon is slightly elastic, and gets deformed in a gravitational field. This couples the orbit motion with the rotational motion, and dissipates energy, until both motions are synchronized. The earth undergoes also tidal effect from the moon (ocean tides), and this slows down earth's rotation, and in the end, the earth will rotate with the speed of the orbit rotation of the moon (that means: always the same side of the earth will face the moon). But this has not settled yet, because the earth is bigger, and has much more rotational energy than the moon had. But this slowing down is measurable (although small).

malty

Wow, cool! So does this mean that the days are going to get longer for us from here on out, but the year will get shorter? Awesome vanesch that's class to know how on earth (pardon the pun) though did they measure it, that's even more awesome still. The things people have experimented with and measured never ceases to amaze me!

Shooting Star

How did you deduce that the year will get shorter?

PatPwnt

Heh, the years won't get longer but the days will. I think the moon takes 29 days to travel around the earth, so the days will be about 29 (current) Earth days long.

Shooting Star

The earth will not take 29 days to rotate about its own axis. The total angular momentum of earth plus moon system will remain the same. Details later.

malty

Yeah I know, my standard reasoning was that a year is 365.25 earth days, and seeing as the day gets longer the year would have to get shorter if it were measured in earth days.
Oops, I er forgot to mention that it is actually the same period of time.

russ_watters

That isn't true. The earth and moon will become tidally locked. The angular momentum/energy is dissipated as heat in tidal friction.

D H

You shouldn't conflate angular momentum with energy. They are conserved by virtue of completely different symmetries (time and space). The angular momentum of the Earth/Moon system is conserved as the Moon recedes from the Earth and the Earth and Moon slow their rotation rates. The Earth will become tidally locked with the Moon billions of years from now. Because the Moon will have receded by another 50% by then, a month will be about 50*86400 seconds long (some sources say 45 rather than 50), as will one Earth and one Moon "day".

vanesch

One can directly measure it now. But the funniest indication was that when one calculated solar eclipses back in classical times, they didn't happen at the place where they would have happened if the earth was turning at a steady rotational speed. Now, this is a complicated affair of course, because it takes into account also the orbital rotation of the moon and so on. But it fits perfectly with a transfer of angular momentum from the earth rotation to the moon orbit.

rbj

that's why there are leap seconds.

Shooting Star

Quote:
Originally Posted by Shooting star
The earth will not take 29 days to rotate about its own axis. The total angular momentum of earth plus moon system will remain the same. Details later.

The earth and the moon will become tidally locked. The kinetic energy is dissipated as heat in tidal friction. The earth and the moon can exchange angular momenta, but the angular momentum of the earth-moon system remains conserved, as long as there is no preturbation by other planets. One earth day will not become equal to the present value of 29 earth-days.