h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

i know that the gravitational attraction balances the centrifugal force and thats what keeps the planets in orbit, but in order for there to be a centrifugal force, the planets need to be moving at a certain velocity, around the sun, what causes this forward motion?

while you're about it, tell me what causes the rotational motion of planet.

In the begining, there was a cloud of dust. This cloud began to collapse under it's own gravitational attraction. As it collapsed, it began to spin. If you've ever watched water go down a drain, you know that this happens. The energy from the spinning of the dust cloud was transferred into the planets formed from it. As the planets formed from the dust, a smaller-scale version of the same thing happened. Thus, planets rotate both around the sun and around thier axis.

Is there a pattern to planetary rotation?
Some people say that it is random, but if a planet is within 30° of alignment with its orbit and this is a probability of 1:12 (30:360) Then would 6 out of the 8 recognized planets falling within 30° be too great a coincidence? If planetary spin originates from collisions at the latter stages of accretion, should not this have produced a more random effect with collisions outside of the 30° plane of rotation if the disc from which the planets formed was not two dimensional?

As far as I can tell, if you've got a spherical cloud of dust, then the particles of dust will attract each other. If the particles are moving by random motion, then they may be:

1. moving fast enough to orbit each other in which case they will orbit the centre of the dust cloud
2. not moving fast enough, in which case they will condense at the centre of the cloud
3. moving too fast, in which case they will fly off and leave the dust cloud.

The cloud will settle down with a big mass in the centre and the rest of the cloud in a sphere around it.

The next thing that happens is that there is some axis around which on average the dust particles are rotating. Gravity will cause the cloud to condense in a direction parallel to this axis, flattening the sphere into a disc. The whole disc rotates around the centre, preventing it from falling into the centre.

Now the local gravity causes the dust in the disc to clump together. The central lump turns into a star and the outer clumps turn into planets.

God asked Job
"where was thou when I laid the foundations of the earth?
...Whereupon are the foundations thereof fastened?" Job 38:4-6
the answer to the question is gravity.
God decreed the laws of gravity and those laws built the Solar System in the way the previous posts described.

Saying that God decreed gravity is a nice thought but it doesn't actually explain anything. The universe would work just as well if gravity just existed as opposed to God decreeing it.

Many Christians (myself included) accept the bible as the ultimate authority. When it says "Where we you when I laid the foundations"
we infer that He laid the foundations. Thinking that the Universe has always existed and its laws just came whole cloth out of nowhere may be comfortable for you but not for me.

The Bible promotes incest, which is why I don't consider it the ultimate authority.

I don't remember that bit, GB. But it wouldn't surprise me.

http://listverse.com/religion/top-6-incestuous-relationships-in-the-bible/

The fact that all of the angular momentum of the bodies in the solar system came from the rotation of the initial cloud also helps explain why everything spins and orbits the same direction.

(More or less, with some exceptions.)

The fact that biblical figures had incest does not mean god promoted it.
The book just tells it like it was. As to god's opinion of it he gave that in Leviticus 18:6-21 where he goes to some length to forbid it.
Getting back on the subject of orbits I always thought it was interesting that the planets rotate CCW yet men made clocks with hands that go just the opposite. It would have been just as easy to make clocks with hands that moved CCW like the planets do.

The sun rises in the east, goes around the south of the sky and sets in the west. This causes the shadow on the sundial to go round in a clockwise direction, so clocks were naturally made to go the same way, although not at the same rate - a sundial shadow takes 24 hours to do a full revolution, while a clock hour hand only takes 12.

The rotation of the earth is only counterclockwise in the northern hemisphere. It is clockwise in the southern hemisphere.

so, is the orbit of the earth still CCW in the southern hemisphere or is the perception of that also reversed.

If Gnomon stood on the Sundial in New Zealand he would be its gnomon
http://www.tikitouring.co.nz/images/wellington-sundial.jpg

The orbit of the earth is the opposite in the southern hemisphere that it is in the northern hemisphere. If you stand on your head it is the opposite as well.

Do you like the image I had slapped on your update, Shagbark?

Where did it come from, GB?

About 800 million kilometres thattaway!

Zube