Solar system stuff.

[peter]

If I was looking down from above on the Sun, with the Earth (North Pole up) revolving around it, and the moon revolving around the Earth, the Earth would be spinning on it's axis in an anti-clockwise direction. (Can we forget the tilt for a minute - this is going to be hard enough for me to get across/grasp as it is!). This spin on it's axis would give us our days and nights. So much I get.
The Earth would also be revolving around the Sun, and one of these revolutions would give us our year. BUT (first question) from our looking down position would the Earth be going in a clock-wise, or anti-clockwise direction.

The moon would be revolving around the Earth and each complete revolution (correct me if I'm wrong) of the moon would give us one month. BUT (second question) (as we look down) is the moon revolving around the Earth in a clockwise, or anti-clockwise direction. As the moon revolves once around the Earth it revolves once on it's own axis and hence always presents the same 'face' toward us. BUT (third question) does the moon revolve on it's axis in a clockwise or an anti-clockwise direction (the 'same face' thing could only happen if the moon's spin on it's axis is in the same direction as it's spin round the Earth [ie both clock- or anti clockwise] couldn't it?). But as we look down - would (fourth question) the Moon's passage round the Earth be in the same 'plane' as the Earth's passage round the Sun or would it be skewed at some random angle (perhaps the moon revolves around the Earth passing through the 'Poles' [ie at 90 degrees to the equator] - I've always assumed the moon moves around the Earth with the Earths equator as it's 'plane' and this would of course tilt it by what, 23 or something degrees to the plane of the Earths revolution round the Sun; I'm starting to realise how little I know!).
As the moon goes around the Earth the shadow of the Eatth on it's surface gives us our view of the 'phases' of the moon, and common sense would suggest that these have to be the same the world over at a given time. ie Full moon in Dheli will also be full moon in Aukland or London.

Lastly (whatever question - I've lost count) looking down, would the Sun be revolving anti or clockwise and as all of the Suns in the Universe are moving away from eachother, it must be possible to extrapolate backwards all of their relative motions in relation to each other untill, bingo, you hit the spot where it all started - the centre of the Universe.

[I'm Murrin]

The diagram at "Names of Lunar Phases" here suggests the earth and the moon orbit counterclockwise viewed from North.

The phases are the moon's own shadow, not the shadow of the earth (that's a lunar eclipse).

Not sure about the sun's motion.

As for extrapolating the beginning point of the universe: Nope. That point is everywhere. All parts of the universe are moving away from all other parts at the same rate, not all moving from a fixed point - space itself expands. Every point in space was at one time the centre.

[Hashi Lebwohl]

I think the Moon's plane relative to the Sun is slightly different than its plane relative to the Earth. The difference is slight or we would probably have phases of the Moon that are more chaotic--sometimes the full Moon would be visible in the Northern Hemisphere and sometimes not.

I am uncertain about the other questions.

[Vraith]

The Earth...all the planets, actually, orbit counter-clockwise around the sun.

IIRC all the planets except Venus and either Uranus or Neptune also rotate counter-clockwise.

The Moon orbits Earth counter-clockwise, and it rotates counter-clockwise, and yes, locked as you say, 1 orbit=1 rotation.

The moon's orbit is inclined relative to earth-sun plane. If it wasn't we'd get a lot more eclipses.
[and is also part of the reason that some monument....I think stonehenge?...only has a perfect Earth/moon/sun alignment once ever 17 or 19 years or somesuch I'm pretty sure...I think that same cycle is important in old Hebrew calendar??]

The phases are the same everywhere on earth at a given time...but the "shadow" isn't really a "shadow" exactly...it is just the half that is in "night,"....just the backside. The phase is our angle of view...the lit part is the "front" facing the sun, the dark is the "back," and we're seeing it from front/side/back at different phases.

The sun also rotates counterclockwise.

And as Murrin said, odd as it is, it's all moving AND it's all the "center"...it can't be reverse calculated to a centerpoint.

Fairly sure all of that is correct.

[Cozarkian]

The moon would be revolving around the Earth and each complete revolution (correct me if I'm wrong) of the moon would give us one month.

The moon only takes ~27.5 days to orbit the Earth, but since they are both also orbiting the sun, it takes ~29.5 days (about 1 month) to go from Full Moon to Full Moon

[peter]

Counterclockwise seems to be the order of the day then - odd that there should be just a few 'objects' that break the rule (bit like left-handed people ).

I don't get the moon thing really - I guess if the sun is behind us (ie it's night) and the moon is......no - I'm going to have to check that out.

re the Universe - again I don't get it. Ok all of space is expanding and carrying the stars with it, but if it's expanding then it's expanding i) in a direction and ii) from something smaller to something bigger. This means a point has to exist from which it could get no smaller. Ok - that point is every point in our Universe - but beyond our Universe it's not. It is a fixed place in a 'where' of somewhere. Just as in the multiverse theory, if there are many Universes there has to be a somewhere for them to exist and it no longer makes sense to think in terms of our Universe being the be all and end all of where there is. I get it - I just don't understand it.

[Vraith]

Counterclockwise seems to be the order of the day then - odd that there should be just a few 'objects' that break the rule (bit like left-handed people ).

I don't get the moon thing really - I guess if the sun is behind us (ie it's night) and the moon is......no - I'm going to have to check that out.

I get it - I just don't understand it.

On the first: That is likely to happen in every system...at least every system that forms from dust condensing...barring outside collisions/influence...I mean, it won't always be counterclockwise...but the system will be consistent throughout. [our two rogues were probably crashed into by very large objects, originally they likely matched the rest.]

On the second...lay a flashlight on a table, put a tennis ball in front of it a couple feet away, then circle it with a ping pong ball. You'll see. You'll probably say "duh!...of course!" to yourself.

On the last...right there with ya, dude. Sometimes in my head I try to trick myself into believing I really really get it by saying "It's ALL the center. It's just a bigger center than it used to be..."
It comforts me. Doesn't mean accurate or helpful.

[I'm Murrin]

re the Universe - again I don't get it. Ok all of space is expanding and carrying the stars with it, but if it's expanding then it's expanding i) in a direction and ii) from something smaller to something bigger. This means a point has to exist from which it could get no smaller. Ok - that point is every point in our Universe - but beyond our Universe it's not. It is a fixed place in a 'where' of somewhere. Just as in the multiverse theory, if there are many Universes there has to be a somewhere for them to exist and it no longer makes sense to think in terms of our Universe being the be all and end all of where there is. I get it - I just don't understand it.

The trick is to realise that "space" doesn't exist outside our universe. "Space" is property of our universe. The only space there is is the space that's appeared since the big bang. So there's nothing for the universe to be expanding "into". It's just plain getting bigger.

[High Lord Tolkien]

Great question peter.
My kids asked me this question and after doing some googling I came with nothing.
KW is better than google!


I heard once that the Moon and the Earth revolve around each other in some way.
Is that true or was I smoking too much crack that day.

[I'm Murrin]

Two bodies in an orbit actually orbit around the centre of gravity of the two, rather than just one strictly around the other. The earth's mass is so much greater than the moon's that the centre of gravity of the two is within the earth's core, but the earth would have a slight "wobble" due to the moon's gravity.

One of the things that made Pluto's status as a planet questionable was that its mass is similar enough to that of Charon that you can't quite class Charon as Pluto's satellite.

[Vraith]

Yea, what Murrin said.
The differences in mass makes it look like the small orbits around the big like swinging a ball on a string around yourself.
What's really happening is more like a waltz...you and your partner revolve around a point between you.

[Avatar]

If anybody wants to actually see things like how the planets revolve around the sun, and the moons around the planets, I highly recommend Celestia, a free open source seamless 3d Solar System (and galaxy and universe) browser.

Position your view so you're looking down at the plane of the ecliptic, switch on the orbit lines, and speed time up to see it all move.

This really is an awesome little prog, especially if you download the additional objects, like satellites etc. If you set it to the date of a known eclipse, you can actually see the moons shadow move across earth. You can follow Halley's Comet around the galaxy, even look at the galaxy from the outside if you zoom out far enough. A must for anybody interested in celestial bodies.)

--A

[peter]

I'm off to find my torch and tennis ball as we speak and then to check out Av's link. Thanks Guys.

[peter]

Sorry for the DP guys but it occured to me, re Murrin's comment that 'space doesn't exist outside our Universe - it is a property 'of' our Universe. Does that take into account (perhaps more recent) ideas in respect of 'the Multiverse. I saw a short on YouTube explaining that it is concievable that some of these 'parralell' universes could exist within the hidden dimensions within our own Universe - indeed (IIRC) possible even within black-holes and other inaccessable places. Does that not beg the question whether our own Universe might not exist within the coiled up dimensions of yet another, and thus the word 'space' should always be preceeded by a qualifying word such as 'our' space to distinguish it from other spaces in other places. Indeed it seems possibly at least to give one answer to the question of where the 'where' that our Universe formed in might be.

[Hashi Lebwohl]

There could indeed be "space" outside our observable universe that comprises the volume in which our universe sits; however, that space might not be space like we experience space. That seems odd, I know, but it still makes sense.

[Vraith]

There could indeed be "space" outside our observable universe that comprises the volume in which our universe sits; however, that space might not be space like we experience space. That seems odd, I know, but it still makes sense.

Yea...though I'd go a step further from speculations on this sort of thing I've heard, and say if there IS a "space" containing "our space," it must be/have at least some different property of spaceness.

peter...if there are spaces and not just space, I think it would only address the "where" issue in a relative sense, in relation to that other space or spaces, but not in an absolute sense

Everywhere in our where would still be the center and moving away from everywhere else.

[peter]

I think the thing that 'confused' me at the beginning of this comment thread was that while infinities by thier very nature have no end (or if they are circular then they are not really true infinities, just closed systems), they do have beginnings. I've always uderstood the expanding Universe best with the 'balloon analogy'. ie mark a balloon with spots, continue to blow it up and all the spots move farther away from each other as it gets bigger. I see each point in space in the Universe as one of those spots if you like, all moving apart. But any moving object has a past position and a future position (relative to all other positions) and the stars give us a visible measure of the expanding space as they move appart. Now why can we not extrapolate back something we can observe moving, though the history of it's past movement - this I don't get.

[I'm Murrin]

It isn't moving. The space around it is getting bigger. Every galaxy is moving away from every other galaxy at the same rate. You can't extrapolate a beginning from that - the beginning would just look like whatever point you're observing from.

[peter]

Are there truly no other explanations for red-shift than the expanding Universe one?

[Vraith]

Are there truly no other explanations for red-shift than the expanding Universe one?

Not any good ones that I've heard...because it doesn't have to only cover the red-shift, it has to be consistent with other known things. [including, but not limited to, the cosmic background, for instance...the other big, popularly known piece of evidence for expansion]