Why is there day and night?

Paulibus

I'm hoping that some kind astronomer or astrophysicist here can tell me which physics law, or laws, (if any) mandate that many astronomical structures revolve or rotate, from asteroids though planets, pulsars, stars to galaxies. I guess they all develop with conserved angular momentum to a lesser or greater degree, but is there a mechanism for its acquisition, as it were, that is accepted in terms of specific features of physics laws? Those involving gravitational condensation, maybe?

I'd prefer an answer other than "Why not?", or "Because it was so ordained" .

TrickyDicky

There is a thread in the cosmology subforum http://www.physicsforums.com/showthread.php?t=613674 where this is lucidly addressed by DH.
The thing is unfortunately there is not a final general answer to your question about space objects angular momentum origin as of now.

D H

The question in the title of the thread ("Why is there day and night?") and the body of the original post are a bit orthogonal to one another. Note: I'm assuming that "why is there day and night" means "why does a point on the surface of a planet have day and night". In other words, "why don't planets have one fixed side always lit by its sun, the other side, never."

A planet which is not rotating would have day and night. A planet on which one side perpetually receives sunlight, the other side is perpetually dark, is rotating. Such a planet is rotating at a rate equal to its orbital rate. In other words, such planets would be tidally locked to their central star, just as the Moon is tidally locked to the Earth.

We have day and night here on Earth because we did have day and night 4.5 billion years ago and because the Earth has not had enough time to become tidally locked to the Sun. The Earth won't become tidally locked to the Sun until long after the Sun runs out of fuel and dies.

For red stars it's a different story. That planets are tidally locked is conjectured to be the norm for Earth-sized planets in the habitable zone of a red star. Tidal forces are an inverse cube force, and this means the time needed to become tidally locked is an inverse r⁶ relationship. That inverse r⁶ relationship means Earth-sized planets in the habitable zone of a red star should become tidally locked in short order because the habitable zone of a red star is very close in.

Paulibus

TrickyDicky: Thanks for the pointer to the current thread Re: What causes angular momentum in rotating bodies ? I found helpful the post # 9 by D H , especially the comments on "gravity gradient torque". This is closest to my wandering thoughts, despite the caveat that it is not the final answer but is: "....still the preferred explanation as the cause of angular momentum because the mechanism is well understood and is known to exist." I also hasten to add that I wasn't thinking of anything as grand as the Universe as a whole, or its origin. Just of smaller stuff.

Perhaps I may be allowed to hazard a guess about gravity and rotation? Consider a toy model of a stable non-viscous fluid disc of free test-masses revolving around some central mass, each in its own circular orbit. Each is held in orbit by gravity and its tangential speed must therefore be inversely proportional the square root of its orbital radius. In contrast, if the test particles were part of a frozen, rigid rotating disc their tangential speeds would be directly proportional to their orbital radii.

It seems to me that such a toy disc (or any more realistic fluid structure?) must be sheared by gravity as it rotates.

Now fluids (like gases and dust clouds) don't support shear stresses. Their response to shear stresses is to form revolving sub-structures; well-known examples are gyres, whirlpools, vortex streets, hurricanes and tornadoes. I suggest that fluids that rotate while gravitating cannot be stable, but must break down into revolving substuctures.

The toy disc is an oversimplified example of how a one in a hierarchy of such nascent astronomical objects could be formed by fluid instability to shear, inherent in rotating gravitational collapse. Maybe?

Paulibus

D H: I missed your post#3 while concocting #4. Thanks for your comments. I admit to being misleading with a title that was more for drawing attention than apt. I also missed Re: What causes angular momentum in rotating bodies ? in which I should have posted. Coincidences happen. I apologise for this, but appreciate the answers you took the trouble to reply with.

If you have time, I'd also appreciate a commment on #4. I have a copy of Lee Hartman's Accretion Processes in Star formation (1998) but oddly didn't find any mention of vorticity or shear there. Maybe I'm nuttily regressing to Victorian Vortex theories of everything, a pet of one Bill Thomson.