Society for Popular Astronomy

dynamics of galaxies.
Page 3 of 3

Author:  Cliff [ Thu Mar 31, 2011 9:33 pm ]
Post subject: 

Dear Maurice
I just tried typing a long reply to you - but to my frustration it disappeared just before being finished - so I hope you'll excuse me being brief.
Basically I think agree with some things you say. However, unless I mis-understand what you said I disagree with you about Saturn's Rings Spokes.
As far as I am concerned my understanding is they are probably levitated particles above the main body of the "proper" rings.
As it happens I am acquainted with an amateur observer who observed the spokes visually with a 10inch scope a few years before the spokes were imaged by space probes.
I gather the "spokes" were first observed in the late 19th century and possibly very occasionally observed after (?).
My acquaintances spoke observations were not accepted by the astronomical community at the time when he reported them.
I think the spokes are not always imaged by space probes and are believed to be transient and have no confirmed observations suggesting they act like rotating spokes in say a stage coach wheel.
As far as I know the spokes have still not beeimaged by any amateur astronomer imagers but I may be wrong about that.
Best wishes from Cliff

Author:  Cliff [ Thu Mar 31, 2011 11:13 pm ]
Post subject: 

Dear Maurice
I've made two attempts to reply to your wednesday post but both my long winded attempts seem to disappear.
Basically I think i may agree with some of your ideas. However, if I read\understand your comments related to Saturn's Ring Spokes Ithink I very much disagree with you there.
I know a chap who observed the spokes using a 10 inch scope before they were imaged by space probes.
I undersatand the so-called spokes are not always found on space probe images. The spokes are probably transient in nature and have no proved "fixed" rotation period. My understanding is the spokes are probably particles levitated above (or below!) the main rings.
I don't understand your comments about the orbital speeds of moons that have cleared gaps in the rings and in particular your suggestion that the moons change in speed.
That said I know something about the two satellites that swop positions and also about the braiding in some parts of the ring system.
Best wishes from Cliff

Author:  maurice [ Sat Apr 02, 2011 9:58 am ]
Post subject: 

Hi Cliff. Sorry for the delay, I’ve been doing battle with this darn computer again. With regard to the spokes query, after reading up on Saturn’s rings I now know the spokes are above the rings and rotating independently, so don’t indicate how the rings are rotating. They seem to be tied in with Saturn’s magnetic field. I wonder if they are similar to Earths Aurora.
The braided ring idea has taken a knocking too. The two moons involved are one each side of the ring and don’t repel the particles but attract them in the normal gravitational way.
My last card in the “rings not rotating at orbital speed argument” has disappeared. I saw somewhere that a moon was gaining angular momentum so was moving out from the system. Like you Cliff, I can’t find any mention of it now.
One last thing, I can’t find any mention of the two moons that change places. I don’t think they could be the moons, Prometheus and Pandora, as they are each side of the braided ring. If you know which they are please let me know. Are they too small to be moons so are satelites?
All the best. Maurice.

Author:  Cliff [ Sat Apr 02, 2011 2:40 pm ]
Post subject: 

Dear Maurice
I needed to do some checking in my astro-books before being reasonably sure about the answer to your query.
"The Atlas of the Solar System" by Patrick Moore & Gary Hunt s
says on page 360.
"They(Prometheus & Pandora) move in orbits either side of Ring F and they are known as sheperd satellites because their gravitational forces keep the particles of the Fring in a stable orbit.
Further out still 151,500 Km from Saturn and 10,000 Km beyond the F Ring approx midway between Ring F and G are two very interesting satellites JANUS and EPIMETHEUS. Each is irregular in shape, the former ....90 x 40 Km. its companion 100 x 90 Km.
They give every indication of being fragments of a former body which broke up.................................................
Which leads to an extraordinary state of affairs........likened to musical chairs.................
At present (when the book was published ?)Janus is slightly closer to the planet and its revolution period is 16.44 hours as against 16.672 for Epimetheus.
This means it will slowly catch up with its companion but as they approach there will be mutual interactio such that the inner faster moving boby will slow down and the outer slower one speeds up .the end result is that the two satellites interchange places.
The interval between successive encounters is abou 4 years. Collision cannot take place otherwise the two bodies would not continue to exist independently"................
Best wishes from Cliff

Author:  maurice [ Sun Apr 03, 2011 7:51 pm ]
Post subject: 

Thanks again Cliff, your better than Google. I have” Atlas of the Universe” and, when I looked, it is in there, but I’ve got into the bad habit of just relying on the internet. I will have to do another search there though, to see if I can find something on the moon or satellite that is gaining angular momentum. If it isn’t something I have dreamt up it could be pretty important.

I wonder if we could clear up the problem of the satellites or any heavenly body, orbiting in formation, one above or below the other or side by side. This is what I mean by the thickness of the rings, or asteroid belt, or galaxies. Especially the side by side aspect, they should all be crossing paths, so there would be countless collisions.
Oh yes, fair warning. I will be in Manchester next weekend, so hide your scope!
All the best, Maurice.

Author:  David Frydman [ Sun Apr 03, 2011 9:10 pm ]
Post subject: 

I think the pairs of moons are side by side but for some reason don't collide, but I may be wrong on this as I saw it quite a while ago

This sort of stuff was in magazines maybe Sky and Telescope years ago.

Regards, David

It could be something like resonances that don't allow them to collide, but I really don't know.

Author:  Cliff [ Mon Apr 04, 2011 5:49 pm ]
Post subject: 

Dear Maurice
The little mathematics I've done with regards satellites orbits are only enough for me to decide not to get too involved in trying to solve the complex issues myself.
"The Atlas of the Solar System" (Moore & Hunt - Chancellor Press) is an excellent book. I cann't find the date it was published, but As far as I know there have been two editions, the first early 1980s I think, the second early 90s (?). I have owned both but gave the first to a friend when I got the later edition.
I must confess I tend to prefer looking up things in books - I find Wikipedia overwhelming and I don't feel happy about all its information.
Best wishes from Cliff
PS I just found first editition published by Mitchell Beazley 1983 Revised 1990. This edition published in 1997 by Chancellor Press.
I don't know if there are any later editions but if there are I will be tempted to get it.

Page 3 of 3 All times are UTC
Powered by phpBB® Forum Software © phpBB Group