whiskers

This proto area is very asymmetrical so the degree one applies the shell theorem is limited. My point there was the parts of the earth for instance can be circling the center of the earth without being attracted to it. Can you indicate what definition of 'orbital energy' you are referring to.

Here's an interesting thing. Imagine that the Earth is a hollow sphere, the mass of which adds up to the true earth mass. If you were outside of it, you would feel that attraction "down" towards the center of the earth. But! If you were *inside* of it, you would feel no gravitational attraction at all! http://en.wikipedia.org/wiki/Shell_theorem Nasa and all these other places describe the bodies going around the barycenter, but this is the kind of language which is more like saying that the Sun is setting - it is a casual description which does the job, but it can lead to misinterpretations.

The big problem here is "what is an orbit" - it is used in 2 different ways: 1) the center of the orbit is something fixed, and other things move relative to it This is the most general definition which works for the Barycenter, or approximately for the Sun, and earlier the earth was seen to occupy this place 2) the massive thing that attracts everything else to it so that they orbit around it This fits the Sun more or less, but it doesn't fit the barycenter in the slightest

Nope. Behind only. There's no "there there" at the SSB. In terms of movements of bodeis inside the solar system, it is a very practical origin for coordinates. In terms of gravitational pull of the SS for a point beyond the SS, the SSB is slightly more accurate single direction of the gravity vector than the Sun itself.

What is gravity strength? The force of gravity as experienced? To get absolute and total accuracy would require summing up the gravitational forces from everything in the universe - understood? For an ant on a hammer - treating the hammer as only the CoM will yield a very poor approximation. If this hammer were perfectly symmetrically spherical then u could use the CoM.

it's all about context. When you are on the surface of the earth, there are plenty of little variances in the gravitational field - trees and rocks exert subtle gravitational forces on each other and on you.. The further you get away from the earth, the more the gravitational field can be treated as a single vector pointing toward the center of the Earth. so it is in the solar system, only so much more so. If you are near Jupiter, the gravitational attraction of the Sun will take a back seat to Jupiter's. So it only makes sense to see the SSB as "exerting gravitational attraction" if you are far away from the solar system.

Any. the SSB doesn't have *any* mass. "When using the Newtonian gravitational force formula one says all the mass is at the CoM and that is at r distance between them," For the SSB this only applies to gravitational interactions between this solar system and other stars. Inside the solar system, Newtonian gravitation is calculated between bodies and other bodies - the SSB is not used for Newtonian forces.

Feynman on inertia, which a local accelerometer is measuring. >> I went to my father and said, “Say, Pop, I noticed something. When I pull the wagon, the ball rolls to the back of the wagon. And when I’m pulling it along and I suddenly stop, the ball rolls to the front of the wagon. Why is that?” “That, nobody knows,” he said. “The general principle is that things which are moving tend to keep on moving, and things which are standing still tend to stand still, unless you push them hard. This tendency is called ‘inertia,’ but nobody knows why it’s true.” Now, that’s a deep understanding. << also with video: http://www.haveabit.com/feynman/2

Can you explain the difference between the SS CoM and the SSB ? There is no gravitational pull from the SSB, since It has no mass. Motion is relative, so there is no objective definition of "fixed position" vs "moving" without a frame of reference. Can you affirm that you understand these propositions?

I don't completely understand, but here's my guess: "Nope".

I wasn't thinking in terms of orbits. The SSB, at any one moment, is at the average of all of the positions of the matter in the solar system, at that moment. Here's an example of how people figure out solar system motions with the best accuracy - primarily there are no "orbits" here: http://www.moshier.net/ssystem.html

The SSB is at rest with - and defined by - an *average* position of the matter in the rest of the SS. It is not over time at rest WRT the Sun or any other particular body or particle in the SS.

(oh also forgot to state the obvious: the viewer in the station will determine that the book - during that period of time it is all present - will appear to be shorter in the direction of motion than the observer in the train does)

It is common to say that the ISS orbits the earth. If they say that it is orbiting the Sun or the SSB they are not wrong. Also, if they say that the ISS and the earth orbit a common ISS/earth barycenter (perhaps imperceptibly close the center of the earth) that is also not wrong. The fact that you "never hear" people saying something only means that people have no reason not to use the most practical and intuitive description of a situation. Also most people don't know much about physics and are just using the description that is in common usage. You "always hear" people talking about the Sun rising and setting - but then you sometimes hear people say that is false it is the earth rotating.

Motion is relative. There is no way to "determine" that something is unmoving vs anothing thing being moving. This is a good topic for your research. You might start here with old-fashioned relativity. https://en.wikipedia.org/wiki/Galilean_invariance

If you say the planets orbit the earth, you will have to make a lot of awkward exaggerated "corrections" (old term is epicycles). Changing center to the Sun causes a revolution of simplicity and yet some irregularities remain. Moving on from there to the barycenter is another leap of improvement. If you move to the GC you remove some more irregulairites. You say that it is "generally accepted" that the planets orbit the Sun, it might be more accurate to say that it is "often stated". If you have to pick one physical body in the Solar system from which the orbits of other bodies appear to move in the simplest repeating movements the most like ellipses, you would definitely want to pick the Sun, which has 99% of the mass of the solar system in it. Science has many ways of defining centers. I have found people to be quite hard-pressed to explain what they mean by "orbit" and "center" and are satisifed with statements liike "people used to be wrong saying planets orbit the Earth, now we are right saying the planets orbit the Sun." Can OP follow that? or is there too much reliance on SSB as if was a real physical thing?

There is no center, really. You can "put the Sun at the center" if you like, but there is no objective reality which you are describing. There is no reality to "A orbits B" therefore no answer to the question "does the Sun orbit the JSB or does JSB orbit the Sun." There is no center, and there are no orbits, but as a convenience, these things are approximately true. "So how can we tell if the JSB proscribes a circle (follows a circular path) around the Sun or whether the Sun orbits the JSB?" In physics you adopt some practical coordinate system. The coordinate system you adopt determines the values of many of the measurements you make, and none of these CS are more real or true than another. Are you familiar with relativity - motion is relative. You can say that the Sun is moving or you can say that it is not moving. Right?

SSB is like a 3-dimensional average. Any collection of particles has a 3-dimensional average or center of mass. So some of this collapsing nebula will form planets. The center of mass of the particles which are in the "proto sun" and the center of mass of all particles in the collapsing nebula are not exactly the same point.

Can you start over, breaking it down a bit? Perhaps a few statements and one question?

To understand how truly strange length contraction is, you have to contemplate the "ladder paradox" https://en.wikipedia.org/wiki/Ladder_paradox At the beginning it is easy to forget that length contraction is so closely related to the relativity of simultaneity - there's no separating them. It might be easier to remember this if you think of the object as moving or changing. Let's say that for a viewere on the train, a red book pops into existence in his hand, and then gradually turns greener, then disappears. From a perspective of a viewer on the station, here is what is determined to be the order of events: * the part of the book towards the back of the train pops into existence * parts of the book forward from there appear until the frontmost part of the book has popped into existence * the part of the book towards the back of the train starts to turn green * then parts of the book forward from there turn greener until the frontmost part of the book has turned green * the part of the book towards the back of the train pops out of existence * then parts of the book forward from there disappear until the frontmost part of the book has disappeared When you think of a relatively stable object such as a train, a spaceship, or a ladder experiencing "length contraction" it is easy to forget this at first and it can be confusing.

If he used some variant of this word, I'm pretty sure he didn't apply it to Astronomy, eh?

Also there's the fact that the parts of the solar system "ahead" of the barycenter in it's general "orbit" relative to the GC and those "behind" are subjected to attraction to the GC which are not parallel and thus distort their relative motion relative to one another and the barycenter.

Hello can someone tell me who first coined this word? The concept of an orbital barycenter is implied by Newton's laws of motion - (BTW was he the first to suggest that both the Sun *and* the Earth move) ? I've tried some google searches and haven't found this information, which surprised me. Thank you.

The purpose of the Einstein quote in the image/link above was to suggest that yes you could in fact justify such a thing - in physics terms if not in practical or budgetary terms. You end up with a more convoluted space metric needed to describe the motions of nearby objects. Using the barycenter by contrast is the simplest - and this is why it is used. Every time you pick a center of greater mass you lose "epicycles" in your calculation - starting Earth, then Sun, then barycenter, then galactic core, etc. The *real* formulas used in today's big computers are moment-to-moment integrations of sums lf all pairs of opposing forces, not a "theory" of "orbits". From the point of view of a single binary star, the other star certainly does appear to orbit around it. You can never completelyuse Newtonian mechanics if you consider your point of view to be "unmoving" - things will never fully add up. Even if one is located at the SSB and attempting to account for all forces inside the solar system, there will be tiny variances from what the formulas would suggest (ignoring relativity for a moment) which are due to gravity & momentum related to SSB relationship to other matter in the galaxy.

For a lot of physics problems, what people often don't get at first is that there is no true or proper frame of reference. Before Newton brought in equal-and-opposite forces, people used an archaic religious model which included a divine "center" around which other bodies moved. Whether you call it the Earth or the Sun or the barycenter, you're still just using a particular model. "Orbiting around" is a convenience, not something which you can determine or measure. To use the barycenter in calculations is to seek a point in local space which is the closest to being an inertial frame of reference. It is a better not-actually-real accounting trick than saying that things are orbiting the Sun. An orbit is a relationship between 2 bodies. Here's an interesting simulation of an "orbit" of two equal-mass bodies around a common center. Note there is nothing there in the center which is attracting the bodies to it, they are not orbiting around it in the pre-Newtonian sense.