Question Where moment of inertia came from? Interaction with primordial matter that may be dark matter?

[Catastrophe]

IG, as per dictionary definitions:
ROTATE vb

• to turn or cause to turn around an axis; revolve or spin.
• to follow or cause to follow a set sequence.

ROTATION n

• the act of rotating; rotary motion
• a regular cycle of events in a set order or sequence
• etc

The second meanings include rotation (orbit) around a body in a set order of sequence.
As I pointed out, it favours spin, but can include orbit.

Furthermore, Google rotate:

Definition of rotate (Entry 2 of 2) intransitive verb. 1a : to perform an act, function, or operation in turn. b : to pass or alternate in a series. 2 : to turn about an axis or a center : revolve especially : to move in such a way that all particles follow circles with a common angular velocity about a common axis.

It is ambiguous. It contributes to the spin/orbit misunderstandings.

Cat

 

[IG2007]

Well, Cat, google tells to me that:

• MATHEMATICS
  the conceptual operation of turning a system about an axis.

In the sense of mathematics, rotation means that.

 

[IG2007]

Well, Cat, google tells to me that:

• MATHEMATICS
  the conceptual operation of turning a system about an axis.

In the sense of mathematics, rotation means that.

 

[Catastrophe]

IG, We are talking astronomy, not mathematics. Saying it twice does not make it more true. In mathematics, as you quote, there is no comparison with one body orbiting another, so the cinfusion is not there to be seen. See my comments about spin = one body orbit = at least two bodies.
I told you that it is not wrong in astronomy - just misleading and ambiguous. The fault is not in the words, it is in the people who misuse them. As I said, that is why I avoid misinterpretation by using spin and orbit. If you are not very careful, you may find yourself getting trapped by the ambiguity. Knowing you, you won't - but then most people are not as bright as you.

 

[Catastrophe]

See Merriam-Webster (different dictionary from previous quoted):
Examples of rotation in a Sentence
the rotation of the Earth around its axis
the rotation of the Moon around the Earth my emphasis.
See More

These include:
Alfalfa and corn are planted in rotation.
the rotation of the job of club president
The car needs a tire rotation.
None of these implies that the subject of the sentence is spinning on a n axis - more that they are being moved around.

If I ask you what you understand by "the rotation of a imaginarymoon takes 15 days", what would you reply? Equally, "the rotation of imaginaryplanet takes 8 hours?

 

[IG2007]

the rotation of the Moon around the Earth my emphasis.

That's revolution.

 

[vagor]

This discussion appears to show the value of Gödel's incompleteness theorem

 

[Catastrophe]

That's revolution.

That is Merriam-Webster Dictionary.

This is not a war. It is English.

 

[Catastrophe]

IG, I apologise for shouting but this is not a contest. Is is trying to get the English language used correctly.
Just to complete the investigation, Merriam-Webster gives:

Quote
1 : the action by a heavenly body of going round in a fixed course The revolution of the earth around the sun marks one year.
2 : a spinning motion around a center or axis : rotation A light push started the globe's revolution.
Quote

That is also ambiguity.
QED

Cat

 

[IG2007]

Well, for the sake of making it unambiguous, can we please make a small sort of "Agreed terms help Sensible Discussion" for it. Like, Revolution is defined as a body orbiting another body. And Rotation is defined as a body spinning around its axis. Can it not be done?

 

[Catastrophe]

IG, thank you for that suggestion. I will give it my attention. The problem is that I cannot mediate the English language and, even with my proposed usage, I have seen examples such as "spinning around the Sun", which could still be taken (by some) as meaning orbiting the Sun, as well as spinning (on its axis) as it orbited the Sun. It may well be, that the best I can do, is to point out the ambiguities, and hope that people will choose their words more carefully, so as to avoid them.

Cat

P.S. I could invent the new words (vb) axiate and orbitate, but I don't think I will see them in any dictionary any time soon

 

[Catastrophe]

IG, from Wiki Rotation:
Quote
A rotation is a circular movement of an object around a center (or point) of rotation. The geometric plane along which the rotation occurs is called the rotation plane, and the imaginary line extending from the center and perpendicular to the rotation plane is called the rotation axis (/ˈæksiːz/ AK-seez). A three-dimensional object can always be rotated about an infinite number of rotation axes.
If the rotation axis passes internally through the body's own center of mass, then the body is said to be autorotating or spinning, and the surface intersection of the axis can be called a pole. A rotation around a completely external axis, e.g. the planet Earth around the Sun, is called revolving or orbiting, typically when it is produced by gravity, and the ends of the rotation axis can be called the orbital poles.
Quote

This agrees with the suggestion of spin and orbit. However, they are still going to be used incorrectly. Do you still think that an Agreed Terms would be useful? It might draw attention to the avoidance of ambiguity.

Cat

 

[vagor]

IG, from Wiki Rotation:
Quote
A rotation is a circular movement of an object around a center (or point) of rotation. The geometric plane along which the rotation occurs is called the rotation plane, and the imaginary line extending from the center and perpendicular to the rotation plane is called the rotation axis (/ˈæksiːz/ AK-seez). A three-dimensional object can always be rotated about an infinite number of rotation axes.
If the rotation axis passes internally through the body's own center of mass, then the body is said to be autorotating or spinning, and the surface intersection of the axis can be called a pole. A rotation around a completely external axis, e.g. the planet Earth around the Sun, is called revolving or orbiting, typically when it is produced by gravity, and the ends of the rotation axis can be called the orbital poles.
Quote

This agrees with the suggestion of spin and orbit. However, they are still going to be used incorrectly. Do you still think that an Agreed Terms would be useful? It might draw attention to the avoidance of ambiguity.

Cat

This may be correct mathematically, but it is certainly not the case from physics point of view. Particularly:
- Physical body may not have a uniform density. Than center of mass will mot coincide with geometrical center of the body,
- A subset (or superset, depending how you look at it) of the above is a case when you have multiple bodies rotating around common center of mass,
- Physical system may rotate around multiple axis simultaneously.
On more generic note: I think that ambiguity is fine as long as a reader and writer can disambiguate the message by parsing the context of the sentence. In this sense, may I suggest to use these terms interchangeably as synonyms with further disambiguation in mind of an educated reader?

 

[Wolfshadw]

Well, for the sake of making it unambiguous, can we please make a small sort of "Agreed terms help Sensible Discussion" for it. Like, Revolution is defined as a body orbiting another body. And Rotation is defined as a body spinning around its axis. Can it not be done?

In regards to, "revolutions", I would note that the acronym RPM stands for "Revolutions Per Minute". Whether you are speaking of a car engine or a vinyl record, the object is spinning around it's axis and not orbiting another object.

-Wolf sends

 

[Catastrophe]

Wolfshadw, you are, of course. quite correct - but the Wiki quote includes:
"A rotation around a completely external axis, e.g. the planet Earth around the Sun, is called revolving or orbiting." My emphasis.

As you see here, revolving is also used as = orbiting.

The best distinction I can see, is that if only one body is concerned, it can only refer to spinning/whatever about an axis, As vagor has correctly pointed we must also consider reality. Thus Earth is subject to precession, nutation and aberration. It is not perfect spin around a single axis through time. But that is getting a little abstract (or real?).

If more than one body is concerned, most words are open to ambiguity, since there is rotation about the other body (centre of gravity can be between bodies - not in either) AND there is rotation about the axis.

I don't know which dictionaries are most respected here. I have several scientific dictionaries, but these words are also in common usage.

Cat

 

[Catastrophe]

This may be correct mathematically, but it is certainly not the case from physics point of view. Particularly:
- Physical body may not have a uniform density. Than center of mass will mot coincide with geometrical center of the body,
- A subset (or superset, depending how you look at it) of the above is a case when you have multiple bodies rotating around common center of mass,
- Physical system may rotate around multiple axis simultaneously.
On more generic note: I think that ambiguity is fine as long as a reader and writer can disambiguate the message by parsing the context of the sentence. In this sense, may I suggest to use these terms interchangeably as synonyms with further disambiguation in mind of an educated reader?

Agreed. Vide precession, nutation and aberration. Cat

 

[Catastrophe]

May I please enter a caveat?
This is a forum of space.com, and I therefore consider it reasonable that we think of spin, rotation, etcetera in the context of space - or planets or moon spinning or orbiting. Is that a reasonable suggestion?
If we increase the discussion to general vocabulary, we shall all die of old age before we get anywhere.

Cat

 

[Wolfshadw]

Ok. Does the Earth rotate or does it complete a revolution each day?

-Wolf sends

 

[IG2007]

Ok. Does the Earth rotate or does it complete a revolution each day?

-Wolf sends

I thought it does both.

 

[Catastrophe]

Ok. Does the Earth rotate or does it complete a revolution each day?

-Wolf sends

Earth rotates about its axis and it completes a revolution each day. It also revolves around the Sun. It has been said to rotate around the Sun.
Do you think we should limit this discussion to astronomy, rather than usual parlance?

Cat

 

[Wolfshadw]

Do you think we should limit this discussion to astronomy, rather than usual parlance?

Per forum rules, the discussion SHOULD be limited to space, science, and astronomy, but I wouldn't report my car engine example as a violation of forum rules.

Wolfshadw
Moderator

 

[IG2007]

P.S. I could invent the new words (vb) axiate and orbitate, but I don't think I will see them in any dictionary any time soon

We all know the good old Occam's Razor, “plurality should not be posited without necessity.”

 

[Catastrophe]

Per forum rules, the discussion SHOULD be limited to space, science, and astronomy, but I wouldn't report my car engine example as a violation of forum rules.

Wolfshadw
Moderator

I am happy with that