# QuestionWhere moment of inertia came from? Interaction with primordial matter that may be dark matter?

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#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
IG, as per dictionary definitions:
ROTATE vb
• to turn or cause to turn around an axis; revolve or spin.
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.

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

#### IG2007

##### "Don't criticize what you can't understand..."
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

#### IG2007

##### "Don't criticize what you can't understand..."
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

#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
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

##### "Science begets knowledge, opinion ignorance.
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?

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#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
That's revolution.
That is Merriam-Webster Dictionary.

This is not a war. It is English.

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#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
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

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IG2007

#### IG2007

##### "Don't criticize what you can't understand..."
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

#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
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

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IG2007

#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
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

IG2007

#### 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?

Moderator
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

##### "Science begets knowledge, opinion ignorance.
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

##### "Science begets knowledge, opinion ignorance.
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

##### "Science begets knowledge, opinion ignorance.
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

IG2007 and vagor

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

-Wolf sends

IG2007

#### IG2007

##### "Don't criticize what you can't understand..."
Ok. Does the Earth rotate or does it complete a revolution each day?

-Wolf sends
I thought it does both.

Catastrophe

#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
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

IG2007

Moderator
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.

Moderator

#### IG2007

##### "Don't criticize what you can't understand..."
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

#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
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.

Moderator
I am happy with that

IG2007

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