# Why Equator only exists for a rotating body like Earth?

avito009
I read that a non rotating planet like Lubricon VI does not have equator because it does not rotate. So why only rotating planets have equator?

Staff Emeritus
Per wiki:

An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and midway between the poles.

Without an axis of rotation, there's no way to assign the necessary plane and it can't have an equator.

Ophiolite
I am having trouble getting my head around the notion of a planet that does not rotate. If it is tidally locked, like the moon, then it certainly rotates. If it is not tidally locked, then it also must rotate. What am I missing? Or am I still diverting blood flow from my brain to digest Christmas Dinner?

Staff Emeritus
If it is not tidally locked, then it also must rotate.

Why must it rotate? It must orbit its star, but there's not requirement that it must rotate about its own axis. (Though it's extremely unlikely to have absolutely zero rotation)

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TurtleMeister
I am having trouble getting my head around the notion of a planet that does not rotate. If it is tidally locked, like the moon, then it certainly rotates. If it is not tidally locked, then it also must rotate. What am I missing? Or am I still diverting blood flow from my brain to digest Christmas Dinner?
That's interesting. The moon is rotating at the same rate that it is orbiting the earth. Since it is rotating, then it must have an axis of rotation. This axis of rotation is located inside the earth (earth moon barycenter). So does the moon not have an equator, even though it is rotating? Is it a requirement for the axis of rotation of a body to be located within the body itself in order for it to qualify as having an equator?

This axis of rotation is located inside the earth (earth moon barycenter).
No, you're thinking of revloution, not rotation. The Moon does both.

TurtleMeister
No, you're thinking of revloution, not rotation. The Moon does both.
So the axis of rotation for the earth moon system is the earth moon barycenter, but the axis of rotation for the moon is inside the moon? So does it have an equator?

Staff Emeritus
So the axis of rotation for the earth moon system is the earth moon barycenter, but the axis of rotation for the moon is inside the moon? So does it have an equator?

Exactly. The Moon rotates around its own axis of rotation in addition to orbiting the Earth-Moon barycenter.

TurtleMeister
In post #7 I stated that the axis of rotation of the earth moon system is the earth moon barycenter. However, after further thought I realized that rotation is not a good way to describe an orbit.

Here is an image from Wikipedia showing the orbit and orientation of the earth moon system:

Notice that a lunar axis of rotation is shown along with a lunar equatorial plane. So it would seem that the moon does have an equator. I'm guessing that if an object is truly not rotating then it's orientation must not change with respect to very distant bodies such as quasars.

Mentor
If the planet doesn't revolve as seen from Earth, it definitely looks like revolving for its inhabitants (with day being equal to a year) - so it has a well defined equator.

If the planet doesn't revolve for its inhabitants (no daytime), it definitely looks like revolving from the Earth - so it has a well defined equator.

Homework Helper
In reality, by the OPs definition, there are no planets with zero angular momentum, so all planets have an equator.

Nick O
In reality, by the OPs definition, there are no planets with zero angular momentum, so all planets have an equator.
You mean the definition Drakkith posted? I'm not seeing how that conclusion logically follows from the definition...

Homework Helper
You mean the definition Drakkith posted? I'm not seeing how that conclusion logically follows from the definition...
The same definition was implicit in the OP as well. Planets, by definition, are quite massive. The only way for a planet to have zero angular momentum would be to start with one with near-zero angular momentum and attempt to bring its rotation to a stop. Even that would be difficult since there are probably other gravitational bodies in the area - so you would need to compensate for tidal forces as well.

ime0n
Actually the equator is not a line it's a surface and there's a line ,around which the mass rotates, which is perpendicular to the surface for a mass that is not rotating around itself there is no such line!

Staff Emeritus