- #1
avito009
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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?
Why Equator only exists for a rotating body like Earth?
In summary: 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.
- #2
Drakkith
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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.
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.
- #3
Ophiolite
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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?
- #4
Drakkith
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Ophiolite said:
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)
Last edited:
- #5
TurtleMeister
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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?Ophiolite said:
- #6
Bandersnatch
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No, you're thinking of revloution, not rotation. The Moon does both.TurtleMeister said:
- #7
TurtleMeister
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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?Bandersnatch said:
- #8
Drakkith
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TurtleMeister said:
Exactly. The Moon rotates around its own axis of rotation in addition to orbiting the Earth-Moon barycenter.
- #9
TurtleMeister
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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.
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.
- #10
Borek
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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.
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.
- #11
.Scott
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In reality, by the OPs definition, there are no planets with zero angular momentum, so all planets have an equator.
- #12
Nick O
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You mean the definition Drakkith posted? I'm not seeing how that conclusion logically follows from the definition....Scott said:
- #13
.Scott
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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.Nick O said:
- #14
ime0n
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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!
- #15
Drakkith
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As complicated as people appear to be making this, it seems to be a simple concept that I believe I answered adaquately in the 2nd post.
- #16
ime0n
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Drakkith I'm sorry you're right I didn't read the posts
- #17
Borek
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ime0n said:
Nope. Equator is a line on the surface of the sphere. Compare http://www.britannica.com/EBchecked/topic/190632/Equator
- #18
ime0n
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Borek you'r right, i just wanted to emphasize
1. Why does the Equator only exist for a rotating body like Earth?
The Equator is an imaginary line that divides the Earth into the Northern and Southern hemispheres. This line only exists for a rotating body like Earth because the rotation of the planet creates a bulge at the center, making the distance from the center to the Equator greater than the distance from the center to the poles.
2. How does the rotation of Earth affect the existence of the Equator?
The rotation of Earth causes the centrifugal force, which counteracts the force of gravity at the Equator. This results in a bulging effect at the center of the planet, creating a larger circumference at the Equator compared to the poles. As a result, the Equator only exists for rotating bodies like Earth.
3. Is the Equator the only line of latitude that exists on a rotating body?
No, the Equator is not the only line of latitude that exists on a rotating body. Other lines of latitude, such as the Tropic of Cancer and the Arctic Circle, also exist on a rotating body. However, the Equator is the most prominent line of latitude and is used as a reference point for navigation and mapping.
4. Are there any other factors besides rotation that affect the existence of the Equator?
Yes, other factors such as the Earth's shape and composition can also affect the existence of the Equator. The Earth is not a perfect sphere and has an oblate shape, which means it is slightly flattened at the poles and bulging at the Equator. The composition of the Earth's layers, particularly the mantle and crust, also play a role in creating the Equator.
5. Can the Equator exist on other planets or celestial bodies?
Yes, the Equator can exist on other rotating bodies such as other planets and celestial bodies. However, the bulging effect and the exact location of the Equator may differ depending on the size, shape, and composition of the body. For example, the Equator on Saturn would be much wider due to its larger size and faster rotation compared to Earth.
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