Hill sphere shape

[Zura]

Hi all.

It's my first post here in this forum.

I came with a question to the professionals of this field.

I'm interested in a shape of Hill sphere (for example of Earth). Is it known for sure that it has spherical shape? Or may be it is ellipsoid or something like it? Was any measurements of it performed?

[studiot]

8 hours ago, Zura said:

Hi all.

It's my first post here in this forum.

I came with a question to the professionals of this field.

I'm interested in a shape of Hill sphere (for example of Earth). Is it known for sure that it has spherical shape? Or may be it is ellipsoid or something like it? Was any measurements of it performed?

No the Earth's Hill sphere is not spherical.

Wikipedia has a good article on this, including excellent diagrams.

 

https://en.wikipedia.org/wiki/Hill_sphere

[Zura]

1 hour ago, studiot said:

No the Earth's Hill sphere is not spherical.

Wikipedia has a good article on this, including excellent diagrams.

 

https://en.wikipedia.org/wiki/Hill_sphere

I read it again more thoroughly. So it is stretched on L1L2 direction? Did I understand right?

Edited April 20, 2021 by Zura

[iNow]

12 hours ago, Zura said:

for example of Earth

I've always heard our planet best described as an oblate spheroid. It's not perfectly spherical, has differences in dimension and density (it's a spheroid, not a sphere), and is a bit squishy around the middle due to its spin (it is oblate). 

[studiot]

2 hours ago, iNow said:

I've always heard our planet best described as an oblate spheroid. It's not perfectly spherical, has differences in dimension and density (it's a spheroid, not a sphere), and is a bit squishy around the middle due to its spin (it is oblate). 

The Hill Sphere is not about or much to due to the shape of the body.

It is the region of space around the body where gravity is controlled by that body.

As such the region is subject to purturbation by other bodies, so the Moon purturbes the Hill Region (Hill was an astronmer) and the Earth purturbes the Sund's region etc.

Within this region a smaller orbiting body will remain orbiting and not escape.
It is said to be locked or gravitationally locked or tidally locked to the larger one.

It may be noted that there is a small region round the Moon where the Moon has greater influence than the Earth and so a satellite could potentially orbit there.

[iNow]

Thank you, @studiot.  I clearly misunderstood the core element of the question and yours was a helpful and welcome clarification. +1

[beecee]

7 hours ago, iNow said:

I've always heard our planet best described as an oblate spheroid. It's not perfectly spherical, has differences in dimension and density (it's a spheroid, not a sphere), and is a bit squishy around the middle due to its spin (it is oblate). 

 

2 hours ago, iNow said:

Thank you, @studiot.  I clearly misunderstood the core element of the question and yours was a helpful and welcome clarification. +1

Funny, last night when I saw this question, was about to also describe the Earth as an oblate spheroid. I have never heard of the Hill sphere and after searching see it is tied in with the Roche limit. Clearly like you, I also misunderstood. Still, I have learnt something though!

Edited April 20, 2021 by beecee

[Zura]

5 hours ago, studiot said:

The Hill Sphere is not about or much to due to the shape of the body.

It is the region of space around the body where gravity is controlled by that body.

As such the region is subject to purturbation by other bodies, so the Moon purturbes the Hill Region (Hill was an astronmer) and the Earth purturbes the Sund's region etc.

Within this region a smaller orbiting body will remain orbiting and not escape.
It is said to be locked or gravitationally locked or tidally locked to the larger one.

It may be noted that there is a small region round the Moon where the Moon has greater influence than the Earth and so a satellite could potentially orbit there.

What about my question about L1L2? 

And how physics explains the asymmetry of the Hill sphere. Gravity, on the other hand, has central symmetry.