- #1

- 1,102

- 6

I guess my question is, is dipolar attraction ever considered in these sorts of models?

- Thread starter maverick_starstrider
- Start date

- #1

- 1,102

- 6

I guess my question is, is dipolar attraction ever considered in these sorts of models?

- #2

- 15,393

- 685

The Earth (and any real astronomical object) is not a perfect sphere. A precision gravity model requires a treatment of the non-spherical shape. Spherical harmonics are one obvious choice. The obvious origin for such a spherical harmonics expansion is the center of mass. The center of mass spherical harmonics expansion has a zero dipole moment because a non-zero value would require a negative mass. So, no dipole moment for Newtonian gravity.

Higher order terms can be quite important, at least at close range. For example, the Earth's zonal quadrupole moment, J2, is one of the major perturbing factors in satellite orbits and dominates all other perturbing factors at geosynchronous satellites. Quadrupole moments only go so far. People have developed 360x360 spherical harmonics models for the Earth's gravitational field (http://cddis.nasa.gov/926/egm96/egm96.html" [Broken]. JPL has developed Lunar gravity models up to degree and order 160.

Higher order terms can be quite important, at least at close range. For example, the Earth's zonal quadrupole moment, J2, is one of the major perturbing factors in satellite orbits and dominates all other perturbing factors at geosynchronous satellites. Quadrupole moments only go so far. People have developed 360x360 spherical harmonics models for the Earth's gravitational field (http://cddis.nasa.gov/926/egm96/egm96.html" [Broken]. JPL has developed Lunar gravity models up to degree and order 160.

Last edited by a moderator:

- #3

- 1,102

- 6

- #4

- 15,393

- 685

What difference does it make, conceptually? There is a zero dipole moment in the center of mass spherical harmonics expansion of the gravitational field for any collection of mass because there is no such thing as negative mass.

One problem with a dust cloud is that the spherical harmonics model will be time-varying, but if you want to get real picky, so is the Earth's. The Earth rotates once per day, but this problem can be addressed by developing the spherical harmonics in a rotating frame that rotates with the Earth. Even so, the Sun and Moon cause tides, which results in a daily variation in the rotating frame spherical harmonics coefficients. Mass migrates northward during winter (northern hemisphere) and southward during summer, causing an annual variation in the coefficients. Mountains rise up and erode and continents move around, causing longer term variations.

Bottom line: An extremely detailed Earth gravitational model will have time-varying spherical harmonics coefficients. The same is true for a dust cloud.

One problem with a dust cloud is that the spherical harmonics model will be time-varying, but if you want to get real picky, so is the Earth's. The Earth rotates once per day, but this problem can be addressed by developing the spherical harmonics in a rotating frame that rotates with the Earth. Even so, the Sun and Moon cause tides, which results in a daily variation in the rotating frame spherical harmonics coefficients. Mass migrates northward during winter (northern hemisphere) and southward during summer, causing an annual variation in the coefficients. Mountains rise up and erode and continents move around, causing longer term variations.

Bottom line: An extremely detailed Earth gravitational model will have time-varying spherical harmonics coefficients. The same is true for a dust cloud.

Last edited:

- #5

- 1,102

- 6

- #6

- 1,102

- 6

- #7

- 15,393

- 685

- #8

- 15,393

- 685

- #9

- 1,102

- 6

I just remember in my undergrad astro courses iit being taught how accretion disks form and how mass accumulates and then when particles start being ejected from the accretion disk plane by random thermal fluctuations you start to see spherical masses and we calculated approximate thresholds for when this kind of activity would occur all assuming the primary force involved in holding these masses cohesive was gravity when really, it seems to me, the most important factor would be van der waals forces.

Especially since, I remember there being a problem explaining the angular momentum a solar system has and one of the possible explanations was solar winds but the solar winds would also excite the particles which would further increase the importance of electronic intermolecular forces by introducing coulombic forces as ions are formed. But never once do I remember us actually talking about the role that electronic forces play in solar system and stellar formation.

Especially since, I remember there being a problem explaining the angular momentum a solar system has and one of the possible explanations was solar winds but the solar winds would also excite the particles which would further increase the importance of electronic intermolecular forces by introducing coulombic forces as ions are formed. But never once do I remember us actually talking about the role that electronic forces play in solar system and stellar formation.

Last edited:

- Replies
- 18

- Views
- 5K

- Last Post

- Replies
- 4

- Views
- 2K

- Replies
- 7

- Views
- 986

- Replies
- 4

- Views
- 2K

- Replies
- 0

- Views
- 2K

- Replies
- 8

- Views
- 1K