Supernova Detonation Aimed at Earth? Astronomers Say "Maybe"

[KrazyHorse]

No, the Moon actually acts like a gyroscope, and tames the Earth's wobble down. Without the Moon, Earth would tumble.

???

[KrazyHorse]

Much of Berzerker's post appears to be based on numerology rather than any scientific principle.

[Lancer]

the wobble thing...

http://solarsystem.nasa.gov/planets/profile.cfm?

[KrazyHorse]

Again, you guys seem to be ignoring the possibility that the sun's plane of rotation has shifted over time. Or that the apparent plane of rotation of the sun's surface is different than the plane of its total angular momentum (since it is not a rigid body).

Either way, using the sun's plane of rotation as a reference frame is a terrible idea.

Most of the angular momentum of the solar system is in the planets, specifically Jupiter and Saturn; for circular orbits due to a single central mass the angular momentum scales as the square root of the radius of the orbit and linearly with mass; saturn's mass is 1/3 that of Jup (IIRC) and its orbital radius is twice that of Jup, so it contributes ~ sqrt(2)/3 ~ 1/2 as much to angular momentum as Jup does.

[KrazyHorse]

Che, I don't understand how a body which is significantly disaligned with the earth's rotational plane (23.5 degrees to plane of ecliptic and moon is only 4 degrees from plane of ecliptic, so there is at least 19.5 degrees in between the two) REDUCES precession...

[chequita guevara]

tides, I'm guessing.

I'm talking about the axial tilt, not the orbital eccentricity of the Earth.

I've heard this mentioned on a number of shows about the history of the Earth.

Lancer's link http://solarsystem.nasa.gov/planets/...fm?Object=Moon to Nasa also mentions this.

[chequita guevara]

Again, you guys seem to be ignoring the possibility that the sun's plane of rotation has shifted over time. Or that the apparent plane of rotation of the sun's surface is different than the plane of its total angular momentum (since it is not a rigid body).

Did I forget to mention that?

[KrazyHorse]

tides, I'm guessing.

Tidal forces disaligned with the axis of rotation of the Earth would CAUSE precession, if my memory of rigid body dynamics is any good. That is precisely my point.

[KrazyHorse]

http://en.wikipedia.org/wiki/Precession

The gravitational tidal forces of the Moon and Sun apply torque as they attempt to pull the equatorial bulge into the plane of the ecliptic. The portion of the precession due to the combined action of the Sun and the Moon is called lunisolar precession.

The other type of "wobble" would be nutation

http://en.wikipedia.org/wiki/Nutation

In the case of Earth, the principal sources of tidal force are the Sun and Moon, which continuously change location relative to each other and thus cause nutation in Earth's axis.

In both cases, the Moon is a (the largest, actually) source of perturbative torque on the Earth (again, in both cases due to tidal forces). I don't understand how this "stabilized" the Earth in any sense.

I'm no astronomer, but I do understand classical mechanics pretty well, and I truly don't understand what you're saying.

[KrazyHorse]

By the way, by precession and nutation I AM referring to the movement of the earth's axis of rotation. I am not referring to orbital precession (completely different effect; not part of the dynamics of rigid bodies).

[chequita guevara]

http://www.astronomytoday.com/astronomy/earthmoon.html scroll down to stable axial tilt.

Also

http://skepticsplay.blogspot.com/2008/12/axial-tilt-milankovitch-cycles.html

Interestingly, it has been shown that if the moon didn't exist, the Earth's axial tilt would change chaotically from 0 to 60 degrees, causing climate changes that would possibly be fatal to life. This is often used to argue that we're pretty damn lucky to have a moon. On the other hand, current theories say that a mars-sized object crashed into early Earth, and the resulting ejecta coalesced into the moon. If that collision had never occurred, the Earth would be spinning much faster now, and its axial tilt would be stable as a result.

Still, your questions are good ones, and I'd like to know the answers also. If nutation causes wobble, then how can the moon also stabilize the axis?

[chequita guevara]

One neat thing I learned while looking up info on this is that Earth has three or four quasi-satellites, i.e., objects which are in orbital synchronicity with the Earth, but don't actually orbit the Earth.

http://en.wikipedia.org/wiki/3753_Cruithne

http://en.wikipedia.org/wiki/2002_AA29 looks like it actually does orbit the Earth though, from the diagram.

[Krill]

looks like =\= really does