Current theories?

I'm wondering what the currently accepted thinking in the scientific
community is on a couple of items, and hope y'all will indulge &
enlighten me.

1) _Why_ is the earth's axis tilted? I understand the result (seasonal
weather, etc.), but how & when did it get tilted in the first place?

2) How did the moon form? When?

3) Is 4.5 billion years still the best estimate of the age of the
earth, and how was it arrived at?

TIA for any and all reasoned responses.

Jim, "Entropy never sleeps; do y'all?"

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Jim wrote:
I'm wondering what the currently accepted thinking in the scientific
community is on a couple of items, and hope y'all will indulge &
enlighten me.

1) _Why_ is the earth's axis tilted? I understand the result
(seasonal
weather, etc.), but how & when did it get tilted in the first place?

Why not? Mars' axis is also tilted, and so are the axes of other
planets. The way in which the planets condensed isn't necessarily that
precise.

2) How did the moon form? When?

I think the most favored current theory is that early in the formation
of the solar system, the proto-Earth got hit by something the size of
Mars.

3) Is 4.5 billion years still the best estimate of the age of the
earth, and how was it arrived at?

The oldest known rocks are about 3 billion years old. They are dated
through long-lived and common radioisotopes such as those of thorium,
found in small quantities in granite. Thorium decays to substances
chemically distinct from it, so they wouldn't end up in exactly the
same spot except through decay.

Perhaps it would take a molten Earth about 1.5 billion years to cool
down enough to form rocks, but I do suspect there is more to it than
that.

John Savard

On 8 Apr 2005 09:35:41 -0700, wrote:

The oldest known rocks are about 3 billion years old. They are dated
through long-lived and common radioisotopes such as those of thorium,
found in small quantities in granite. Thorium decays to substances
chemically distinct from it, so they wouldn't end up in exactly the
same spot except through decay.

Perhaps it would take a molten Earth about 1.5 billion years to cool
down enough to form rocks, but I do suspect there is more to it than
that.

Better than dating Earth rocks is dating meteorites. Many of these
remain largely unchanged since their formation. Such dating places the
age of the Solar System at about 4.65 billion years.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Jim wrote:
snip
1) _Why_ is the earth's axis tilted? I understand the result
(seasonal
weather, etc.), but how & when did it get tilted in the first place?

A planet "tipping" over can also be the result spin-orbit resonances
building up over geologic time. Think of a wobbling spinning top you
used as a child. The general rule might be the opposite from
common-sense intutition. Unless there is some force working against
spin resonance, most planets will "tip over" relative their star
system's ecliptic plane. For example the obliquity of moonless Venus is
178=B0; Uranus is 97.9=B0.

The Earth's Moon contributed to the habitability of Earth that we now
enjoy. The Moon acts as dampener that soaks up the spin-resonances of
the Earth's wobble. This constraints the Earth to a relatively even
keel with a maximum tilt of around 24=B0 with respect to the ecliptic.
That keeps the seasons habitably even. The Earth would not be a very
nice place to live if the South Pole pointed toward the Sun.

Table of the obliquity of planets and their major moons
http://www.physics.vanderbilt.edu/as...tary_data.html

See thread and my post in this usenet group for further references:
Topic: If We Had No Large Moon, would we Rotate like Venus?
Begun by: TerryS Aug 16, 2004, 1:30am
My response with references: Aug 16, 2004, 6:37pm
Google usenet engine Message-ID:


Use search terms "obliquity planets" in internet search engines for
more information.

A good general reading level book -

Comins, Niel F. 1993. What if the moon didn't exist : voyages to earths
that might have been. HarperCollins.=20

- Peace Canopus56

1) _Why_ is the earth's axis tilted? I understand the result (seasonal
weather, etc.), but how & when did it get tilted in the first place?

See below.

2) How did the moon form? When?

Early collision between earth and smaller object --- roughly the size of
Mars. Collision would scatter lighter mantle material of both earth and
impactor into earth orbit where it would become the moon. This also produces
the tilt of earth axis. Look around at the other planets and notice unusual
tilts and/or "day" orbit and you can see where it might have happened to
others.

At one point this was thought to be ridiculous as it needed a big
catastrophe. But if you stop and think of the accretion process, it is
inevitable that you have some big collisions.

3) Is 4.5 billion years still the best estimate of the age of the
earth, and how was it arrived at?

Yes. Dating metorites.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/

Are you interested in understanding optics?
Try http://groups.yahoo.com/group/ATM_Optics_Software/

To reply, remove Delete and change period com to period net

TIA for any and all reasoned responses.

Jim, "Entropy never sleeps; do y'all?"

The above address is invalid; send email to fesser at same domain name.

http://community.webtv.net/IronDuff/SpringBreak
http://community.webtv.net/a968/ContinentalDivide
http://community.webtv.net/a968/TennentMountain