Time deplacement since ancient Babylons question.

Dear Jonathan Silverlight:

"Jonathan Silverlight"
wrote in message ...
In message Dj5%c.86376$4o.80797@fed1read01, "N:dlzc D:aol T:com (dlzc)"
writes
Dear Jonathan Silverlight:

"Jonathan Silverlight"
wrote in message ...
Something else which I'm sure I've seen answered, but can't remember.
The
Devonian measurements depend on the year being the same length, because
you're really just counting days in a year, and perhaps months. If the
year _wasn't_ the same length, how could we tell?

You'd have to either alter the Sun's mass, or our position wrt it.
Either
one would leave indelible marks on the Earth.

David A. Smith

Of course. I don't believe it happened in the last billion years, but
Sackmann and Boothroyd have proposed a "bright young Sun" to explain
liquid water on the early Earth and Mars (and as you would expect, their
idea has already been hijacked by creationists !)
Oddly, they don't mention the planets' orbits or the year, but if you
alter the mass of the Sun in order _not_ to leave marks on the Earth, how
do you measure the length of the year?

Tidal rhythmites make some assumptions, and take us back pretty far (I
think 2 or 2.5 Gy, but I may be wrong). The fact that there is liquid
water says something about how close or how far we are from the Sun.

David A. Smith

In article , Jonathan Silverlight
wrote:
Did I read somewhere that the angular momentum of the Earth may have
changed in the past - if the core changed state, for instance? There's
presumably an upper limit on any such change which is set by these
measurements.

During the assembly of the Earth, the moment of inertia would
certainly have been changing all the time. That's trivial. The present
popular models of the formation of the Moon indicate that there were
iron-rich cores to both proto-Earth and proto-Moon. Well, the impactor at
least. That's the major rearrangement of the internals of the Earth
accounted for before 4.5 billion years ago. Any such changes since have
been relatively trivial - the crystallisation of the solid core, maybe
continued segregation of iron-rich material from silicate-rich material
in the lowermost mantle. They don't add up to much.
In comparison, there are noticeable shifts (of a metre or so) in
the intersection of the rotation axis of the earth with it's surface (as
defined by a fixed geographic grid based on a number of observatories
worldwide). Much of this correlates with large earthquakes once you've
taken out the systematic effects (Chandler wobble etc.). Causes like this
near the Earth's surface are going to have a larger effect than the same
movement of mass in the core, because they're effectively on the end of a
lever.

--
Aidan Karley,
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233