Re: [Phys-L] [SPAM] Re: Physics, Errors and Different Teaching Styles

[John Denker <jsd@av8n.com>]

On 06/29/2012 09:35 AM, Hugh Haskell wrote:
> I wasn't talking about rotation *of* the earth, but rotation of the
> earth *about* the sun, as I thought my wording should have made
> obvious.

Wow, that was not obvious at all.  The wording did not include the
word "sun".  It did however mention the space station, which is in
orbit around the /earth/.

> The earth is "on the end of a string" in its trip about the sun, and
> I was assuming that the effect of that situation was negligible.

1) Tidal stress due to the sun's effect on the earth is directly
observable.  Inferring the /quantitative/ stress from the height
of the ocean tides is tricky, but it's obvious that there's 
something going on.

This is relevant to questions about "rotational" i.e. centrifugal
effects, as follows:  Expand the sun's gravitational field in a 
Taylor series.  The orbital centrifugal effect cancels the first-
order term, leaving us to observe the second-order term, namely
the tidal stress.
  http://www.av8n.com/physics/tides.htm#sec-Taylor-pot

2) The ECEF reference frame rotates once per /sidereal/ day.
A laser ring gyro is plenty good enough to see the difference 
between a solar day and a sidereal day.  This has real-world
practical significance for inertial navigation systems.

In principle you could observe the same thing using a Foucault
pendulum, but the average high school probably doesn't have the
resources to build a Foucault pendulum with that kind of accuracy.

Note ECEF == "earth-centered, earth-fixed"


On 06/29/2012 09:57 AM, Chuck Britton wrote:
> > the orbital effect couldn't very well be 'tabulated'.
> > It would have a 24 hr variation wouldn't it?

The first-order term "would" have a 24-hour period, but it gets
canceled as discussed above.  The second-order term (tidal stress)
has a 12-hour period.  The magnitude of this stress (and higher
order corrections to boot) have definitely been tabulated.