[Phys-l] gravity field mapper ... and why folks should care

[John Denker <jsd@av8n.com>]

IMHO, here is a very nice piece of science writing:

http://news.bbc.co.uk/2/hi/science/nature/6919016.stm

> The planet is far from a smooth sphere; the radius of the globe at
> the equator is about 20km longer than at the poles. This ellipsoid is
> then marked by tall mountain ranges and cut by deep ocean trenches.
>
> The Earth's interior layers are also not composed of perfect shells
> of homogenous rock - some regions are thicker or denser.
>
> Such factors will cause the gravitational force at the surface to
> deviate from place to place by very small but significant amounts.
>
> The Gravity Field and Steady-State Ocean Circulation Explorer (Goce)
> will map these differences. This information will then be used to
> fashion what is, in essence, an idealised globe. Scientists call it
> the geoid.
>
> The geoid is of paramount interest to oceanographers who study the
> causes of the "hills" and "valleys" on the sea surface.
>
> If local gravity differences are not creating these features, then
> other factors such as currents, winds and tides must be responsible.

For the rest of the story, see
  http://news.bbc.co.uk/2/hi/science/nature/6919016.stm

I like this because 
 a) It is accessible to a wide audience,
 b) it does a good job of explaining the science, and
 c) it explains why people should care.

Students often have no clue what physicists do for a living.  This
article gives multiple good answers:
 -- You have to be a pretty good physicist to measure g (or almost
  anything else) to one part in 10^12.
 -- It takes some cleverness to design an electric motor for a
  spacecraft.

There's a syllogism that goes something like this:
  You want to understand weather and climate.  (Information
  about El Niño and suchlike is worth many, many billions of
  dollars to farmers and others.)
  ==> You want to understand currents etc. in the ocean.
      ==> You want to know the fine details of the geoid.
          ==> You want a satellite that can measure g to
              tremendous accuracy.
              ==> You want some smart physicists.


There's a separate but similar syllogism that starts with trying
to track volcano activity.