[Proj] How to convert a sphere to ellipsoid with correct datum?

[Heiko Klein]

Hi,

I'm working with meteorological model-data. The models usually use a 
spherical earth. Since the resolution of the models is now ~1km, I have 
to think more about converting those data for people using GIS systems. 
I have only a very rough understanding of datum, geocentric and 
geodetic. Up to now I've only used the ellps-parameters of proj to 
describe the earth.

If I understand it correctly, a spherical earth is completely described 
by the major axis, and geodetic and geocentric coordinates are the same?


Using the following does not change the coordinates

./cs2cs +proj=latlong +ellps=sphere +a=6371000 +e=0 +to +proj=latlong 
+ellps=WGS84
10 60
10dE	60dN 0.000

since it calculates completely in geocentric coordinates?



Giving a WGS84 datum on both input and output gives geodetic coordinates?
/cs2cs +proj=latlong +ellps=sphere +a=6371000 +e=0 +datum=WGS84 +to 
+proj=latlong +ellps=WGS84 +datum=WGS84
10 60
10dE	60d9'58.075"N 8921.648


Why should the sphere-datum be WGS84? Using NAD27 gives me geocentric 
coordinates?
./cs2cs +proj=latlong +ellps=sphere +a=6371000 +e=0 +datum=NAD27 +to 
+proj=latlong +ellps=WGS84 +datum=WGS84
10 60
10dE	60dN 0.000

Does there exist a correct 'datum' for a sphere? Isn't it all the same 
for a sphere? Why do I have to give it at all?
If I have a measurement station with GPS coordinates, latitude and 
longitude, are they geocentric or geodetic?

And the final and most important question: What is the best description 
of a spherical earth to convert from with cs2cs?


Best regards,

Heiko