Poles

[wasdale]

One knows all about geographic and magnetic poles, but what and where are the geometric poles one has been hearing about recently?? :confused:

[divdby0]

Geographic or Geodetic latitude : angle between equitorial plane and normal to the observers horizontal plane.

Geometric or Geocentric latitude : angle between line joing the observer to the geocentre of the earth. Geocentre is the point where earths axis crosses the equitorial plane.

The difference between them at poles and equator is 0 and maximum difference occurs at 45 deg lat where it is approx 11.4 deg., geographic higher than the geometric lat.

And from what I can recall, the precession rate varies as well, as the axis of earth's rotation is along geometric axis.

For a more detailed and very complicated explanation, visit http://www2.ncdc.noaa.gov/docs/klm/html/i/app-i4.htm



Regards,

Divdby0

[Mad (Flt) Scientist]

maximum difference occurs at 45 deg lat where it is approx 11.4 deg.
That seems a huge discrepancy; surely the earth isn't that far from spherical?

edit: yes, it's way off.
At sea level, the relation ship between geocentric latitude C and geodetic D is:

C = arctan ( (1-F)^2 * tan D)

where F is the "flattening", which for earth is 1/298 approx

That means that the following are some examples:

D=0 -> C=0
D=30 -> C=29.83
D=45 -> C=44.81
D=60 -> C=59.83
D=90 -> C=90

So the difference 45 deg geodetic is 0.19 degrees - which is 11.4 MINUTES approx, not degrees.

[divdby0]

hats off flt scientist.:D 11.4' it is.

Regards,

D.

[wasdale]

I still don't see that that has answered my question. :confused: :confused:

[Mad (Flt) Scientist]

Since 90=90, they are technically in the same place. For a symmetrical earth.

BUT - if the earth isn't symmetrical, so that at the geodetic north pole - where the horizon is at right angles to the pole on the equator - you could drill "straight down" i.e. at right angles to the local horizon and NOT hit the centre of the earth, then they wouldn't be the same place.

I can't see how that's a practical possibility for the earth, but maybe some smart geographer has found the earth's a bit squint.