Still studying!

Question

I have The Macmillan & Silk Cut Nautical Almanac from 1981 which pre-dates WGS84 and they make reference to the NM being 6046 feet at the equator and 6108 feet at the poles. Conversions to metres = 1842.82m and 1861.71m. Remarkably similar to WGS84 derived figures of 1842.9m and 1861.57m. Just goes to show the old ways take some beating!

I also have to hand "The Oxford Companion to Ships & The Sea" by Peter Kemp printed 1976. There is a passage included in it I nearly understand which I think is very relevant and might clear some of my confusion up if I understood it correctly as follows:

"Geographical latitude, also called true latitude is equivalent to the true altitude of the elevated celestial pole at the place. This is always greater (except at latitude 0 or 90) than the corresponding angle at the earths centre, which latter angle is called the geocentric latitude of the place."

I do understand the part about the elevated celestial pole but it is the suggestion that any random point on earth can have 2 latitudes depending on the method used in calculating it.

Can anyone offer further explanations.

Thanks

Mike

Answer 1

The difference in the two latitudes may be due to the oblateness (nonsphericity) of the earth. The plane tangent to local sea level is not perpendicular to a line to the center of the earth, except at the poles and the equator. The same oblateness is the reason for the difference in nautical miles measured at the equator and along a meridian.