# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Sextant Positions versus Map Datums?**

**From:**Bill Murdoch

**Date:**2002 Jan 20, 7:39 AM

The way I look at this, and I am not sure I am right, is best described by thinking of a cross section of the earth drawn as a circle and as a ellipse with common centers. The circle would represent a spherical earth and the ellipse an ellipsoidal earth. At the north pole and at the equator, tangents to the circle and to the ellipse are parallel to each other. Lines normal to those tangents all pass through the common center. The normals from the poles intersect those from the equators at right angles giving the poles a latitude of 90 degrees. The normals are the orientation of plumb bobs at the equator and at the poles. A line drawn outward from the center of the earth half way between the equator and the north pole strikes the circle at a point where a tangent to the circle would be at right angles to that line. A pumb bob at that location would be parallel to that line from the center of the earth. The location would be 45 degrees north. The same line from the center of the earth would not strike the ellipse at the point where a normal to a tangent line would be 45 degrees to the equator. That point would be just a little bit farther south. (I can not do the math, but maybe for the slightly flattened earth 11'32"). The plumb bob at the location called 45 degrees north latitude on the ellipse would not pass through the center of the earth, but would be parallel to a line from the center of the earth at 45 degrees to the equator. Bill Murdoch