Introduction Page 5

Introduction

Consequences of Rotating Earth

The earth rotates once per day. This throws the matter outward away from the spin axis - the line between the north and south poles. The force is proportional to the square of the distance from this line. This makes the force maximum on the equator and zero at the poles. This is a "force" felt only by objects held to the earth and rotating with it, but that is all common object including people. Satellites do not feel this force because they are not fixed to the earth. This force, or pseudo forces as some people call it, is the centrifugal force.

On the earth we feel the sum of the attraction of gravity explained by Isaac Newton and this rotational effect. (This effect was also first described by Newton.) This causes the earth to bulge out at the equator making the radius of the earth a function of latitude. The earth is 21 km bigger in the equator direction than in the pole direction. The local Up vector is essentially perpendicular to this bulged surface.

This means that there are now at least two kinds of latitude, that measured from a line that goes to the center of the earth (geocentric latitude), and that using the local Up/Down line as a reference (astrodetic latitude). These can differ by 0.19 deg (11 arc minutes).


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