I have a question that I'm hoping some crack phys-ler can answer. This may
not be the proper forum for this question, but I don't subscribe to any
geography or cartography mail lists.
What is the "official" definition of latitude?
If the Earth were a sphere the answer to the question would be straight-
forward. Since the Earth's shape is much closer to an oblate ellipsoid of
revolution than a sphere we can define at least two inequivalent latitudes at
a given place P at its surface.
Definition 1: The latitude at P is the measure of the angle by which the plane
tangent to the local surface of the Earth is tilted with respect to the
corresponding local plane at the point Q on the equator on the same meridian
as point P. An equivalent definition is that the latitude at P is the measure
of the angle between the direction of the local "vertical" at P and the
direction of the local "vertical" at Q. Here "vertical" means the direction a
motionless plumb bob hangs. This direction is determined by the direction of
the vector sum of the of the local gravitational force of the Earth acting on
a test particle and the effective centrifugal "force" acting on the test
particle when it is at rest with respect to the Earth's surface (caused by the
rotation of the Earth). The local horizontal plane is always perpendicular to
the local vertical using the above definition of vertical.
Definition 2: The latitude is the measure of the angle made between a line
connecting the Earth's center of mass O with the point P and the line
connecting O with the point Q on the equator.
Personally, I think that I like Definition 1 better than Definition 2 as it
can be easily determined by simple astronomical measurements and a plumb bob.
It is what an sextant would measure as the angle between the celestial pole
and the horizon. Latitude using Definition 2 is harder to determine from only
local measurements. My guess is that satellite GPS-type measurements can just
as easily determine either kind of latitude.
What do any of you know about the "official" definition? When I see a
detailed map with the iso-latitudinal curves drawn on it, which definition are
the cartographers using?
David Bowman
Georgetown College
dbowman@gtc.georgetown.ky.us