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Lu wrote-

| This seems quite silly, but I realize that I don't know the "official"
| definition of a degree of latitude.
|
| I'm sure most on this list know that the earth is an oblate spheroid --
| it's fatter than it is tall.   This means if I cut the earth in half
| through its poles, the resulting cross-section looks like an ellipse,
| wider than it is tall, rather than a perfect circle.   And this
| elliptical cross-section can lead to two possible definitions of a
| degree of latitude.
|
| If take a cross-section of the earth and draw an angle one degree up
| from the equator, is the place where this line intersects the surface of
| the earth the first (degree) parallel?   Or is the first parallel one
| ninetieth of the way from the equator to the pole?

=============

From George.

Not a silly question at all. I'm not certain what is meant by "draw a line
one degree up from the equator" (through where?) but I suggest neither of
these definitions is the true one. What matters is the direction of the
vertical that passes through the point, at right angles to the sea surface.
When that vertical is 1 degree in angle from the plane of the equator, then
the latitude of the point is 1 degree. That vertical does NOT pass through
the Earth's centre.

Defined in that way, it allows the latitude to be precisely measured by the
altitude of a culminating body with known declination, independent of the
shape of the Earth. The price you have to pay for this useful "fiddle" is
that lines of latitude are a bit unequally spaced, but that doesn't present
much of a problem.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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