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So is this of some help to us land-locked navigators who want to practice
with our sextants without the hassle of an artificial horizon? 

-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Frank Reed
Sent: Tuesday, March 26, 2013 12:25 PM
To: dcortes@rwlw.com
Subject: [NavList] Finding the true horizon

The true horizon is invisible but easily defined. You find the vertical
(gravitationally, with a level or a plumb line or similar) at some point on
the Earth's surface and then you locate all points that are 90� from that
vertical line. That's the true horizon. It's important to note that this
horizon and the vertical it's derived from are locally variable. It's only
good luck for us earthling navigators that the Earth's gravity field is
rather smooth, closely approximating a spheroid. If we lived on a
gravitationally lumpy world, celestial navigation would still be possible,
but the connection between coordinates drawn on the sphere and the readings
of celestial navigation would be much more complicated. 

There's a trick for finding the true horizon that works extremely well in
some cities. Buildings are constructed "plumb". The floors are supposed to
be level. If you live in a city with modern buildings, and if those
buildings aren't in danger of collapse or settling over many decades, then
you can count on floors being level. If you look into the distance and sight
down the side of a building, perspective will lead you right to the true
horizon. This is especially easy in photographs. I've attached a photo
trimmed from a random photo looking down an avenue in New York City. The
lines drawn from floors on the building on the left converge in the
distance. That crossing point is located right on the true horizon for this
observer's location. Note that a building at great distance wouldn't work
since there would be some rotation due to the curvature of the Earth.

You can use this trick in photographic experiments in astronomical
position-finding. Get the horizon from the lines of neighboring buildings.
Note that this technique finds the true horizon so, of course, there's no
correction for dip, refracted or otherwise.

-FER


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Attached File: http://fer3.com/arc/imgx/true-horizon.jpg


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