NavList:

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Hi Don

When I take sights to the north, I'm sighting across Long Island Sound with Connecticut in the distance.  Sometimes the line between the sea horizon and the land behind it IS washed out, other times not.  What is not visible is where the water touches the Connecticut shore, that is below the horizon.

( Additionally, although Frank is on that far shore, I lack the visual acuity to see him, since he is only about 13 arc seconds tall! )

But in celestial, we are always subject to the vagaries of an indistinct horizon, as well as odd ball refraction and cloud cover.  This applies to open ocean sights as well as our straw man, the Bonneville Salt Flats. 

Lu was looking for a specific place on earth where the celestial navigator 'could' take land observations without an AH.  I think perhaps we have that.  Is it perfect?  No, but then, what is?

Brad

On Oct 15, 2013 10:21 AM, "Don Seltzer" <timoneer{at}gmail.com> wrote:

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I wrote: Perhaps in selected directions. However, much of the horizon is dotted with distant mountains.

Lu Abel replied: But you still have a horizon, right?? Remember, the distance to the horizon is roughly 8/7 times the square root of height of eye. So for someone standing on the flats themselves, the distance to the horizon would be less than three miles. Sure, you can see mountains in the distance, but their base is hidden by the curvature of the earth. This is similar to, for example, taking shots on a large lake or bay where there is shore in the distance but beyond the natural horizon.

I was wondering if you could reliably distinguish the boundary between those distant mountains and the salt flat 'horizon'. Frank posted an example of some nearby mountains (to the north?) that sharply contrasted with the salt flats.

But if one were doing a noon sighting from the same spot, there are mountains directly south, about 60 miles away but rising 5000' above the plain. The tops of these mountains are well above the 'horizon', and because of the distance will often appear as a very washed out gray. I question whether they provide enough contrast with the salt flats to make an accurate observation.

Checking some topographical maps, I think that one might be OK with sightings to the SE and east. The mountains are distant enough that even the tops are not visible from Bonneville Salt Flats.

Don Seltzer
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