|
|
Subject: Re: Winter Sextant Sight Accuracy?
From: Jared Sherman (jared.sherman@XXX.XXX)
Date: Fri Jan 11 2002 - 17:08:36 EST
George, for the sake of brevity I omitted the things that I was not seeking discussion of. But since you asked...
<1. I am puzzled by what Jared says about checking index error. >
I did check the index error before taking sights. Perhaps you are familiar with the Cassens & Palth design which puts a large index error adjustment screw at the end of the vernier? That's what I was using, and in the cold with gloves I *suspect* that I loosened the screw and tightened it again as I went back and forth, so that I unlocked the wheels and the zero slipped. This *after* I had checked that it was zeroed, during the sightings.
I have now tightened that screw strongly enough so that it will not accidentally come undone.
<It is bad practice (in my view) to check the index error
against a star at night, then go out next day and use that index error to
correct a Sun altitude>
Why? If it is the same temperature at both times, what should change? And given that a high sighting of a pinpoint object is going to be sharper than a low sighting of a somewhat large object in disturbed air...Why not?
Note that I am not relying on the zero from the previous night--I am checking it against the horizon on the following day anyway. But I have found that in the same temperature, a zero from a pinpoint light source will be easier and more accurate than from any other source.
One of the other things that I didn't mention for brevity is that I have some sight damage from LASIK. The pinpoint light source from a star is also easier for me to use than a horizon or sun disk.
2. <These are NOT the effects of winter.> Nope, didn't say they were. The winter means that I will be dealing with a lower sun though. I said late winter afternoon, and around here that means the sun is near 28 degrees at high noon, 14 degrees at mid afternoon, and somthing like ten degrees or less in the late afternoon.
Compare the atmospheric effects for a sighting at ten degrees to a typical summer midday sighting at altitudes that neer come near ten degrees. THAT's the effect of winter.
<<If you are looking at a rough horizon, from on shore, errors could be quite
significant. You are seeing the tops of the waves only.>>
Thank you George! That's the point that I was asking for a response on. Although, you still haven't responded on the main point: What kind of position accuracy do YOU get with a sextant under similar circumstances? How tight a cluster and what absolute position accuracy? Can you get 2/10th's of a mile with a rough horizon and an Ho on the order of 10 degrees? 1/2 mile? 1 mile?
<The Sun's semidiameter varies> Yes, and that is accounted for.
|