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From: David Weilacher (no email)
Date: Thu Aug 12 2004 - 08:48:44 EDT
Well George. I guess I'll pitch out the puptent and eastern horizon picture and go at this a different way.
Say it is LAN. I can see both the southern horizon and northern horizon just fine. Using the southern horizon, I get a lower limb sight of 60 degrees exactly after correcting for dip and IC. I get an upper limb of 60 degrees, 32 minutes.
Now what I know is that if I do a backsight by using the northern horizon, I need to end up with 60 degrees exactly for a lower limb reduction, as opposed to any other number.
If I use the northern horizon and bring what looks like the lower limb of the sun in the eye piece down to the horizon, it is actually the top edge of the sun that I'm using. The angle I'll get, after dip and IC, will be 119 degrees 28 minutes.
If I subtract this from 180, I get 60 deg 32 min. By golly, thats an upper limb correction needed.
I don't mind being wrong nearly as much as I mind you being right yet again.
Dave W
-----Original Message-----
From: George Huxtable <>
Sent: Aug 11, 2004 6:08 PM
To:
Subject: Re: Reducing back sights
Dave Weilacher wrote-
>I picture it this way.
>
>Instead of an octant, or a sextant, I'm out there with a halftent (I'm
>pretty sure this would be called a puptent).
>
>Sun comes up in east I have east horizon. Every hour I take a lower limb
>sight using the east horizon only.
>
>LAN comes and goes.
>
>I am still bringing the same limb of the sun down to the same horizon even
>in the afternoon. With my puptent, I can do this all the way to sundown.
>
>Problem is that in the afternoon, the limb that I am bring down is the
>upper limb, refraction wise, but appears as lower limb against east
>horizon through the eyepiece of my puptent.
>
>Our marine almanac combines semidiameter and refraction together as one
>figure. Thats what makes our lower and upper limb corrections different.
>
>Do I have this pictured to your satisfaction?
=================
Response from George.
Yes, that's fair enough. In the afternoon, measuring angles greater than
90deg from the Eastern horizon, Dave is free (as he did) to use the true
upper-limb of the Sun (which happens to look as if it's a lower-limb in his
sextant-view, BUT IT ISN'T). After subtracting index error and dip, then
subtracting from 180deg (not 90- Bill Noyce made an uncharacteristic slip
there) his sextant will have given him the apparent altitude of the
upper-limb above the unseen Western horizon. Now he will have to subtract
the corresponding refraction and also the semidiameter, which is just the
same combination as for any normal sextant correction of the Sun's upper
limb.
Alternatively, after noon, Dave could just as well have observed, from the
Eastern horizon, the true lower-limb of the Sun, which actually involves
switching to the Sun's opposite edge at noon (if there was a visible
sunspot to show it up). Now, this happens to look in the sextant view as if
it's the upper-limb, BUT IT ISN'T. After subtracting index error and dip,
and then subtracting the answer from 180, that will provide the apparent
altitude of the Sun's lower limb, above the unseen Western horizon. Again,
he will have to subtract the refraction corresponding to that apparent
altitude, but this time ADD the Sun's semidiameter; which is again exactly
the same combination of corrections as for a normal lower-limb Sun sight.
So for these sextant backsights, the only thing that has to be done
differently is to subtract the angle from 180deg, after correcting for
index error and dip in the normal way. I think that the problem bugging
Dave Weilacher is an unreal one. However, it's essential not to be confused
by the appearance in the sextant view, and to make sure that an upper-limb
observation really is treated as an upper limb observation, no matter what
it may look like in the sextant.
Another matter, which an observer will become aware of as soon as he tries
to take such a backsight, is that as he rocks the sextant, he won't see the
usual picture of the Sun dipping down to just kiss the horizon when he has
it right. Instead, as he rocks the sextant, the Sun appears to ascend from
below the horizon, and what he is aiming for is for the appropriate limb to
just brush the horizon at its highest point, not its lowest.
Bill Noyce and I seem to agree, but have we convinced Dave, I wonder?
George.
================================================================
contact George Huxtable by email at , by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
================================================================
Dave Weilacher
.US Coast Guard licensed captain
. #889968
.ASA instructor evaluator and celestial
. navigation instructor #990800
.IBM AS400 RPG contract programmer
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