|The difficulty with using tables such as H.O. 229 when working out practice sight is that they are not convenient to work from your known position which you want to do so that you can compare your sextant observations with what should have been measured at your known position. You can, of course, plot the LOPs from the AP and then measure the distance from the LOP to your known position, the distance representing the error in your sextant work. Older tables such as H.O. 214 provided a way to interpolate for your exact longitude and latitude and I have uploaded these tables which can also be used with H.O. 229, see:|
Or, you can use a slide rule that also allows you to work from your known position and the
instructions for making it are available here:
It's really quite easy, your merely print out two scales and move them against each other.
Or you can use the rule of cosines on a calculator. I posted a step by step key stroke list here:
It solves the cosine formula this way:
((Cos LHA X Cos Dec X Cos Lat )+ ( Sin Lat X Sin Dec)) Arc sin = Hc
(1/Cos Hc X Sin LHA X Cos Dec)Arc sin = Z
--- On Thu, 9/29/11, Randall.F.Morrow@kp.org <Randall.F.Morrow@kp.org> wrote:
From: Randall.F.Morrow@kp.org <Randall.F.Morrow@kp.org>
Subject: [NavList] Re: Home made artificial horizon
Date: Thursday, September 29, 2011, 9:18 AM
I am recently self taught
at celestial navigation, using the Dutton's and Hobbs texts over the summer
so please forgive my questions being very basic. Finding
a forum like this one is a great 'land-fall' for me and I sincerely appreciate
the opportunity to discuss navigation. First, being land locked
in Bakersfield California an artificial horizon is all that's available
to me and the results often disappointing. My sextant is a Davis
Mark 15 and I now use a metal plate based, screw adjusted mirror horizon.
I have just changed to a first surface mirror but though the sights
are free of ghost images, the accuracy does not seem to have improved.
What steps can I take
to improve my accuracy? Can you give me the date of a post on this
subject so I may read in your archives?
I level my horizon mirror carefully
but the digital level is only accurate to 1/10 degree. Is a more
accurate level available?
I have tried colored water as a reflector
but only the sun or very bright moon is reflected enough to use.
Does another liquid such as oil reflect star light?
Will changing to a metal instrument
such as an Astra markedly improve results?
Do direct calculations using the law
of cosines give better results than Pub. 229 tables?
Again, thank you for the
opportunity to be a part of this forum and for your patience with my entry-level
questions. I find this to be a fascinating hobby I was introduced
to by my Dad some 30 years ago when he let me tag along at the USPS Seamanship
and Advanced Piloting classes he attended. He had done some celestial
navigation in B-17's in the pacific theater during WWII.
Randall F Morrow PT
Senior Physical Therapist
Ergonomics Safety Consultant
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Department of Physical Medicine
Kaiser Permanente - Kern County - Bakersfield
Phone: 661-852-3677 (Tieline - 378)
Fax: 661-852-3516 (Tieline - 378)
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Well, I built a set-up much like this, as followers of
a previous thread on artificial horizons will recall. Results were
not pretty. With the help of others on this list, who supplied me
with precision leveling legs and a very very accurate level, who
put me on to a good source of front surface optical quality mirrors and
who directed me away from plastic as a base, I got somewhat better results.
Still not great. It has been weeks since I have seen a decent night
sky. If they appear again, I will take some systematic sights using
this a.h. and share the statistics. Do the same for sights using
the river and correcting for dip short and compare.
|Patrick Goold <firstname.lastname@example.org>
Sent by: email@example.com
09/29/2011 05:59 AM
Please respond to
|[NavList] Re: Home made artificial horizon|
On Thu, Sep 29, 2011 at 6:58 AM, Randall Morrow <firstname.lastname@example.org>
If you are fortunate enough to live near the ocean you
will have a natural horizon to use to take your sextant sights. If you
are land-locked as I am an artificial horizon is a necessity. Davis Instruments
sells a liquid-based artificial horizon with its own sun shades. But any
shallow tray or dish with water or oil can also be used as an artificial
The problem with both of these lies in the nature of liquids. First they
take time to settle before the reflected image can be used. Second, even
the slightest breath of wind will disturb the surface making sights impossible.
The Davis unit does have walls and a cover making the wind less of an issue,
however, the angled walls of this device could restrict you to a narrow
angle of view of bodies you can observe without moving the unit. Neither
of these will allow you to make sights of faint light sources like stars.
Any liquid, whether water or oil, will absorb much more light than it reflects.
The sun, moon and very bright planets might appear as a reflection but
most stars will not. Early explorers such as Lewis and Clark would use
a tin of mercury as a reflector but as mercury it is highly toxic and difficult
to manage it is neither practical nor recommended.
The solution is to use a leveled mirror as a reflecting surface. A mirror
horizon can only work properly if the reflecting surface is absolutely
level. Since a liquid seeks its own level no device is needed for perfect
level to be achieved. For a mirror though, a leveling system is needed.
I have found that a satisfactory level can be achieved with a 3 screw leveling
system in a 6 to 10 inch diameter plastic or metal plate. The accuracy
of leveling can be checked by eye with a quality torpedo level and careful
inspection. Select a level with the lines being close to the edges of the
bubble. Wider gaps in the lines on a spirit level make judging accurate
alignment more difficult.
A better solution is to use a digital level. I found a Craftsman digital
torpedo level that sells for under $40.00 and measures to within 1 tenth
of a degree. You don’t need the flashlight it takes to see a bubble level
since the digital display is lighted. To set up and level the mirror before
a run of sights takes about 5 minutes. The display numbers bounce around
for a few seconds but with a gentle touch, a few turns of the leveling
screws can bring you to 0.00 quickly.
The plate and leveling system can be made at home in 30 minutes and for
about $20.00. A ¼ 20 drill bit and tap set can be purchased for about $7.
A plastic cutting board ¼ inch and about 9 x 12 inches is less than $3.00.
The screws and nuts will be under $5.00. A 6 in square or round hand mirror
is around $5.00. The tools are a saw, hand drill and a cheap T-handle for
the tap. A protractor is used to mark the angles between the holes 120
degrees apart but if it’s not exact it won’t matter.
First mark off the cutting board to change it from a rectangle to a square.
Saw the excess off with a jigsaw or hand saw. The plastic is very easy
to cut. Next use the protractor to mark the holes locations at 120 degree
angles. Drill the three holes and clean off any plastic scrap with a pocket
knife or sandpaper. Use the tap in the T-handle to cut the threads into
the plastic. The mirror is attached to the base with squares of foam double
stick tape at each corner. Thread the 1&1/2 in wing bolts though the
holes and screw on the cap nuts and tighten them. The rounded heads of
the cap nuts give a centered footing so the board does not move as the
bolts are turned while leveling.
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Dr. Patrick Goold
Department of Philosophy
Virginia Wesleyan College
Norfolk, VA 23502
757 455 3357