|
Author
|
Topic: Artificial Horizon Sights
|
David Burch
|
posted October 28, 2008 11:55 AM
We have had the pleasure of working with a group that is reenacting the Morten Stanley search for Dr. Livingston (I presume). His navigation from Zanzibar to the west some 600 nmi was by cel nav and artificial horizon (mercury in his case), which has led us to readdress this type of sight.
The technique and procedures are covered in Section 11.15 of our course book. In re-reading that section we note that there is not a clear enough statement that you measure the angle from the sextant, then do the IC correction, then divide the results by 2. Then we propose overlapping the suns and sight reduce as if it were a star sight.
For those that are interested, here is a recent example we did with a plate of molasses.
Sun center-center on molasses at Starpath front door 10/27/08
DR = actual = 47 40.5 N, 122 23.9 W
IC = 1.0' On the scale
HE (artificial horizon) = 0
PDT Hs (Hs-1.0')/2 a @ Zn by StarPilot
14 59 22 46 07.5 23 03.25 0.1' A 213.6
15 03 56 45 16.2 22 37.6 0.1' T 214.7
15 05 48 44 54.1 22 26.55 0.1' A 215.1
15 09 16 44 14.8 22 06.9 0.6' T 216.0
The last sight is obviously not good, and the others are very good.
Notes:
Molasses works very well. it can tolerate a slight breeze very well, way beyond what water can do. the reflection is very good. It takes a few minutes to come to a flat surface after moving it. If it sits out for a few days a few bugs get in it. You can revitalize it with 10 seconds in a micro wave. In short, molasses is a great artificial horizon.
I made these sights grabbing an old sextant in our office in a hurry as there is not much sun this time of year! and i made the mistake of not checking IC or side error before the sights. after doing the sights i turned the sextant toward the sun for a solar method index correction (Section 11.6) and first noticed that the side error was very large... in fact too large to make the check. so i took the sextant to the beach and did it with the horizon. Note that you would not want to remove the side error at this stage as it will alter the IC and the data would all be lost.
So the message is we should practice what we preach. Always check the index correction before doing sights.
A reminder that artificial horizon sights not only cannot be done when the sun is too high but also when the sun is too low. since the angle doubles in the sextant you are limited to about 100/2 = 50° Hs, but you are also limited by the narrow angles. it is hard to see the reflected image when the angle is much below 40°, is Hs = 20°.
Though we have panned this method of practicing cel nav (pardon the pun) in the past, after doing it again here after many years, we might weaken that and suggest it might be instructive to try a few sights.
From: Starpath, Seattle, WA
|
|
Capt Steve Miller
|
posted October 28, 2008 12:34 PM
I do a lot of artificial horizon sights using the Davis artificial horizon. In my classroom course here at Chapman that is the main type of sights that we do in the class. The students are able to get the Lat & Lon of the school as an exercise for the course. The Suns elevation is one reason that I do the classes in Feb and Oct. I have also done sights in the mountains of NH, in SC, and in MA. I have not tried molasses but I have tried various oils and glycerin all with good success.
From: Starpath
|
|
David Burch
|
posted October 28, 2008 12:37 PM
A big advantage of molasses is if you get some on your fingers you can lick it off!
From: Starpath, Seattle, WA
|
|
David Burch
|
posted November 05, 2008 05:14 PM
After doing a few more of these, we start to recall more tricks.
-- Important! Sometimes it is difficult to get the two suns lined up in the plate, especially when using a higher power scope and when the the sun is relatively low. Thus remember the trick to just take the telescope off and get all set up with the two suns beside each other or on top of each other, then put the scope back on and finish normally.
-- A plate of molasses that sits out for several days or a week or so, begins to thicken some and a film develops on the surface. To resolve this, you can put the plate in the microwave for 20 seconds or so. Also as it gets thicker, you can add a small amount of hot water to thin it out again... but if it gets too thin it acts just like dark water and you will see the ripples from the wind. Remember it takes just a very light breeze over the surface to shut down the process.
-- a last point is that on some sextant models you may find that the horizon shades may not have an adequate range of filtering for this type of sight. It does pay in most cases to have a distinctively different color so can remember which is the direct view (off the molasses) and which is the reflected view that moves when you turn the dials.
We have found for example that one model of sextant we have uses two crossed polarizers for the filers on both horizon and index arm. This is in principle a nice way to do the shades, but often these are not installed properly or not made quite right so they do not offer the full range of shading they should offer. The one we have will not get dark enough for the direct view on artificial horizon sights, for example.
From: Starpath, Seattle, WA
|
|
|
