C Plath Sun Compass

16 10 2014
Figure 1: Dan LaPorte's sun compass.

Frontispiece: Dan LaPorte’s sun compass.

This post was preceded by  “A Fleuriais’ Marine Distance Meter” A Stuart Distance Meter”;“A Russian Naval Dip Meter”; and  “An Improvised Dip Meter”

Sometimes, kind people lend me their precious instruments for me to deconstruct and examine so I can post details on this site. I invited Dan LaPorte to contribute a “guest blog post” and he has kindly obliged. Dan’s contribution is in blue, and my comments and additions are in black.

Last year I obtained a WWII Plath sun compass via an e-bay purchase.    At the time I really didn’t know exactly what I had bid on and won, but I did know it was something out of the ordinary.

First, a little about me and why I would be remotely interested in such an item.  I am a retired  US Merchant Mariner having sailed for some thirty-five years at sea, twenty of those as a Ship’s Master.  During that time, I acquired many skills and interests, one of them being magnetic compass correction.  For years I’ve used an Abrams and an Astro sun compass for such duties.  Both work on the same basic principal of local time or hour angle to obtain a true bearing of the sun or other celestial body.  Hence I was immediately interested in the Plath sun compass.

Upon delivery of the item I was saddened to find that the clock work no longer functioned (the Plath uses a Junghans 30 clock work  with optical sight for taking a bearing of the sun).  In fact the Junghans 30 movement was also used in the ME 109 fighter of the same era.   The idea is to set the correct time (more on this later), so the sun compass will track the sun’s path and hence a constant bearing using the sun can be obtained.  When functioning and set correctly a true bearing can be recorded of a landmark to obtain a position, or drive (or fly) from a known position to another by following the desired course.  Another use of the Plath would be to check and correct an aircraft’s magnetic compass while on the ground.  

After a bit of research I found that this model was used almost exclusively by  German troops deployed to the  North Africa corps during WWII.   Of course all this would have been unknown to me if not for the assistance of Mr Malcolm Barnfield.   By contacting Malcolm via his web site: http://www.sundials.co.za , I was able to obtain a wealth of information on the Plath.   Malcolm is without a doubt one of the most knowledgeable people in the world on the topic of sundials, sun compasses and their use.   Without Malcolm’s expertise on the topic I would have certainly been lost for much longer, and perhaps forever.  Malcolm was also good enough to put me in contact with other very talented men, highly regarded on the same topic, such as Mr Konrad Knirim who provided a manual for the Plath, and Mr Kuno Gross, who translated it from German to English for me.  These highly accomplished men in their fields were good enough to assist me in my search for information regarding the Plath. 

While history of the Plath was very interesting, it did nothing to solve the one large remaining problem – it simply didn’t keep time and thus was nothing more than an interesting item to marvel at and only ponder at its use.  

Enter Bill Morris.   Bill and I had communicated for months on various topics related to celestial navigation both air and sea.  Bill is regarded by all that know him as one of the most knowledgeable people in the world where navigation instruments and their structure are concerned.   Bill has written books on the topic and provides detailed manuals for repair of several  sextant types both aeronautical and nautical.   His manuals are truly works of art and allow the layman to repair and bring sextants back to working order.  Bill had in the past repaired an old A10A aircraft sextant for me that works perfectly to this day.  Given his talent for repairs, knowledge of machine tools and ability to work on intricate and complex antiques with a sure touch, I asked if he would be good enough to have a look at the workings of the Plath.   I should state at this point that I was, and remain very protective of the Plath and would not allow just anyone to begin repairs on it.  Bill was my first and only choice that I would trust enough to allow any attempt at repairs.  As luck would have it Bill was to travel to Katy in Texas, not all that far from my home.  Add to this I would be able to finally meet Bill face to face.  In short it was a perfect and fortunate turn of events. 

I was able to meet Bill and his lovely wife for a visit in August of this year.  We enjoyed a very nice chat and lunch, covering topics ranging from navigation to what should be seen while in Texas.   I left the Plath safely in Bill’s hands, with hopes he could repair it.  A mere week later I had the Plath in my hands and working perfectly.

At this point the Plath was repaired, and I had a basic knowledge of how to use it.   When the Plath arrived I at once set it to local standard  time adjusted with the EQT ( Equation of time) from the nautical almanac.  To my dismay it did not point to North or any other direction.  In fact is seemed to be some 15 degrees off to the East at best.  That is, I would have to be another time zone to the East for the Plath to be anywhere near correct in obtaining a true bearing. Adding to my frustration, I was not entirely clear on how to orientate it to obtain a true bearing (the manual giving scant information in the translation). I set both the Abrams and Astro Compass in a hope to clarify the situation, this only proved to entangle my thoughts even more, at least for the moment. 

A few words about the equation of time are perhaps appropriate. Our daily life is governed mainly by the sun and its passage across the sky is not perfectly regular. It slows at some times of the year and speeds up at other. This is partly because the Earth’s orbit is slightly elliptical, so that it speeds up when nearer the sun and partly because the Earth’s axis of rotation is inclined at about 23 1/2  degrees to the plane of its orbit, so that the component of the Sun’s apparent velocity parallel to the equator  varies with the seasons. It is very difficult to make clocks to follow these variations, so the concept of mean solar time was invented, the average time for the Sun’s apparent rotation around the Earth. The difference between the apparent time on a given day and the mean solar time is known as the equation of time, often shown as a graph as in Figure 2, and in the bottom right hand corner of the daily pages of the Nautical Almanac. In the sun compass, it has to be applied as a correction on a given date to the mean time so that the alidade will point correctly to the sun.

Figure 2: The equation of time.

Figure 2: The equation of time.

 After further study I found the problem. To outline what the problems was I first have to explain the use of the two sun compass types I was more familiar with.   As stated previously, my tools used for obtaining a bearing of the sun or other celestial body was the Abrams or Astro Compass.   The Abrams uses local standard time, adjusted east or west of the standard time meridian, the observer’s approximate latitude and an adjustment for EQT provided on the face of the sun compass.  When all these details are known and set the instrument will provide the desired bearing by using the shadow of the sun.  The instrument has to be updated by moving the time marker one mark on the scale every four minutes.  This of course is due to the movement of the sun covering one degree of longitude every four minutes.  Simple when you know how.  The Astro Compass uses the settings of:  Local Hour Angle (LHA), declination of the body and latitude of the observer.  The declination of the body is obtained from the Nautical Almanac, LHA is calculated from your known longitude and applying it to the GHA of the sun or other celestial body.  Latitude of the observer would also need to be known and set on the instrument.  As with the Abrams the Astro needs to be constantly updated by moving the LHA scale in keeping with the sun’s motion across the sky.  

Why am I boring the reader with these details?  Simply to drive home the use of the Plath and the ingenious setting of the unit.  Unlike the Abrams and Astro the Plath is set to GMT standard time (not DST).  The user would also need to apply EQT to the time setting in order to obtain solar time with the EQT sign ( -/+) reversed due to the correction from a local time to GMT.  Once set to GMT – Solar time (GMT with EQT applied) the user then simply sets his latitude and longitude on the Plath.  No further corrections  and no almanac entries are needed.  As long as the Plath keeps correct time, and the user updates the estimated position of latitude and longitude, the unit will continue to function.  My mistake was in setting the Plath to local time.  I had wrongly assumed local time would be used as with my other instruments.  The Plath’s use of GMT is a perfect solution when one has time to reflect on the subject.  

Needless to say when the details of correctly setting the Plath were known and understood another test was in order.   So, one afternoon with the sun high and bright in the sky I set the Plath.  It should be noted that as with all other sun compasses it needs to be mounted securely to a stable platform and levelled with the provided spirit level. I also set the Abrams and Astro compass at the same time, a kind of a sun compass smorgasbord if you will.    To my amazement the Plath indicated true north as checked by my Abrams and Astro Compass (any course could have been selected for the test).  Given that the ultimate test of the Plath was to maintain a true bearing for hours or even throughout the night a test for the rest of the evening continued. I allowed the Plath to run for a few hours checking it now and then.   As per the design, the Plath displayed a constant true bearing until sunset due to the clock works keeping time and following the transit of the sun across the sky.  The Abrams and Astro compass would have had to be manually corrected continually for the entire event.  The value of the Plath became even more clear when you imagine using it in a desert with no natural land marks.  Given the successful test I personally would not have a problem using it for land navigation across a desert to this day if I knew what course I needed to travel from my location to destination.  No need for GPS signals or the like.  Just simple old style navigation would serve the user very well indeed. In fact I’d prefer to use the Plath instead of the Abrams or Astro compass due to the Plath’s ability to constantly maintain the required bearing, thanks of course to the Junghans clock works. 

The final test was the following morning.  As the sun rose in the East the Plath tracked perfectly still displaying the true bearing of North as she was set the evening before.  Perfect!   After seventy plus years the Plath with the assistance of Bill Morris worked as she worked many years ago in the North African desert.

 I wish to thank Dr Bill Morris,  Mr Malcolm Barnfield, Mr  Kuno Gross, and  Mr Konrad Knirim with having similar interest, assisting me, answering my questions, being patient,  and at times commiserating with me on this project.   These men:  doctor, military historian, engineers, authors, experts in their fields, took the time to assist a retired sea Captain with his quest to restore an antique sun compass to operational status.  Simply put, without them and their assistance the Plath would have remained locked away in my study with other odd instruments, not used, not understood and in a non-functional condition.  It would have been an unfortunate end for such a fine instrument.  As it turns out she may well run another seventy plus years.

 Captain Dan LaPorte (ret)

Figure 3: General arrangement.

Figure 3: General arrangement.

Now for a few anatomical details. The compass is mounted on the vehicle or aircraft via a universal joint, which can be quickly locked or unlocked in order to level the base plate (which I have labelled “compass card” in Figure 3 above) using a circular level in the centre of the plate. The base plate index corresponds to the lubber’s line in a magnetic compass and the line can be correctly aligned with the fore and aft axis of the vehicle using a sort of iron sight. In Figure 3 this can be seen as a thin vertical rod in a gap in the trunnion above the W of the base plate. On the other side is a point, just visible in Figure 1. The two trunnions support the horizontal axis of the compass and this axis is provided with a latitude scale on one end and a knurled locking knob on the other. A moment’s thought shows that when the scale is set to the local latitude, the equatorial axis, which I have drawn as a red line, will be parallel to the Earth’s axis. The horizontal axis bears a clock with a twenty-four hour dial and the clock can be rotated inside an equatorial mounting ring provided with a longitude scale and locked at the local longitude.

Figure 4: Equatorial mounting ring and longitude scale.

Figure 4: Equatorial mounting ring and longitude scale.

The watch is provided with a rather unusual hour hand in the form of an alidade (Figure 5a) and also has a conventional minute hand.

Figure 5a: Structure of the alidade.

Figure 5a: Structure of the alidade.

The alidade is made of Perspex (Lucite). One end has a vertical cylindrical lens that projects an elongated image of the sun on to a ground screen at the other end. The screen has two vertical setting lines and a lower, transverse extension to help in initial setting. When the time is set to the correct Universal or Greenwich Mean time, adjusted for the equation of time,  and the latitude and longitude scales set to their local values the base plate is rotated so as to bring the elongated image of the sun between the setting lines. If the vehicle is pointing north, the base plate index will then indicate north. If the vehicle turns and the base plate re-adjusted to bring the sun back between the setting lines, the direction of travel will be indicated by the base plate reading. The clock will keep the alidade tracking the sun correctly as long as the direction of travel does not change and if the direction of travel  does change  it is necessary only to bring the alidade back into alignment to get a correct indication of the new direction of travel. Amateur astronomers will recognise this as an adaptation of the familiar equatorial telescope mounting. The whole is enclosed by a protective Perspex cover. Figure 5b gives another view

Figure 5b

Figure 5b: Further view of alidade and scales.

I do not propose to give many details of the clock mechanism except to point out that, somewhat unusually, it is wound up by rotating the spring barrel rather than by rotating the spring arbor. This allows some simplification of the internal power train and also allows setting of the hands by means of a co-axial arbor (Figure 6).

Figure 6: Winding and setting knobs.

Figure 6: Winding and setting knobs.

Figure 7 shows how power is transmitted to the train from the spring via the central winding gear.

Figure 7: Winding gear detail.

Figure 7: Winding gear detail.

Dan and I would be glad to know of any errors or significant omissions and to hear from other owners about their experience with this ingenious instrument.

Bill Morris

Pukenui

New Zealand

 

 

 

 

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Ilon Industries Mark III Sextant

1 10 2014

John Pazereskism kindly sent me an e-mail, but my replies to him have bounced. If you’re reading this, John, please send me an e-mail from another address.

Of the many small-size sextants, possibly the rarest are those by Ilon Industries Inc of Port Washington, N.Y.  Victor Carbonara, a prolific inventor of navigational instruments prior to  and during the Second World War and one-time president of Kollsman Instruments, manufacturer of altimeters and bubble sextants, may have had something to do with its design. However, I have not been able to discover any details about Ilons. I was very pleased, then, when an Australian friend, one of the many friends I have yet to meet, entrusted me with his Ilon Mark III sextant and invited me to describe it, deconstructing it in the process if need be. The sextant is in a stout leather case with a metal zip fastener (Figure 1)

Figure 1: Case

Figure 1: Case

The interior of the case is lined with red velvet, with compartments for the sextant body, an adjusting key, the handle, a sighting tube and a tiny prismatic telescope, as shown in Figure 2. A clearer view of the individual parts is given in Figure 3, which shows the parts outside the case.

Figure 2; Kit of parts in the case.

Figure 2; Kit of parts in the case.

Figure 3: Parts out of case.

Figure 3: Parts out of case.

Figure 4 names some of the parts of the sextant for those who are not very familiar with sextant structure. The telescope is six power with an aperture of 15 mm.

Figure 4: Some of the main parts.

Figure 4: Some of the main parts.

While it is quite usual for sextants to be stored without their telescope in place, it is very unusual for the handle to be a separate part.  The sextant has a front plate which carries the telescope, the index arm with its attached micrometer mechanism, a rack with which the micrometer worm engages, the arc and the two mirrors. The front plate  is attached via two pillars and a plate through which the telescope passes, to a back plate to which are attached the shades and the handle. Figure 5 gives some clues as to how the handle is attached. The upper leg of the handle has a cross pin through it and this is inserted into a slotted hole and rotated through about 30 degrees, at the same time engaging a reduced diameter of the lower leg in a plain hole in the back plate. On tightening the knurled locking nut, the handle is located and  held with sufficient firmness to the back plate.

Figure 5: Sockets for handle.

Figure 5: Sockets for handle.

Figure 6 shows the handle locked into place and also shows the means of attaching the telescope or “zero magnification” sighting tube. The ‘scope or tube screws into a dovetail slide which engages with dovetails machined in the plate that holds the rear of the the two plates together. The ‘scope can be moved transversely to admit more or less light from the horizon, depending on conditions, and then locked into place by means of a locking nut bearing on the upper gib strip. This takes the place of the conventional “rising piece”. While the human eye can just about detect a doubling in light intensity, quite small changes in intensity can improve contrast between the sky and the horizon at twilight significantly.

Figure 6: Handle locked into place.

Figure 6: Handle locked into place.

Figure 7 shows how the index and horizon mirrors are tucked away safely between the plates. It also shows how up to three index shades and two horizon shades can be rotated on brackets attached to the inside of the back plate to reduce light intensity from the observed body and the horizon respectively.

Figure 7: Shades and mirrors.

Figure 7: Shades and mirrors.

The radius of the arc is a mere 60 mm (about 2.4 in.) It is traversed by an index arm outside the front plate and which rotates about a short plain bearing, the other side of which is a plate carrying the index mirror bracket. A magnifier built into the lower end of the index arm helps in reading the whole number of degrees (Figure 8).

Figure 9: Micrometer index.

Figure 8: Micrometer index.

A rack is machined into the rear of the front plate and is engaged with the worm of the micrometer mechanism (Figure 9). An 18 mm diameter micrometer drum allows single minutes to be read off with ease. The mechanism follows the practice of Heath an Co in swinging the worm out of  the plane of the rack by means of a spring loaded release catch, so that the index arm can then be moved rapidly.. The swing arm that carries the worm in its bearings rotates between cone-ended screws that can be locked in place when all backlash has been removed.

Figure 8: Micrometer mechanism.

Figure 9: Micrometer mechanism.

Axial play of the worm shaft is removed by an adjustable bush that is also locked into place when it is judged that there is no axial play of the worm in its bearings (Figure 10). A tongue (best seen in Figure 9) forming part of the rear swing arm trunnion bearing projects to form a keeper that prevents the index arm lifting off the front plate.

Figure 9: Detail of micrometer mechanism.

Figure 10: Detail of micrometer mechanism.

Figure 11 shows the light path, in red from a heavenly body at about 60 degrees altitude and in yellow from the horizon. Both mirrors have their reflective surface on the front and presumably this was chosen so that they could be cemented firmly into their brackets without the need for clips, which, at this scale would be extremely small and fiddly to fit. The horizon mirror has no clear-glass portion and the light from the horizon simply passes over the top of the mirror to combine with the rays from the observed body. In some respects, this is a disadvantage as the clear portion of the more usual horizon mirror reflects about 10 percent of the light coming to it from the heavenly body and increases the overlap in the view of the body and the horizon.

Figure 10: Light path.

Figure 11: Light path.

Figure 11 shows some of the structure of the horizon mirror bracket. Horizontal and vertical slots almost meet, leaving a slightly flexible diaphragm of metal between, so that the mirror can be adjusted in the vertical plane by two screws, the nearer one of which in the photo, as it were, pushes, while the other pulls. The two are adjusted against each other to correct side error. This is seen in a different view in Figure 13. The index mirror bracket is slotted in a similar way to allow adjustment for perpendicularity in the usual way.

Figure 12: Detail of horizon mirror bracket.

Figure 12: Detail of horizon mirror bracket.

To adjust for index error, the whole bracket rotates about a screw through the plate and two adjusting screws bear against each other on the opposite sides of a post. When adjustment is complete, the screw through the plate is locked.

Figure 13: Horizon mirror adjustment.

Figure 13: Horizon mirror adjustment.

It is not clear for whom this interesting little sextant was intended. Produced in small numbers, it must have rivaled a full-sized sextant for cost. There are plenty of box sextants around, but the Ilon is much easier to read than the crowded vernier scale of a box sextant and though the latter were useful to surveyors for reconnaissance surveys and to artillerymen for setting up their guns, improvements in survey instruments may well have made the sextant redundant for these purposes. The professional seaman is unlikely to have wished to be seen with anything other than a full-sized instrument, leaving only the well-heeled yachtsman or the collector as a possible purchaser. Perhaps this ingenious little instrument was too good for its own good, as they appear to be excessively rare, suggesting perhaps that it did not sell well.

I am grateful to Murray Peake for the loan of his instrument.

3 October 2014: In response to this post, Alan Heldman remarks that the design “…would easily lend itself to giving the user the option of putting the handle on the left side as well as the right side. With the handle on the left-hand side, the user could easily work the micrometer screw with his right hand.” There does not seem any reason why the handle could not be retrofitted to the left plate, though it would have to be almost horizontal and high up to clear the index arm.

Chris White writes “ Just saw the Ilon sextant article. Victor Carbonara was my grandfather and i worked at Ilon for a couple years in the 1970’s. I actually assembled and sold a number of these sextants from the parts inventory years after they had been discontinued.

It is a great sextant for marine use. Fussy to build due to the short arm requiring higher accuracy. “