An Improvised Sun Compass

23 12 2014

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

In October, I and Dan La Porte described a C Plath sun compass in which a 24 hour clock is used to keep the alidade aligned with the sun. Recently, a friend in Australia sent me an old Astro-compass Mark II which someone had attempted to convert to a dumpy level, with only limited success. However, it gave me the opportunity to attempt to make a sun compass of my own along the lines of the Plath instrument, itself a modification of the Bumstead sun compass.

An essential requirement was a 24 hour clock. I had a Hamilton Master Navigational watch which has a 24 hour face, but I was not about to use that valuable instrument. What I did have was a quartz clock with a 24 hour face and, as it cost only a handful of dollars, I was happy to use that. However, I live in the southern hemisphere and as there was no room to mount it upside-down on the Mark II, it would have to run anticlockwise on the top. For those who might wish to copy me, let me say at the outset that reversing the battery will not cause the clock to go backwards. No doubt there is a diode somewhere in the circuit that prevents damaging reverse current from flowing. This led me to explore the mechanical insides of the clock. There is quite a lot of variation between makes, so I will not attempt to illustrate it, but all have an electronic circuit that delivers pulses every second to a tiny stepping motor whose rotor rotates through 180 degrees with each pulse. As it does so, it transmits movement to a gear chain that drives the hands in the correct relationship. There is a coil of fine wire wound around the armature of the stepping motor and I thought that I could simply reverse the polarity of the coil attachments to make the rotor go backwards. To save others quite a lot of difficult de-soldering and re-soldering trouble, let me say now that it does not work. The armature has two loose pieces that embrace the rotor and if the left is exchanged for the right (and vice-versa), this will  make the rotor and the clock go backwards. If you Google something like “How to make a clock run backwards” you will find several videos that show exactly how this is done.

So I had a 24 hour clock movement that ran backwards and now needed a face numbered in reverse. This is relatively easy to make using a drafting program such as TurboCAD and Figure 1 shows my result. Northern hemisphere readers who wish to make the compass will not of course have to go to the trouble of making the clock go backwards or of making the anti-clockwise dial.

Figure 1: 24 hour reversed dial.

Figure 1: 24 hour reversed dial.

It was then necessary to glue it to a suitable piece of sheet steel or brass, taking care to ensure that the central holes coincided exactly. I am happy to send a pdf file of the dial to anyone who might want to follow in my footsteps and who has no drafting program to draw such a dial. The dial is then fixed to the clock using two hexagonal nuts on the central pillar.

To allow easy removal of the movement to change the battery, I made a rectangular clip out of thin and springy sheet steel (Figure 2). Fixing the movement in its clip to the top of the compass will vary according to the maker. Sperti’s version has a smooth top and after removal of the alidade and its bracket, the clip could be glued with contact cement to a spacer glued in its turn to the top of the instrument, taking care to get things well centred with 00 hrs/12 hrs  correctly aligned fore and aft. The spacer is necessary so that the bottom of the clip clears the trunnions. My version was made by Henry Hughes and Son and had the round heads of three 6 B.A. screws projecting from the top, so I exchanged them for longer, countersunk head screws and used them to attach the clip via three spacers. Other improvisations may occur to readers.

The knob on the left in Figure 1 is used to adjust the longitude , using the “True bearing” scale and a little mental arithmetic. For example, I live at 173 degrees East longitude so the True Bearing scale will have to be set 7 degrees anti-clockwise. At 00 hours GMT, the sun will still have 7 degrees to go before it bears true north at local noon.

Figure 2: General arrangement from above left.

Figure 2: General arrangement from above left.

Attention now has to turn to the alidade. This could be simply a vertical bar arising from the end of an hour hand, or even simply a long hour hand bent up at a right angle so that its shadow falls on the face, passing through its centre. I chose to imitate a little the Plath arrangement as I had some Perspex (Lucite) available to cut drill and glue into shape, as shown in Figure 3. I trimmed a minute hand to length and bent it to clear the alidade.

Figure 3: Face of clock.

Figure 3: Face of clock.

Figure 4 shows the latitude setting knob and scales, set at my latitude of 35 degrees south. While the declination of the sun as I write is just under 23 1/2 degrees, there is enough length in the shadow bar to make it unnecessary to allow for this, though the original Mark II alidade had a separate declination scale to use with its sighting arrangements.

Figure 4: View of instrument from right and above.

Figure 4: View of instrument from right and above.

In use, the instrument is levelled , the clock is set to read GMT (or UTC which amounts practically to the same thing) and placed in its clip, 00 hours upwards, the latitude set and the longitude (after a little careful thought) allowed for on the True Heading scale. If the north on the compass scale is aligned with the lubber line and directed at true north, you should find that the shadow of the shadow bar passes right down the middle of the clock face and between the two lines scribed on the face of the screen. While the shadow remains aligned, the North point on the compass card will remain pointed at true north and any other desired course can be read off against the lubber’s line. The equation of time reaches a maximum of 16 1/2 minutes on November 5. This amounts to just over a degree in direction, so if you do not need direction to this precision, it can be ignored. Otherwise it has to be applied and the clock offset from GMT as outlined in the preceding post.

As an aside, I found that bringing out the instrument on its tripod was an excellent way of causing the sun to disappear behind clouds. It needed only for me to take it back inside to make the sun re-appear…


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: , 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


New Zealand





Fleuriais’ Marine Distance Meter

4 11 2012

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

In 1890, Admiral Georges Ernest Fleuirais (1840 to 1895) published a description of his “Micrometre à réflexion”. At the time, he was Director of the Cartographic Department of the French Navy and he had had a distinguished scientific career. He had led an expedition to Santa Cruz in Patagonia, as part of an international effort to observe the transit of Venus on December 6th, 1882. The international cooperation led to the solar parallax being established as 8.847 ± 0.012 seconds, allowing the distance of the Sun to be calculated as 92,384,000 miles (148,677,000 km). He also invented a sextant provided with a gyroscopic artificial horizon.

His micrometre was in fact a marine distance meter, in which the angle subtended by an object of known height is used to calculate its distance. For example, if the mast head of a distant ship is known to be 30 metres above the water line and the angle between the masthead and the waterline is 2 degrees, the ship lies  approximately 30/ tan 2 º = 860 metres away. I recently acquired a French Navy Fleuriais Micrometer, as it is a doubly reflecting instrument of the sextant type, and was able to examine it in detail.

Figure 1: View of front (left-hand-side).

It is immediately obvious that it is a sextant-type instrument. It has a radius of about 85 mm, a plate brass frame about 4 mm thick and two mirrors which, for want of better terms have to be called the index and horizon mirror, even though the horizon is not usually viewed through either. Instead of the usual arc, the instrument has a micrometer which  looks much more like an early engineer’s micrometer than the typical sextant micrometer which, in any case, had yet to be invented by C Plath in about 1907. The periphery of the drum is divided into 100 minutes and 12 turns of the screw, as denoted on the index, cover an observed angle of 1200 minutes, or 20 degrees (Peter Ifland in his Taking the Stars incorrectly writes that the drum is divided to read 1/100th of a degree). As the drum is rotated, the micrometer screw advances through an adjustable nut and presses on the capstan head of a screw attached to the end of the index arm, thus rotating the index mirror. A spring takes up any backlash between the head of the capstan screw , the micrometer screw and the nut . The nut can also be closed up to adjust the clearance between it and the screw (Figure 2). Legs on the face of the instrument allow it to be put down without changing hands. The Galilean telescope is x 3 power with an objective lens of 24 mm diameter.

Figure 2: Details of adjustments

The capstan headed screw is used to adjust out index error, while another screw acts on the base of the horizon mirror to adjust out side error if required, though in such instruments a little side error is helpful while having a negligible effect on the accuracy of the observations. The base of the mirror bracket is slotted and a captive screw is used to close or open the slot in order to tilt the mirror (Figure 3). No provision is made to adjust the index mirror for perpendicularity.

Figure 3: Side error adjustment.

Figure 4 shows the almost featureless rear or right hand side of the instrument. The traditionally shaped handle can be held comfortably either way up, so that observer who wishes to hold the instrument in his left hand may do so at the cost of some slight discomfort while operating the micrometer.

Figure 4: Rear (right hand side) of instrument.

Early instruments were provided with a drum fitting around the telescope to convert the angle reading to a distance, but later ones came with a circular slide rule devised by  Commander Émile Guyou,( 1843 – 1915) shown in Figure 5. The index arrow was set against the height of the object on the outer circle and its distance in metres read off the inner scale opposite the angle in minutes on the outer scale. In the figure, the index is set at about 91.8 and if the micrometer had read 60 minutes (1 degree), then the distance could be read off as about 5,260 metres.mile

Figure 5: Guyou’s circular slide rule.

While I continue to acquire, restore and describe sextants, I also have a small collection of chronometers, and have recently completed a book The Mariner’s Chronometer which will be available via from 10 November 2012.

A Stuart Distance Meter

13 07 2012

 This post was preceded by  “A Russian Naval Dip Meter”; and  “An Improvised Dip Meter”

When sailing in company with other ships, as for example, in a convoy, or when maintaining a safe distance when rounding a danger, it was useful to know the distance of one’s ship from the other objects. Until the advent of radar, a variety of distance meters was used. The most obvious is perhaps the sextant, likely to be found aboard every ship of any size. If some dimension of the object is known, like height of the mast above the waterline, and the vertical angle subtended by the dimension is measured, its distance can be calculated, but every self-respecting set of nautical tables had a table of “Distance by vertical sextant angle” to obviate calculation. A variety of distance meters was invented in the late nineteenth century to eliminate even the incovenience of looking up tables by giving a direct reading of the distance once the mast height had been set.

Most, like that of Fiske, in effect used a modified sextant that read through a relatively small angle while having a scale that gave the distance directly in yards. Recently I came into the possession of a Stuart distance meter that uses a different measurement principle, somewhat similar to that of the N5 dip meter described in the preceding post. The instrument was very dirty and the ivorine scales had shrunk and torn away from their screws, but happily no parts were missing and I anyway paid very little for it. Figure 1 shows the meter after cleaning and restoration.

Figure 1 : General view of distance meter.

The height of the object, say, a ship, up to 200 feet, is first set against the left edge of the transverse height scale. This need not necessarily be mast head to waterline. The note pad on the other side of the meter has provision for noting also the distance from the mast head to the “lower top” and “Upper speed(?) to stern lt.” The ship is then viewed through the telescope, when a field split vertically is seen. The image of the head of the mast in one half is brought alongside the image of the waterline in the other half by rotating the knob, when the distance in cables (a cable is one tenth of a nautical mile) can be read against the index on the distance scale. In Figure 1, the height is set to 60 feet and the distance is one cable.

Figure 2 shows the somewhat shrunken note pad on the front of the instrument and Figure 3, showing it with the telescope removed, begins to reveal some of its workings. In front of the left half of the telescope objective is a fixed slice of a negative lens of about -2.3 dioptres (about -440 mm) and a similar but longer slice is in front of the right half of the field. This latter lens is attached to a slide that carries the scale and as the slide moves through a usable distance of about 60 mm, the images separate as shown in Figure 3. Note that in Figure 2 “Patt 498” is probably a naval designation and cetainly not a reference to a patent. The telescope is about x 3 power and has an interrupted thread that allows it to be fitted in its bracket with just one sixth of a turn

Figure 2 : Front of distance meter

Figure 3 : To show split image.

Figure 4 shows the effect of time and sunshine on ivorine. I replaced the scale with a sheet of brass 1.6 mm thick and glued to it a paper scale copied from the original scale. It does not allow for shrinkage and the meter is probably no longer accurate, but it does allow the principle of the meter to be illustrated.

Figure 4: New scale for old.

Figure 5 shows the relative complexity of a Stuart distance meter’s competitor in the form of a Fiske-type distance meter or “stadimeter”, invented at  the same time in about 1895. While the Stuart instrument has a single slide machined in an aluminium casting requiring no great precision of manufacture, in the Fiske instrument the distance screw and scale are carried in a close fitting bronze carriage running in a precisely machined bronze frame. There are two mirrors, each needing means of adjustment, two lead screws and a bearing for the height scale, which corresponds to the index arm or alidade of a nautical sextant. It may well be that the Fiske is capable of greater accuracy of measurement, but no great accuracy is required in station keeping in a convoy, while one would err on the side of caution in rounding a danger. It may be that the instruments were originally envisaged as a range-finder for gunnnery or as a rangefinder during a “creeping attack” by two ships hunting U-boats. In this, one attacking ship remained at 1000 yards  astern of the submarine, where the latter was in its asdic cone and guided another ship moving from astern at slow speed so that its approach was masked by the submarine’s own propellor noise, until the distance of the sub by asdic and the distance of the ship by rangefinder coincide.

Figure 5: Fiske-type stadimeter or distance meter (Mark II, Mod 0).

A Russian Naval Dip Meter

24 06 2012

This post was preceded by  “An Improvised Dip Meter”

In the preceding post, I wrote a little about the Blish prism and the Gavrisheff dip meter and pointed out that if the angle between two horizons opposite to each other could be measured, the local dip could be deduced. Through the kindness of Alex Eremenko, I have recently been able to examine in detail an N 5 Russian dip meter which has several interesting design features. It is probably easier to appreciate these if an idea is had of the optical path of the instrument (Figure 1). I explained a little about dip and its importance in celestial navigation in the preceding post.

Figure 1 : Light path of Russian N5 dip meter.

The horizon is viewed simultaneously to the left and right of the observer. Taking the rays from the left side first, shown in green,  they pass through an adjustable slit, used to make the brightness of the two horizons equal, through a watertight window in the side of the instrument and thence to a roof prism, where they are deflected downwards at 90 degrees. They then pass through a semi-reflecting junction, and are again deflected by a second prism through 90 degrees in a plane at right angles to the first, into the objective of a x 4 Keplerian (inverting) telescope.

The rays from the right hand horizon also pass through a window and then through a weak positive (convex) achromatic lens of 1.5 dioptres power. They then enter a negative (concave) lens whose power is such that the two lenses together have no net power, so that they behave like a piece of plane glass. However, the positive lens can be moved up or down from a central position, so that the light path also moves up or down – but not by very much. In fact, the total travel of the lens is 12 mm and the total deflection of the rays is 15 minutes of an arc each way. The right rays continue on through the right angled prism and are reflected off the common, semi-reflective face, off the opposite face and thence into the telescope objective, where they are combined to be viewed through the eyepiece. A yellow filter can be attached to the latter to reduce glare. Figure 2 shows the practical realisation of Figure 1.

Figure 2 : Practical light path

Projecting from the bottom of the slide that carries the sliding lens is a boss which carries an adjustable cam follower. This cam follower is held by two anti-backlash springs against a large cam (Figure 3). The cam is rotated by means of the adjusting ring, which carries a scale graduated plus or minus 15 minutes from zero, each minute being subdivided to 0.2 minutes (Figure 4).

Figure 3 : Adjusting cam.

Figure 4 : 3/4 Plan view.

Figure 5 shows the 4-power telescope. The images of the two horizons are seen to be vertical and, by rotating the adjusting ring they are brought into coincidence, when the dip can be read off the scale. The zero is set using two autocollimators, aligned as described in the preceding post An Improvised Dip Meter.

Figure 5 : The telescope.

The instrument is well-sealed against the ingress of moisture by greased felt rings where there are rotating parts and by heavy wax at metal-to-metal and glass-to-metal joints. It is provided with a stout leather carrying case. Figure 6, modified from a drawing in the original Russian handbook, shows the general arrangement of the parts. It can be seen at a larger scale by clicking on the picture. Use the back arrow to return to the text.

Figure 6 : General arrangement drawing.

To use, the window with the slit is directed to the brighter horizon and the slit adjusted to make its brightness equal with the opposite horizon. The adjusting ring is rotated to bring the horizons (which will be seen to be vertical) into coincidence and the reading noted.. The observer then rotates through 180 degrees (about a vertical axis, of course) and also rotates the instrument through 180 degrees on a horizontal axis. A second reading is taken and the dip taken as the mean of the two.

I have provided this post to support my book The Nautical Sextant, which covers solely the nautical sextant. If you have enjoyed reading this and others of my posts, I am sure you will enjoy reading the book, available from the publishers, from Amazon and from good booksellers.

An Improvised Dip Meter

5 04 2012

On 19 March this year (2012) on NavList, Alex Eremenko reported some strange results for observations made by him and a friend from the shores of Lake Michigan. Much discussion followed about abnormal refraction conditions that can cause large errors in the dip of the horizon and the possibility that clocks corrected by radio signals could occasionally be in error by a whole minute. As correcting the observations for an error of a whole minute in time then gave results that were uniformly as good as these experienced observers normally obtained, it seemed to Alex (and to me) that the clock hypothesis was the correct one. However, discussion of the matter then moved on (28 March) to how to determine whether there is abnormal dip of the horizon, a condition likely to occur when there is warm air over cool water, which is particularly common and severe in arctic regions. Uncertainty about dip swamps most other potential errors in measuring the altitudes of heavenly bodies at sea.

Figure 1: Dip with and without allowance for refraction

For those not familiar with the concept of dip, Figure 1 shows that the height, h, of the eye of an observer O affects the apparent horizontal. This is shown in dotted red. But, especially close to the horizontal, light does not travel in straight lines, because the density of the atmosphere decreases with height. Generally the light path, shown in full green, is bent or refracted so that it is convex upwards. This has the effect of making the distance to the horizon greater and the angle between the apparent and true horizontals, the dip, smaller. However, when atmospheric conditions are abnormal it can be much greater or even reversed. For a much fuller treatement of dip and its abnormalities, see

 If the angle between the horizon in front of the observer and the horizon behind him can be measured, then dip can be deduced directly, as it is half the value of that angle.This is not a new problem. In 1900, John Blish of the United States Navy applied for a patent for an attachment to add to a normal sextant. The patent can be viewed on Google Patents by searching for Patent number 714,276. Figure 2 shows one of the patent drawings.

Figure 2 : Blish prism attached to nautical sextant.

Essentially, the device is a prism that diverts light rays through 180 degrees so that the horizon directly behind the observer can be viewed at the same time as the horizon in front and the amount of dip read out directly from the sextant’s scale.

Several other inventors devised instruments or attachments to do the same thing. Among the more complex dedicated instruments was one patented by Boris Gavrisheff in 1961 (US Patent number 2,981,143). A telescope views via two prisms light coming from one horizon behind the observer at the same time as the light from the horizon in front of the observer. One of the prisms is rotatable so that the deviation from 180 degrees, i.e. the dip, can be directly read off a micrometer drum. A third prism, labelled 12 in the diagram, diverts the rays into a telescope (Figure 3).

Figure 3 : Gabrisheff’s dip meter.

It occurred to me that expensive prisms are not needed. They are not used in nautical sextants though for some reason most American bubble sextants used them. I had a box of wreckage from three Hughes and Son Mark III survey sextants that a kind friend gave me, so I set about building a dip meter as an exercise to illustrate the principle rather than as a serious sea-going device, though it could certainly be made more robust for sea-going use. An index arm extension carries a mirror that receives light from the rear horizon over the top of the observer’s head and diverts it through 45 degrees into another mirror that diverts it another 45 degrees into the telescope. The telescope receives light from the front horizon over the top of the second mirror so that, provided the mirrors are correctly aligned, both horizons can be viewed at once and the deviation from a straight line be measured using the micrometer drum. Figure 4 shows the layout of parts and Figure 5 the path of the light rays from front and back.

Figure 4 : Parts of the dip meter

Figure 5 : Ray path of dip meter.

Figure 6 shows how the dip meter is adjusted. Two autocollimators are set up facing each other and their axes adjusted so that they coincide in the vertical and horizontal planes and are parallel to the surface of the surface table. One is then “shuffled” sideways using a mirror, so that its axis is displaced from but remains parallel to the axis of the other. The plane of the sextant’s arc is set parallel to the table using a dial indicator and shims under the feet of the sextant. The sextant is set to zero and the two mirrors adjusted until the images of the crosswires of the autocollimators coincide when viewed through the telescope. The dip meter is then ready to measure the angle of dip directly.

Figure 5 : Light paths from the autocollimators

Figure 6 shows how the mirrors are adjusted, the same way that an horizon mirror in a standard sextant is adjusted, with screws bearing on the back of the mirror opposite spring clips bearing on the front.

Figure 6 : Mirror adjusting screws.

If you enjoyed reading this post, you will enjoy reading my book The Nautical Sextant, available from the joint publishers, Paradise Cay and Celestaire and via Amazon. Readers in Australia and New Zealand may Contact me, as I am able to offer them a discount on the published price.