The Nautical Sextant

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.

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.

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 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.

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…