The Nautical Sextant

In about 1963, Captain Svend Simonsen, a merchant seaman officer who had retired from the sea, is said to have tired of selling shoes instead, so he set up a navigation school, at first teaching students in their own homes and then by correspondence. By 1971, it was a thriving business, trading in Santa Barbara, California, as the Coast Navigation School. Captain Simonsen arranged with Tamaya of Tokyo to provide him with sextants named “Simex”, for him to on-sell to his students and others. There were, it seems, several models for sale, with various telescope, shade, mirror and frame options, reflecting the options that Tamaya offered under its own name, hence the plural in the title of this post. Recently there came into my hands a sextant named “Simex Mariner”. It had plainly been dropped, as a leg and the telescope stud had been broken off, but it had been a high end sextant, as it has a bronze frame, polarising shades, scale illumination and a micrometer vernier reading to 0.1 minute. Figures 1 and 2 show it after overhauling and  checking, and replacing broken parts.

Figure 1 : Front (LHS) of Simex Mariner sextant

Figure 2 : Rear (RHS) of Simex Mariner sextant.

The bronze frame is of a standard 162 mm radius with a black crackle finish. The index mirror measures 32 x 48 mm and the round horizon mirror is 50 mm diameter. A  contemporary German sextant has mirrors of 41 x 56 mm and 56 mm diameter respectively. The design of the latter’s micrometer mechanism is almost identical to that of the Simex, following the design principles devised by C Plath. While most post-1950 sextants either omit  a micrometer vernier altogether or divide it to 0.2 minutes, the Simex vernier is divided to 0.1 minutes (Figure 3). Given the uncertainties of the dip of the horizon and that the absolute limit of the sextant’s measurement accuracy may be no better than 12 seconds (0.2 minutes), the vernier is probably an unecessary extra. Possibly it was added to attract traditionalists.

The scale lighting system follows Tamaya’s earliest practice and does its job perfectly adequately. It is easy to change the bulb and batteries. The latter lie diagonally across the width of the shaped hardwood handle and the handle itself is canted at 20 degrees to the vertical to give a slightly more comfortable grip. The switch mechanism is simple and easy to access by removing two woodscrews, unlike in many later battery handles, particularly those by C Plath, where is is nearly impossible to remove the switch when it is in need of servicing.

Figure 3 : Detail of micrometer and illumination.

The shades use crossed polarising filters to give continuously variable darkening of the view. The idea was first mooted in a patent of about 1938 and some versions of the US Navy Mark II sextant were fitted with polarising shades, but the material seemed to fade after a few years. In my Simex, the index shade worked perfectly well, but one of the filtersof the horizon shade had lost all its polarising properties. Access is easy, by unscrewing the retaining ring shown below in Figure 4. The material is sandwiched between two sheets of plain glass, and by soaking the filter in acetone overnight it is usually possible to slide the two apart and clean off the remnants of the polaroid material. As mine had faded uniformly, I simply stuck a layer of self-adhesive polaroid film to the outside of the glass, a solution that has worked well.

My Simex also has an astigmatiser added to the index shade. This is simply a weak cylindrical lens whose axis is horizontal when swung into place. Its purpose is to draw out images of stars or the sun into horizontal lines and it probably finds its main use in combination with a bubble horizon though, at least with stars, it may be helpful to remove tilt of the sextant by lining up the extended image of the star with the horizon.

Figure 4 : Astigmatiser and polarising shade.

There is quite a variety of ways of attaching shades to their mounting brackets and the method may appear mysterious to someone seeking to overhaul or tighten a shade that has become annoyingly loose so that it does not stay where it is put. Figure 5 shows the horizon shade in its mounting and Figure 6 shows it exploded. At a casual glance, the two grub screws may be overlooked and the heads of the mounting pin or adjusting screw mutilated in futile attempts to turn them. Each grub screw must first be backed out, using a well-fitting 1.5 mm screwdriver to do so, as if the slot is destroyed, the overhauler will be faced with an even greater problem.

Figure 5 : Horizon shade and bracket.

Figure 6 : Horizon shade and bracket, exploded view.

Mirror mountings, index arm structure and index arm bearing are all conventional.