The Nautical Sextant

This post is preceded by “Tamaya Collimation Blunder” and by “Incorrect assembly of SNO-T shades” While this post is about workmanship rather than a blunder, this seems to be the correct category for it.

.A week or two ago, I was contacted by a new friend who in his younger days had sailed in a yacht to many places, travelling over 20,000 miles. In his maturity and now owner of a substantial business shipping people about New Zealand’s Hauraki gulf, he recently bought a 30 foot yacht to resume sailing. On digging out his sextant from storage, he found it had signs of thirty years of neglect and asked if I could help to bring it back to a good condition. I readily agreed.

The sextant is a bronze-framed instrument by C Plath, a firm of high repute. It had an inspection certificate dated in 1965 and was contained in a heavy black Bakelite case which appears to be almost indestructible. Such cases were common in Plath sextants made after the Second World War, perhaps because there was a shortage of wood workers, and around 1965, wooden cases with box comb corner joints began to appear. The sextant had minor paint chips and widespread areas of minor corrosion, confined mainly to screw heads and the aluminium frames of the shades. Movement of the index arm and rotation of the micrometer were rather stiff, as might be expected of oil and grease which had had thirty years to acquire the consistency of soap. The telescope focussing in particular needed a lot of care to free it without destroying it. I noted a small area of the objective lens where the cement that joins the two elements of the lens has probably shrunk

Usually, when I restore an instrument of this vintage I begin by stripping it down to the last screw and washer, beginning by removing the index arm with its attached micrometer assembly. Removing the telescope bracket allows the arm to swing clear of the frame, prior to releasing the journal from its bearing.The bearing is the part that encloses the rotating shaft or journal. Loosely, the two parts are often referred to as the bearing. As soon as the index arm was free of the frame the lower end sprang forward, revealing a distinct bend in the arm at its junction with the upper, circular end. My first instinct was that it had been damaged and I straightened it, when it then appeared to lie behind its proper place. The lower end of the arm has two keepers that slide in a slot on the front of the limb, to keep the micrometer worm in its proper relationship with the rack, while the upper end of the arm is guided by its bearing(Figure 1).

Figure 1: Showing mis-alignment of index arm.

The index mirror sits on a circular table with the journal projecting from its underside. The journal passes through a close-fitting hole in the upper part of the index arm and the latter is secured to the underside of the table by three screws. When I turned my attention to this area, I found that there was a tapered gap between the index arm and the frame of the sextant, indicating that either the bearing was mis-aligned in the frame or that the journal was mis-aligned with the table(Figure 2).

Figure 2: Upper arm and index mirror table.

Figure 3 shows how the alignment of the bearing in the frame is checked in another sextant, using a mandrel that fits closely in the bearing, and a square. As shown the mandrel and therefore the bearing is accurately aligned with the frame., and this was also the case with the sextant being restored.

Figure 3: Upright mandrel.

This meant that the journal must have been mis-aligned with the table. Usually, the journal is soldered into the table, but in this instance I could see no joint line and even after heating, there was no sign of a soldered joint giving way, so I surmised that the whole had been machined from a casting. The top of the table would have been turned flat by facing in a lathe and then used as a locating face for turning the tapered journal. By holding the table in a three jaw chuck with the top face hard against the chuck jaws it was possible to see a pronounced wobble as the chuck was turned slowly. Bringing up the tail centre illustrated that there was pronounced run-out of the journal(Figure 4).

Figure 4: Run-out.

The next Figure shows that the total indicated run-out was 1.53 mm.

Figure 4: Total indicated run out.

I asked its owner whether the instrument had perhaps been dropped and he did remember a time when it may have received knocks when he was taking a round of star sights to fix his position, to ensure that he could safely round the North Cape of New Zealand in bad weather. However, It is highly unlikely that the mis-alignment could have been caused by other than shaky inspection or workmanship or both. While it is clear that the index mirror could have been set perpendicular to the plane of the rack at only one position, this would not have introduced a great error(proportional to the cosine of the mis-alignment angle?) at other positions and it may well have allowed the sextant to be certified as “…free from error for practical use.” The variation in effort needed to move the index arm from one end of the arc to the other might well have been unnoticed.

Nevertheless, I elected to make a new part by cutting out a disc of 3 mm brass plate, reaming a 10 mm hole in it and turning and fitting a new journal in it by using a modern industrial adhesive rather than by soldering. In the paragraph preceding Figure 3 in my post of 20 March, 2011 I explain how this is done to ensure proper alignment and the figure of the post illustrates the process. Once fitted, the keepers entered the slot in the limb with correct alignment. Having some time spare, I dug out my sextant calibrator, described in my post of 13 February, 2011 and used it to check the sextant’s errors at 15 degree intervals(Figure 5).

Figure 5: Checking sextant calibration.

The post about the calibrator may tell most people more than they want to know, but put simply, the calibrator rotates one way and the sextant is index mirror rotates the same nominal distance in the opposite direction. The auto-collimator measures any angular difference with a precision of better than an arc second. The calibrator’s errors are known and applied to the result. My friend will I hope be pleased that no error exceeded 12 arc seconds or 0.2 arc minutes and that C Plath’s original claim of being free from error for practical use still holds.

Those with sharp eyes will have noted that a mirror is held against the index mirror with a rubber band. The index and horizon mirrors which I replaced are flat within about one wavelength of green light, quite adequate for use in a sextant. The additional mirror is about five times flatter so that the images seen in the auto-collimator are much sharper and easier to measure.

If you have enjoyed reading this post you may well enjoy reading about the detailed structure of the nautical sextant in “The Nautical Sextant“, available from Paradise Cay Publications, Celestaire and many nautical booksellers. Sticklers for detail may also like my “The Mariner’s Chronometer” available from Amazon. Read about them at http://www.sextantbook.com and http://www.chronometerbook.com