The Nautical Sextant

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 amazon.com from 10 November 2012.

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 previous post in this category was “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.

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 http://mintaka.sdsu.edu/GF/explain/atmos_refr/dip.html.

 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.