The visible area around our ship extending to the horizon as a circular plane, we can establish the edge of this plane (the horizon) as the zero point from which to measure heights of objects in the sky. The dome of the sky is a hemisphere sitting on this plane. From one point on the horizon, up through the overhead point and down again to the horizon opposite the starting point would be an arc of 180°. The overhead point or zenith would be the highest spot from the horizon with height of 90°. Celestial bodies can have angular heights of 0° to 90°. Such heights are called altitudes and are measured with a sextant. Sextant altitude is designated as Hs.
To pinpoint a body on the celestial sphere, we need more than just altitude since several bodies in different directions can have the same height. A second coordinate is used called azimuth. It refers to direction and most often is measured from north eastward and is designated by Zn. Since the horizon is a full circle or 360°, Zn can range from 0° to 360°. "Z" which is called azimuth angle but you can use south or north as its origin and may be measured in eastward or westward direction, ranging from 0° to 180° (similar to the longitude system on the earth). This Z must always be converted to Zn for use in plotting. The altitude is measured with a sextant, azimuth is found in mathematical computations.
The sextant has a micrometer drum, used to measure the minutes and seconds (or tenths of minutes) of altitude. The shape of the frame is one sixth of a circle. Because of double reflection achieved with the two mirrors, the actual angles observed may reach 120° instead of only 60°.
In choosing a sextant, the biggest decision is quality versus purchase price. There are two basic "grades" to investigate - plastic and metal - the price ranges vary from $20 to $75 for plastic and $300 to $800 for metal. The plastic sextant's one advantage is price. This should be weighed against several disadvantages. Plastic will expand and contract with varying temperatures, the index correction (instrument error) is constantly changing. This can be partially compensated for by obtaining an index correction each time a set of sights is taken. The navigator may find that even between the first and last sight during a twilight series, the change can be alot. Remember, a minute of error in sextant altitude directly corresponds to one nautical mile on the plot.
Plastic sextants weigh less than a pound and some varieties offer considerable wind resistance, all making it more difficult to hold the sextant vertical when sighting in breezy conditions.
The quality of the components is less - the filters, the mirrors, the zero to three power viewing scopes. And the life of a plastic sextant is shorter, depending on the amount of use. Filters break off, the plastic gearing wears down, the micrometer drum develops slop. Any of the plastic sextants are great for learning.
The advantages of a metal sextant are obvious after reading the disadvantages involved in using a plastic sextant. Index correction is always the same unless the sextant is dropped or mirror ad,justments are made. The weight (2oz to 4 pounds) and openwork frame reduce windage problems, the better optics and filters give accuracy and the life of the instrument is great as long as you care in use and storage.
The metal frame may be made of either brass or an aluminum alloy, lightening the weight from roughly four to three pounds. The size of the frame varies changing the weight. I find that the lighter sextants are easier for me to hold for a length of time.
The telescope power varies from three to eight power, the advantage of the higher power being its ability to pick up the light of a star earlier in the twilight when the naked eye still cannot see it. The disadvantage of greater power is reduce the field of view and this becomes critical when the when you are trying to keep the celestial body in the field while bouncing around on a small vessel.
Lighting is another option. Of course this isn't needed during the day but near the end of twilight, it is convenient to press a button or turn a switch to illuminate the arc and micrometer drum. The battery case, wires and bulb socket are all subject to corrosion at sea and batteries tend to wear down when you need need it. Cases usually come with the sextant and are included in the price.
There are several good makers of metal sextants Weems and Plath, Tamaya, and Plath. Some Navy surplus sextants can be found at reasonable prices.
The final choice of instrument to buy comes down to how much you can afford, how important celestial navigation is to you, how comfortable the instrument is to use, and how experienced you are.
When you get your sextant adjustments to the mirrors may be necessary to reduce the index correction. One or two adjusting screws are located on each mirror for this purpose. Each mirror should be perpendicular to the sextant frame and when the sextant is set at zero the two mirrors should be parallel to each other. Three tests are involved.
The first test is for perpendicularity of the index mirror. Hold the sextant on its side (with handle down) and with the index arm set to 35°.
Place your eye lose to the sextant near the index mirror so that you can see the sextant arc in the mirror (reflected) and also just to the right of the mirror (direct). If these two images are not in a straight or continuous line, the mirror is not perpendicular to the frame. Adjusting the screws will bring the images in line.
The second test is for perpendicularity of the horizon glass. The "glass" is only half glass with the right half of the frame filled with a mirror. The horizon is viewed through the glass, the reflected image of the celestial object viewed in the mirror. If this horizon glass is not perpendicular to the frame, the error is referred to as side error. If a star is viewed both in the glass and in the mirror with the sextant set near zero, by adjusting the altitude, the star should pass over itself and become superimposed. If instead the reflected image of the star passes to the right of the direct image, side error exists and can be minimized by adjusting the two screws at the base of the horizon glass, or you can use a distant terrestrial object.
The third test is for parallelism of the index mirror and horizon glass when the index arm is set exactly at zero. At this setting the horizon or a celestial body appear higher or lower in the mirror than in the glass, the mirrors are not parallel and should be adjusted until they are. This error is called index error.
A sextant is not difficult to use but it takes practice to get a sight quickly and accurately, especially aboard a bouncing vessel. The instrument is held vertically in the right hand and the sighting is made through the telescope. The horizon is observed in the horizon glass while the celestial object is found in the mirror and positioned such that it is in line with the horizon. In the case of the Sun or Moon, the edge of the body is placed on the horizon. If the lower edge is used, the sight is referred to as a lower limb sight. An upper limb sight is less often used with the Sun but is sometimes used with the Moon since the lower edge may not actually be a circular one, depending on the phase.
Once the body is lined up, the sextant is "rocked" or pivoted as if the top of the index arm were attached to the rod of a pendulum and the arc were at the bottom with the swinging action. This is done to so that the sextant is held vertically when the sight is taken. As the rocking is done, the celestial body will seem to trace an arc with to the horizon.
The sextant is vertical or when the body is at the bottom of the arc. The sight is then "marked" the observer says "mark" to his timekeeper or observes the time himself.
The ancular height of the celestial body is read on the arc and on the micrometer drum. The arc dissplays the degrees and the drum displays the minutes and tenths of minutes (or in some cases minutes and seconds).
A arrow on the index arm points to the degrees on the arc. The degree is pick that rests just to the right of the arrow. If the arrow pointing to the micrometer drum lies between two minutes, an estimation is made as to how many tenths of the way between it is or sometimes a vernier is available on the index arm for that purpose.
There are several techniques of getting the celestial body in the field of view, in sighting the Sun, and having good sea conditions, the observer can get the horizon under the Sun in the glass and then move the index arm back and forth, homing in on the glare surrounding the Sun until the Sun's disk is seen. Filters will be needed in front of the index mirror to protect the eye from the Sun's brightness. Also, filters may be necessary in front of the horizon glass if the Sun's sparkle on the water is too bright.
A second method is useful for non-glaring objects such as the planets, stars and daytime Moon. Hold the sextant upside down in the left hand and sight through the glass toward the celestial object. Then move the index arm until the horizon appears in the mirror. The advantage in this method is that it is easier to find the celestial object by direct observing and leave the easily found horizon line for the moving mirrors. Once the object is lined up with the horizon, the sextant is turned right side up and the final adjustments with the micrometer drum are made.
If some mathematical calculations are made ahead of time, the rough altitude of the celestial body can be figured allowing a third method to be used. This involves presetting the sextant to the prefigured altitude and then scanning the horizon with the horizon glass until the celestial body comes into the field of view of the mirror. The rough azimuth of the body can also be figured so that the area of scanning can be limited. For this method sextant would be held right side up the whole time.
Some practical things when using the sextant are, if the instrument represents a considerable investment and happens to be the only sextant aboard. A lanyard attached to the sextant and to the observer saves accidental dropping of the instrument, on deck or to be lost to Davy Jones Locker. Bracing yourself when taking a sight over the rail saves the navigator from the same fates. If rigging and comfort allow, sitting amidships when taking the sight is the most stable position since it is the pivot point of the ship's motions. On a small vessel, the navigator has to learn to rise and fall to counteract the motion.
Sighting when the ship gets to the top of a wave is important to insure that the real sea horizon is used rather than the closer top of a nearby wave. The real sea horizon can vary in distance depending on the height of the observer's eye but corrections for this can be made.
From experience I can say that it is embarrassing to not be able to find a star in the sextant when dozens of them are clearly visible to the naked eye - all because salt deposit from sea spray has not been washed off the mirrors. A sextant should be stored always in its case and wiped over after each use when a lot of spray has been flying. This will not only prolong the life of the sextant but also reduce maintenance and embarrassing situations.
A sextant can also be used in piloting. Held vertically, it can give the angular height of a light or marker above the water. With a little math, distance off of the object can be found.
Second use in piloting involves finding horizontal angle between terrestrial objects. The sextant is held horizontally while one object is sighted through the glass and another is found in the mirror. Combining sights of three objects will give a fix. In both of these piloting situations, index correction must be applied to the reading.
