Wednesday, June 18, 2008

The Marine Sextant (Correctable and Non-Correctable Errors)

Devices for measuring the altitude of celestial bodies above the horizon along a vertical circle have existed for centuries. The earliest devices were cross staffs which consisted of two pieces of wood. Later came quadrants, octants, and sextants which incorporated arcs of one fourth, one eighth, and one sixth of a circle, respectively.

The instruments used by early explorers were made of wood, but by the time of the American Revolution instruments of cast brass or bronze were most common. The modern "micrometer drum" sextant's accuracy can be as close as one tenth mile when used by a skilled navigator in good weather conditions.

Sextants are used to measure the angle between two points. Its main use is to measure the altitude (angle) of celestial bodies above the sea horizon. The sextant is also used to measure the angle (or difference in bearing) between two terrestrial objects by turning it on its side.

The Frame: Constructed of either brass or aluminum. It is the part of the sextant to which all the other parts are attached.

The limb: The bottom part of the frame which is cut with gear teeth.

The Arc: The arc is graduated with a scale representing altitude readings from a few degrees negative to over 120°.

The Index Arm: A movable bar pivoted about the center of curvature of the limb. The index mirror and micrometer drum/tangent screw assembly are fixed to it.

The Tangent Screw: It is the worm gear and shaft which engages the limb.

The Release Levers: These spring actuated clamps hold the tangent screw against the limb and keeps the worm gear engaged. Pinching them togeth­er releases the tangent screw so that the navigator may make large changes to the position of the arm.

The Micrometer Drum: Sixty minutes of arc are graduated around its cir­cumference. It is rigidly mounted on the shaft of the tangent screw and one revolution of the drum changes the altitude measurement one degree.

The Vernier Scale: Adjacent to the micrometer drum and fixed to the arm, it allows reading to the nearest tenth of a minute of arc.

The Index Mirror: Mounted on the index arm perpendicular to the plane of the frame of the sextant. It is directly adjacent with the pivot point of the arm.

The Horizon Glass: One half mirror and one half clear glass. It is mount­ed perpendicular to the frame of the sextant directly in the line of sight of the telescope. The silvered half of the glass is closest to the frame of the sextant.

The Shade Glasses: Those for the index mirror allow observation of the sun or bright moon and the horizon glass shades reduce glare on the hori­zon for low altitude sights.

The Telescope: Amplifies both direct and reflected images observed.

The Handle: Made of plastic or wood and is designed for right-hand use. Often, the batteries for the light which illuminates the arc for night read­ings are in the handle.

One of the interesting things about the sextant is that it can measure an angle of over 120 degrees, even though its arc is actually only sixty degrees. This is possible because of the optical principle of the sextant. The angle between the body and the horizon is about twice the angle between the index mirror and horizon glass. In fact, it is exactly twice the angle, and that is why a sextant with an arc of 60° can read angles of up to 120°.

Reading a marine sextant is not difficult, but must be done in a certain order. First, read the number of degrees indicated by the index arm. Next, note where the zero mark on the vernier is pointing to read the minutes of arc. Next, note where the zero mark on the vernier is pointing to read the min­utes of arc. Finally, to read tenths of minutes, check to see which two marks are most closely aligned between the vernier and the drum.


There are four correctable and three non-correctable errors which may be found in marine sextants. The correctable errors are:

1. Error of perpendicularity - This error results from the index mirror not being perpendicular to the sextant frame.

2. Side Error - This error results from the horizon glass not being perpen­dicular to the frame of the sextant.

3. Index Error - This error results from the horizon glass not being paral­lel to the index mirror when the sextant is set on zero.

4. Collimation Error - An error which results from the telescope not being parallel to the frame.

These errors may be corrected by the navigator using various adjustment points on the sextant.
The non-correctable errors on the sextant are:

1. Graduation Error - These small errors are caused by imperfections in machining the arc, cutting the limb gears, or marking the scale of the arc or micrometer drum.

2. Prismatic Error - This error is caused by the planes of a mirror not being exactly parallel.

3. Centering Error - This error results when the index arm is not pivoted at the exact center of curvature of the arc.

The sum of these errors, called instrument error, may be found on the manufacturer's certificate located inside the lid of the sextant case. The amount of error may change with the altitude being measured. To correct a sextant reading for instrument error, the sign of the error is simply reversed to become the instrument correction, which is applied to the sextant altitude observation.

Of all these errors index error is the most important because it is an error which directly effects altitude measurements. Because sextant readings can be effected by changing temperatures, which tend to expand or contract the metal parts of a sextant, the navigator should determine the amount of index error at least once each day.

Index error is determined by setting the sextant near zero, pointing it at the horizon and then turning the micrometer drum slowly until the actual and reflected images of the horizon are aligned. If the sextant reads zero when the horizons are aligned, there is no error. If not, note the sextant reading when the horizons are aligned. If the reading is a positive angle (greater than 0°-00.0') the error is said to be "on the arc." If the reading is less than zero, or a negative angle, it is aid to be "off the arc." If the error determined is on the arc it must be taken off (the index cor­rection (IC) is negative). If off the arc, put it on (the IC is positive). As a memory aid remember, "If it's ON it's OFF, and if it's OFF it's ON."