Octant (instrument) Totally Explained

Everything about Octant Instrument totally explained

The Octant, also called reflecting quadrant, is a measuring instrument used primarily in navigation. It is a type of reflecting instrument.

Etymology

The name octant derives from the Latin octans meaning eighth part of a circle, because the instrument's arc is one eighth of a circle. Reflecting quadrant derives from the instrument using mirrors to reflect the path of light to the observer and, in doing so, doubles the angle measured. This allows the instrument to use a one-eighth circle arc to measure a quarter circle or quadrant.

Origin of the Octant

Newton's reflecting quadrant

Isaac Newton's reflecting quadrant was invented around 1699. A detailed description of the instrument was given to Halley and the description wasn't published until after Halley's death in 1742. It isn't known why Halley didn't publish the information during his life, however this prevents Newton from getting the credit for the invention that's generally given to Hadley and Godfrey.
   One copy of this instrument was constructed by Thomas Heath and may have been shown in Heath's shop window prior to its being published by the Royal Society in 1742.
   Newton's instrument used two mirrors, but they were used in an arrangement somewhat different than the two mirrors found in modern octants and sextants. The diagram on the right shows the configuration of the instrument.
   The 45° arc of the instrument (P-Q), was graduated with 90 divisions of a half-degree each. Each such division was subdivided into 60 parts and each part further divided into sixths. This results in the arc being marked in degrees, minutes and sixths of a minute (10 seconds). Thus the instrument could have readings interpolated to 5 seconds of arc. This fineness of graduation is only possible due to the large size of the instrument - the sighting telescope alone was three to four feet long.
   A sighting telescope (A-B), three or four feet long, was mounted along one side of the instrument. A horizon mirror, was fixed at a 45° angle in front of the telescope's objective lens (G). This mirror was small enough to allow the observer to see the image in the mirror on one side and to see directly ahead on the other. The index arm (C-D) held an index mirror (H), also at 45° to the edge of the index arm. The reflective sides of the two mirrors nominally faced each other, so that the image seen in the first mirror is that reflected from the second.
   With the two mirrors parallel, the index reads 0°. The view through the telescope sees directly ahead on one side and the view from the mirror G sees the same image reflected from mirror H (see detail drawing to the right). When the index arm is moved from zero to a large value, the index mirror reflects an image that's in a direction away from the direct line of sight. As the index arm movement increases, the line of sight for the index mirror moves toward S (to the right in the detail image). This shows a slight deficiency with this mirror arrangement. The horizon mirror will block the view of the index mirror at angles approaching 90°.
   The length of the sighting telescope seems remarkable given the small size of the telescopes on modern instruments. This was likely Newton's choice of a way to reduce chromatic aberrations. Short focal length telescopes, prior to the development of achromatic lenses, produced an objectionable degree of aberration, so much so that it could effect the perception of a star's position. Long focal lengths were the solution and this telescope would likely have had both a long focal length objective lens and a long focal length eyepiece. This would decrease aberrations without excessive magnification.

The Octant

Two men independently developed the octant around 1730: John Hadley (1682-1744), an English mathematician, and Thomas Godfrey (1704-1749), a glazier in Philadelphia. While both have a legitimate and equal claim to the invention, Hadley generally gets the greater share of the credit. This simply reflects the central role that London and the Royal Society played in the history of scientific instruments in the 18^th century. Those outside the social circles within London and the Royal Society didn't get the historical attention they deserved.
Two others who created octants during this period were Caleb Smith, an English insurance broker with a strong interest in astronomy (in 1734) and Jean-Paul Fouchy, a mathematics professor and astronomer in France (in 1732).

Hadley's versions

Hadley produced two versions of the reflecting quadrant. Only the second is well known and is the familiar octant.

Hadley's reflecting quadrant

Hadley's first reflecting quadrant was a simple device with a frame spanning a 45° arc. In the image at the right, from Hadley's article in the Philosophical Transactions of the Royal Society

Hadley's Octant

Hadley's second design had the form familiar to modern navigators. The image to the right, also taken from his Royal Society publication, shows the details.
   He placed an index mirror on the index arm. Two horizon mirrors were provided. The upper mirror, in the line of the sighting telescope, was small enough to allow the telescope to see directly ahead as well as seeing the reflected view. The reflected view was that of the light from the index mirror. As in the previous instrument, the arrangement of the mirrors allowed the observer to simultaneously see an object straight ahead and to see one reflected in the index mirror to the horizon mirror and then into the telescope. Moving the index arm allowed the navigator to see any object within 90° of the direct view.
   The significant difference with this design was that the mirrors allowed the instrument to be held vertically rather than horizontally and it provided more room for configuring the mirrors without suffering from mutual interference.
   The second horizon mirror was an interesting innovation. The telescope was removable. It could be remounted so that the telescope viewed the second horizon mirror from the opposite side of the frame. By mounting the two horizon mirrors at right angles to each other and permitting the movement of the telescope, the navigator could measure angles from 0 to 90° with one horizon mirror and from 90° to 180° with the other. This made the instrument very versatile. For unknown reasons, this feature wasn't implemented on octants in general use.
   Comparing this instrument to the photo of a typical octant at the top of the article, one can see that the only significant differences in the more modern design are:

The location of the horizon mirror and telescope or sighting pinnula is lower.
The internal bracing of the frame is more central and robust.
The position of the shades for the index mirror is in the path between the index and horizon mirrors rather than at the top of the instrument.
Multiple shades are used to allow for different levels of shading.
Separate shades are provided on the horizon mirror for sighting a low sun position with a very bright horizon.
The second horizon mirror and accompanying alidade isn't provided.

Smith's Astroscope

Caleb Smith, an English insurance broker with a strong interest in astronomy, had created an octant in 1734. He called it an Astroscope or Sea-Quadrant. His used a fixed prism in addition to an index mirror to provide reflective elements. Prisms provide advantages over mirrors in an era when polished speculum metal mirrors were inferior and both the silvering of a mirror and the production of glass with flat, parallel surfaces was difficult.
   In the drawing to the right, the horizon element (B) could be a mirror or a prism. On the index arm, the index mirror (A) rotated with the arm. A sighting telescope was mounted on the frame (C). The index didn't use a vernier or other device at the scale (D). Smith called the instrument's index arm a label, in the manner of Elton for his mariner's quadrant.
   Octants were produced in large numbers. In wood and ivory, their relatively low price compared to an all-brass sextant made them a popular instrument. The design was standardized with many manufacturers using the identical frame style and components. Different shops could make different components, with woodworkers specializing in frames and others in the brass components. For example, Spencer, Browning and Rust, a manufacturer of scientific instruments in England from 1787 to 1840 (operating as Spencer, Browning and Co. after 1840) used a Ramsden dividing engine to produce graduated scales in ivory. These were widely used by others and the SBR initials could be found on octants from many other manufacturers.
   Examples of these very similar octants are in the photos in this article. The image at the top is essentially the same instrument as the one in the detail photos. However, they're from two different instrument makers - the upper is labelled Crichton - London, Sold by J Berry Aberdeen while the detail images are of an instrument from Spencer, Browning & Co. London. The only obvious difference is the presence of horizon shades on the Crichton octant that are not on the other.
   These octants were available with many options. A basic octant with graduations directly on the wood frame were least expensive. These dispensed with a telescopic sight, using a single- or double-holed sighting pinnula instead. Ivory scales would increase the price, as would the use of a brass index arm or a vernier.

Demise of the Octant

In 1767 the first edition of the Nautical Almanac tabulated lunar distances, enabling navigators to find the current time from the angle between the sun and the moon. This angle is sometimes larger than 90°, and thus not possible to measure with an octant. For that reason, Admiral John Campbell, who conducted shipboard experiments with the lunar distance method, suggested a larger instrument and the sextant was developed.
From that time onward, the sextant was the instrument that experienced significant development and improvements and was the instrument of choice for naval navigators. The octant continued to be produced well into the 19^th century, though it was generally a less accurate and less expensive instrument. The lower price of the octant, including versions without telescope, made it a practical instrument for ships in the merchant and fishing fleets.
One common practice among navigators up to the late nineteenth century was to use both a sextant and an octant. The sextant was used with great care and only for lunars, while the octant was used for routine meridional altitude measurements of the sun everyday. This protected the very accurate and pricier sextant while using the more affordable octant where it performs well.

Use and Adjustment

Use and adjustment of the octant is essentially identical to the navigator's sextant, which see for information on these topics.

Other reflecting instruments

Hadley's wasn't the first reflecting quadrant. Robert Hooke invented a reflecting quadrant in 1684 and had written about the concept as early as 1666. Hooke's was a single-reflecting instrument. Other octants were developed by Jean-Paul Fouchy and Caleb Smith (astronomer) in the early 1730s, however, these didn't become significant in the history of navigation instruments.

Further Information

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