Humboldt State University
Robert A. Paselk Scientific Instrument Museum
Micrometer Sextant, English
David Shackman & Sons
for Kelvin & Wilfred White Co.
Boston & New York
Ser. No. 2337
Humboldt State University; 1943
The sextant is the iconic instrument for navigation, however its dominance is fairly recent and short lived. Its development was driven by the need to determine the longitude, and in particular by the “Longitude Prize” offered by the Board of Longitude in the early 18th century. Longitude is determined primarily by finding the local time compared to the time at a known location (today the Greenwich prime meridian). Before the development of precise reliable clocks for use at sea, astronomy provided a number of potential clocks, the most useful being “Lunar distances” or the distance between the Moon and a known star. The early instruments of celestial navigation (such as the mariner’s astrolabe, quadrant, cross-staff, and backstaff) by could successfully measure the altitude of the Sun or other celestial object compared to the horizon, but they were difficult or impossible to use for measuring the lunar distance. The reflecting octant (also called a reflecting quadrant) was invented around 1730 by Hadley and others to solve this problem. The octant could measure up to a quarter of a circle (90°), thus the name quadrant, but due to the reflection process only required an arc of an eighth of a circle (giving the name octant). The octant gradually became the dominant instrument, displacing other angle measuring navigational instruments by the end of the 18th century as production costs for this more complex but easier to use and superior instrument came down. The later development of the sextant occurred because important lunar distances often exceeded the 90° direct measuring capability of the octant.
Most octants were made of ebony, though brass octants became more common as the 19th century progressed. The octant was the workhorse navigational instrument up until the 20th century, with sextants, being more delicate and significantly more expensive, being reserved for determining lunar distances or other precise measurements. Improvements in technology and production methods brought prices down and “sextants” (measuring 120° or more) became the standard for the 20th century.
The modern navigational sextant is designed to precisely and accurately measure the angle between two points. In modern usage it is most commonly used to measure the altitude of a celestial object or the angle between a celestial object and the horizon. The sextant is so named because its arc encompasses one sixth of a circle (60°), however, due to the optical properties of the reflecting system it measures up to a third of a circle (120°).
Micrometer instruments, still usually referred to as sextants, often have an arc graduated to more than 120° using an arc greater than 60°, in this case the graduations extend to 135°, but can only be used to 128°. A close-up image of a Navy sextant micrometer mechanism with labels is available here. The operational principle and basic use of the sextant is summarized in the animation accessed by clicking on the icon below:
This sextant was used as an example and demonstration instrument in science education lectures at Humboldt in the early 21st century.
English micrometer sextant, inscribed "MADE IN ENGLAND FOR KELVIN & WILFRED. O. WHITE Co. BOSTON & NEW YORK." and (see closeup of inscription) and "BRITISH PATENT No 556034" and DS&S in a circle logo over the serial number 2337 (closeup of DS&S logo etc.). A copy of the patent abstract is provided and a copy of the patent itself. An apparently identical instrument is in the NMAH collection and is described, including information about the maker, and illustrated at: http://amhistory.si.edu/navigation/object.cfm?recordnumber=1197558
This instrument has a cast bronze frame with the 6.5 inch radius arc graduated on the limb from -5° to 135°. All exposed metal, with the exception of the scales, is coated with a black enamel "crinkle" finish. The micrometer reads directly to one minute of arc, with a vernier reading to 1/6 minute = 10" of arc, as seen in the image of the vernier. The cylindrical micrometer drive worm is brass. The bronze arc/gear is cut into the limb. There are four 1" diameter, round index shades in "square" mounts and three 1" diameter, round horizon shades in :round mounts" (one blue 'lunar'and the rest neutral density filters in each set) as seen in the sextant image above. The index arm and telescope are of brass. The legs and furniture are brass. The handle is made of a dark hardwood (walnut?).
The 10.75" wide by 10.5" deep by 5.5" high box is constructed of 1/2" solid walnut (see image of case) with box "finger" jointed corners. The 5/16" solid walnut top and bottom are attached with screws. "MADE IN ENGLAND" and "17" are stamped on a corner of the bottom of the case, note that the number 17 matches that stamped on the handle of the sextant. A certificate from the National Physical Laboratory is attached to the interior of the lid of the case, describing the corrections needed for the precise measurements, as seen in the certificate image.* There are places in the box for the telescope and telescope eyepiece filter as seen in the image of sextant in case. A small bent wire asdjusting wrench is also present, seen just below the eyepiece filter. Press tabs are provided to lock the hooks holding the case securely closed.
* Note the signature of C.G. Darwin, grandson of Charles Darwin and a well known physist in his own right, who became Director of the National Physical Laboratory in 1935, when he became Master of Christ's College, Cambridge, a position he retained until 1949.
** http://en.wikipedia.org/wiki/List_of_Liberty_ships_(G–Je) downloaded 4/29/2014
© R. Paselk