Humboldt State University
Robert A. Paselk Scientific Instrument Museum
Micrometer Sextant, US Maritime Commission
Leupold & Stevens Instruments
Ser. No. 1997
Humboldt College; 1945
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°).
Click here to see an image of vernier sextant with the main components labeled. Vernier sextants have arcs graduated to more than 140° (measuring 70+°) because of the 20° overlap required of the vernier scale is seen in this image.
Modern instruments, which often have an arc graduated to 144° using an arc of 72° (a fifth of a circle), read the full 144° since they use a micrometer readout instead of a vernier, as is the case with the sextants at Humboldt, making them quintants. 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:
- Gif animation courtesy of its creator, Joaquim Alves Gaspar, via the Wikipedia Commons
This sextant was used to instruct oceanography students at Humboldt. When the sextant was transferred to the Museum in September 2010 there were a total of nine sextants in the Natural Resources stockroom. The entire collection of sextants may be viewed by following this link
US Merchant Marine micrometer sextant, "inscribed" with raised caset-letters "U.S. Maritime Commission" and "Leupold & Stevens Instruments Portland Oregon." (see closeup of logo).
This instrument has a cast bronze frame finished in black enamel with the 6.5 inch radius arc graduated on the limb from -5° to 145°. The micrometer reads directly to one minute of arc, wih a vernier reading to 10" of arc. The tapered micrometer drive worm is nickel-silver. The bronze arc/gear is cut directly into the arc. There are three, one and one eigth inch diameter, round index shadesor filters (two red of medium and high density and one high density neutral) and two horizon shades (red medium density and high density neutral; see sextant image with red shades in optical path). The index arm, telescope and furniture, including the cone-bearing housing/leg, are of cast white metal (specific gravity = 6.32 for telescope bracket, zinc-aluminum alloy?). The legs and handle furnture are brass The handle is a dark composite material.
The 12.75"w x 11.25"d square by 6" tall box is constructed of 1/2" light-colored hardwood with a mahogony stain on the outside surfaces only, with dovetailed corners (see image of case-front, image of case-side showing dovetails and image of sextant in case). The 1/4" mahogany plywood top and bottom are attached with nails and glue. The sextant case is original to the sextant, the serial number seen on the sextant matchs that of the certificate, as seen in the certificate image. There are places in the box for a spare mirror (missing) and eyepiece filter (missing). The latches on the case have a locking mechanism which can bee seen in the side image of the case where the tab can be rotated to hold or release the latch-hook
© R. Paselk