# How It Works: Science and Technology - Vol. 15

By Wendy Horobin | Go to book overview

Sextant

An early model of a
sextant. This nautical
instrument was introduced
in the 18th century and
revolutionized travel by
enabling accurate mapping
of the world.

Since the earliest ocean voyages navigators have had to fix their position at sea by means of measuring the angles above the horizon of heavenly bodies: the Sun, Moon, and stars. The simplest, and least accurate, of these methods is to measure the altitude (angle above the horizon) of the Pole Star (in the Northern Hemisphere). More accurately, the altitude of the Sun at local noon or of bright stars whose position is known at their highest point will give the latitude after simple calculations from tables.

The first instruments to be used for measuring these angles were astrolabes, cross staffs, back staffs, and quadrants—all variations on the theme of a sighting bar moved along a scale of degrees. In most cases, it was necessary to view both the star and the horizon at the same time from the deck of the ship, and it is not surprising that the observations were inaccurate. In the case of the quadrant, the reference point was not the horizon but a plumb line attached to the scale. This instrument made it possible to concentrate on the star only, but the plumb line could easily swing about, leading to further errors.

The device that replaced these instruments, the forerunner of the sextant, was the reflecting octant invented by an Englishman, John Hadley, in 1731. The principle and design of the octant was the same as that of any sextant in daily use today: the main difference is that the octant had a scale that was one-eighth of a circle, 45 degrees, and a sextant has a scale of one-sixth of a circle, 60 degrees. Because both devices measure an angle that is reflected by a mirror, the octant will measure angles up to 90 degrees, and the sextant, angles up to 120 degrees.

Hadley’s invention used a pair of small mirrors to reflect the image or the star to be observed so that it appeared to be on the horizon. The navigator could keep both in view at the same time, and as the ship rolled, both would move together. The movable mirror was attached to the pivot of the movable index bar at the radius of the scale or arc, so that, as the angle was changed, the mirror would move. This mirror reflected the star’s image to a second mirror, permanently set to view the first one. The navigator looked through a sight to the second mirror, past which he could see the horizon. He moved the first by moving the index bar until the star’s image reflected by both mirrors exactly touched the horizon. The angle was then read off the arc, which was graduated in degrees (but twice as closely as a true scale of degrees, to allow for the mirror’s reflection).

Hadley’s octant was immediately accepted by navigators. In 1757, John Campbell introduced the true sextant, which was capable of measuring a greater angle. Captain Cook, the English explorer, was probably the first to fully apply the potential of the sextant for measuring not only vertical angles but also angles at any inclination. By measuring the angle between the Moon and a given star, he could calculate the precise time using tables of the Moon’s motion, thus enabling him to find his longitude as well as his latitude— he used this method for the charting of New Zealand during the voyage of 1768–1771 The invention of accurate timekeepers made the procedure unnecessary, and the sextant was then used to measure the altitude of stars or the Sun at precise times, thus giving the longitude whenever it was required.

Although the earliest sextants and octants had simple sighting devices, the accuracy was much improved by the use of a small telescope instead. The second mirror, the horizon mirror, would be silvered only across half its width so that the telescope would show both the horizon and the star side by side. Dark filters could be moved into the light paths to cut down the brightness of the Sun or horizon.

-2062-

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How It Works: Science and Technology - Vol. 15

• How It Works® Science and Technology 2017
• Title Page 2019
• Contents 2020
• Salvage, Marine 2021
• Satellite, Artificial 2024
• Schlieren Techniques 2030
• Screw Manufacture 2032
• Seaplane and Amphibian 2035
• Sea Rescue 2038
• Security System 2042
• Seismology 2046
• Self-Righting Boat 2050
• Semiconductor 2052
• Servomechanism 2055
• Sewing Machine 2058
• Sextant 2062
• Sheet Metal 2064
• Ship 2067
• Shutter 2074
• Silicon 2076
• Silicone 2078
• Silver 2079
• Sine Wave 2081
• Siphon 2083
• Ski and Snowboard 2084
• Skin 2087
• Skyscraper 2090
• Slaughterhouse 2096
• Sleep 2099
• Smell and Taste 2103
• Soap Manufacture 2107
• Soft-Drink Dispenser 2109
• Soil Research 2110
• Solar Energy 2114
• Solar System 2118
• Solenoid 2124
• Sonar 2126
• Sorption 2129
• Sound 2131
• Sound Effects and Sampling 2136
• Sound Mixing 2138
• Soundproofing 2140
• Sound Reproduction 2142
• Soundtrack 2146
• Space Debris 2150
• Space Photography 2152
• Space Probe 2156
• Index i
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