Every-day Science: Volume 7. The Conquest of Time and Space

by Henry Smith Williams; Edward Huntington Williams

The instrument with which such astronomical observations are made is known as the sextant. Its purpose is to measure with great accuracy the angle between two objects, which in practice are the horizon line on one hand and some celestial body, usually the sun, on the other. The determination of the latitude of the ship, for example, is a matter of comparative ease, if the sun chances to be unobscured just at midday. The navigator has merely to measure the exact elevation of the sun as it crosses the meridian,—that is to say when it is at its highest point,—and, having made certain corrections for so-called dip and refraction, to which we shall refer more at length in a moment, a very simple calculation reveals the latitude—that is to say, the distance from the terrestrial equator.

That the latitude of a ship could thus be determined, with greater or less accuracy, has been familiar knowledge to seamen from a very early period. It was by the use of this principle that the earth was measured by Eratosthenes and Posidonius in classical times, and the sailors of antiquity probably carried with them a crude apparatus for measuring the height of sun and stars, as the mediæval navigators are known to have done.

The simplest and crudest form of measurer of which the record has been preserved is known as the cross-staff. This consisted essentially of a stick about a yard in length, called the staff, on which a cross-piece was arranged at right angles, so adjusted at the center as to slide back and forth on the staff. An eye-piece at one end of the staff was utilized to sight along projections at either end of the cross-piece. If the apparatus is held so that one of the lines of sight is directed to the horizon, and then the cross-piece slid along the staff until the other line of sight is directed toward the sun or a given star, the angle between the two lines of sight will obviously represent the angle of altitude of the celestial body in question. But the difficulty of using an apparatus which requires two successive observations to be made without shift of position is obvious, and it is clear that the information derived from the cross-staff must have been at best very vague—by no means such as would satisfy the modern navigator.

Even the navigators of the fifteenth century were aware of the deficiencies of the cross-staff and sought to improve upon it. The physicians of Henry the Navigator of Portugal, Roderick and Joseph by name, and another of his advisers, Martin de Bohemia, are credited with inventing, or at least introducing, a much improved apparatus known as the astrolabe. This consists of a circle of metal, arranged to be suspended from a ring at the side, so that one of its diameters would maintain the horizontal position through the effect of gravity. A superior quadrant of the circle was marked with degrees and minutes. A straight piece of metal, with sights so that it could be accurately pointed, was adjusted to revolve on a pivot at the center of the circle. This sighting piece being aimed at the sun, for example, the elevation of that body could be read directly on the measuring arc of the circle. Here, then, was no new principle involved, but the instrument had obvious points of advantage over the cross-staff, in particular because only a single sight need be taken, the horizon line being determined, as already explained, through the action of gravitation.

The astrolabe did not gain immediate favor with practical navigators, and it was at best a rather clumsy instrument, subject to peculiar difficulties when used on a rolling ship. Many attempts were made to improve upon it, but for a long time none of these was altogether successful. The final suggestion as to means of overcoming the difficulties encountered in measuring the altitude of astronomical bodies was made by Sir Isaac Newton. But nothing practical came of his discovery, as it was not published until a long time after his death. Meantime independent discovery of the same principle was made by Thomas Godfrey of Philadelphia, in 1730, and by the English astronomer Hadley, who published his discovery before the Royal Society in 1731. The instrument which Hadley devised was called a quadrant. The principle on which it worked involved nothing more complex than the use of two mirrors, one of them (known as the horizon glass and having only half its surface mirrored) fixed in the line of vision of a small telescope; the other (called the index mirror) movable with the arm of an indicator, which is so adjusted as to revolve about the axis of the quadrant. In operation these two mirrors enable the images of two objects, the distance between which is to be measured, to be superimposed. The telescope may be pointed at the horizon, for example, directly under the position of the sun, and the arm of the instrument, altering the position of the so-called index mirror, may be rotated until the limb of the sun seems just to touch the horizon—the latter being viewed through the unsilvered half of the horizon glass. The scale at the circumference of the instrument is marked in half-degrees, which, however, are registered as whole degrees, and which, so interpreted, give the direct measurement of the angular distance between the horizon and the sun; in other words the measurement of the sun's altitude or so-called declination.

The instrument just described, perfected as to details but not modified as to principles, constitutes the modern sextant, which is used by every navigator, and which constitutes, along with the compass and chronometer, the practical instrumental equipment that enables the seaman to determine—by using the tables of the Nautical Almanac—his exact position on the earth's surface from observation of the sun or certain of the fixed stars. The modern instrument is called a sextant because it has, for convenience' sake, been restricted in size to about one-sixth of a circle instead of the original one-quarter, the small size being found to answer every practical purpose, since it measures all angles up to 120 degrees.

In practice the sextant is an instrument only six or eight inches in diameter. It is held in the right hand and the movable radial arm is adjusted with the left hand with the aid of a micrometer screw, and the reading of the scale is made accurate by the vernier arrangement. The ordinary observation—which every traveler has seen a navigator make from the ship's bridge just at midday—is carried out by holding the sextant in a vertical position directly in line of the sun, and sighting the visible horizon line, meantime adjusting the recording apparatus so as to keep the sun's limb seemingly in touch with the horizon. As the sun is constantly shifting its position the vernier must be constantly adjusted until the observation shows that the sun is at the very highest point. The instrument being clamped and the scale read, the latitude may be known when proper correction has been made for the so-called dip, for refraction, and where great accuracy is required for parallax.

"TAKING THE SUN" WITH THE SEXTANT.

The instrument is held in the right hand, and levelled at the horizon; the left hand manipulating the micrometer screw which adjusts the radial arm carrying the index mirror (at top of figure). The result is read on the Vernier scale (arc at bottom of figure) with the aid of the magnifying glass.